Patent Publication Number: US-11039511-B2

Title: Defrosting apparatus with two-factor mass estimation and methods of operation thereof

Description:
TECHNICAL FIELD 
     Embodiments of the subject matter described herein relate generally to apparatus and methods of defrosting a load using radio frequency (RF) energy. 
     BACKGROUND 
     Conventional capacitive food defrosting (or thawing) systems include large planar electrodes contained within a heating compartment. After a food load is placed between the electrodes and the electrodes are brought into close proximity with the food load, electromagnetic energy is supplied to the electrodes to provide gentle warming of the food load. As the food load thaws during the defrosting operation, the impedance of the food load changes. Accordingly, the power transfer to the food load also changes during the defrosting operation. The duration of the defrosting operation may be determined, for example, based on a timer, which may be used to control cessation of the operation. Some conventional food defrosting (or thawing) systems may require the use of physical weight sensors to determine the weight of a food load. Some conventional systems may forego weight detection entirely, instead depending entirely on user input for the characterization of a food load. 
     For conventional systems that include physical weight sensors, such sensors may add to the cost and complexity of manufacturing the system. Additionally, although acceptable defrosting results are possible using systems that rely on user input for determining load weight, inaccuracies inherent in relying on user-defined weight of a food load may result in premature cessation of the defrosting operation, or late cessation after the food load has begun to cook. What are needed are apparatus and methods for defrosting food loads (or other types of loads) that may result in efficient and even defrosting throughout the load and cessation of the defrosting operation when the load is at a desired temperature without necessarily requiring the use of physical weight sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a perspective view of a defrosting appliance, in accordance with an example embodiment; 
         FIG. 2  is a perspective view of a refrigerator/freezer appliance that includes other example embodiments of defrosting systems; 
         FIG. 3  is a simplified block diagram of an unbalanced defrosting apparatus, in accordance with an example embodiment; 
         FIG. 4A  is a schematic diagram of a single-ended variable inductance matching network, in accordance with an example embodiment; 
         FIG. 4B  is a schematic diagram of a single-ended variable capacitive matching network, in accordance with an example embodiment; 
         FIG. 5A  is a schematic diagram of a single-ended variable inductance network, in accordance with an example embodiment; 
         FIG. 5B  is a schematic diagram of a single-ended variable capacitive network, in accordance with an example embodiment; 
         FIG. 6  is an example of a Smith chart depicting how a plurality of variable passive devices in embodiments of a variable impedance matching network may match the cavity plus load impedance to a radio frequency (RF) signal source; 
         FIG. 7  is a simplified block diagram of a balanced defrosting apparatus, in accordance with another example embodiment; 
         FIG. 8  is a schematic diagram of a double-ended variable impedance matching network with variable inductances, in accordance with another example embodiment; 
         FIG. 9  is a schematic diagram of a double-ended variable impedance network with variable inductances, in accordance with another example embodiment; 
         FIG. 10  is a schematic diagram of a double-ended variable impedance network with variable capacitances, in accordance with another example embodiment; 
         FIG. 11  is a cross-sectional, side view of a defrosting system, in accordance with an example embodiment; 
         FIG. 12A  is a perspective view of a double-ended variable impedance matching network module with variable inductances, in accordance with an example embodiment; 
         FIG. 12B  is a perspective view of a double-ended variable impedance matching network module with variable capacitances, in accordance with another example embodiment; 
         FIG. 13  is a perspective view of an RF module, in accordance with an example embodiment; 
         FIG. 14A  is a flowchart of a method of operating a defrosting system with dynamic load matching, in accordance with an example embodiment; 
         FIG. 14B  is a flowchart of a method of variable matching network reconfiguration, load mass estimation, load mass estimate refinement based on the rate of change of one or more signal parameters, and determining desired RF signal parameters in accordance with an example embodiment; 
         FIG. 14C  is a flowchart of a method of variable matching network reconfiguration, load mass estimation, load mass estimate refinement based on time elapsed between matches, and determining desired RF signal parameters in accordance with an example embodiment; 
         FIG. 14D  is a flowchart of a method of refining an initial mass estimate based on the rate of change of one or more signal parameters and refining desired RF signal parameters in accordance with an example embodiment; 
         FIG. 15  is a chart plotting cavity match setting versus RF signal source match setting through a defrost operation for two different loads; 
         FIG. 16A  is an example of a look-up-table (LUT) that may be used to determine parameters for a defrosting operation and estimate characteristics of a load based on the component values of a variable inductor network; and 
         FIG. 16B  is an example of a LUT that may be used to determine parameters for a defrosting operation and estimate characteristics of a load based on the component values of a variable capacitor network. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the words “exemplary” and “example” mean “serving as an example, instance, or illustration.” Any implementation described herein as exemplary or an example is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description. 
     Embodiments of the subject matter described herein relate to solid-state defrosting apparatus that may be incorporated into stand-alone appliances or into other systems. As described in greater detail below, embodiments of solid-state defrosting apparatus include both “unbalanced” defrosting apparatus and “balanced” apparatus. For example, exemplary “unbalanced” defrosting systems are realized using a first electrode disposed in a cavity, a single-ended amplifier arrangement (including one or more transistors), a single-ended impedance matching network coupled between an output of the amplifier arrangement and the first electrode, and a measurement and control system that can detect when a defrosting operation has completed. In contrast, exemplary “balanced” defrosting systems are realized using first and second electrodes disposed in a cavity, a single-ended or double-ended amplifier arrangement (including one or more transistors), a double-ended impedance matching network coupled between an output of the amplifier arrangement and the first and second electrodes, and a measurement and control system that can detect when a defrosting operation has completed. In various embodiments, the impedance matching network includes a variable impedance matching network that can be adjusted during the defrosting operation to improve matching between the amplifier arrangement and the cavity. 
     Generally, the term “defrosting” means to elevate the temperature of a frozen load (e.g., a food load or other type of load) to a temperature at which the load is no longer frozen (e.g., a temperature at or near 0 degrees Celsius). As used herein, the term “defrosting” more broadly means a process by which the thermal energy or temperature of a load (e.g., a food load or other type of load) is increased through provision of radio frequency (RF) power to the load. Accordingly, in various embodiments, a “defrosting operation” may be performed on a load with any initial temperature (e.g., any initial temperature above or below 0 degrees Celsius), and the defrosting operation may be ceased at any final temperature that is higher than the initial temperature (e.g., including final temperatures that are above or below 0 degrees Celsius). That said, the “defrosting operations” and “defrosting systems” described herein alternatively may be referred to as “thermal increase operations” and “thermal increase systems.” The term “defrosting” should not be construed to limit application of the invention to methods or systems that are only capable of raising the temperature of a frozen load to a temperature at or near 0 degrees Celsius. In one embodiment, a defrosting operation may raise the temperature of a food item to a tempered state at or around −1 degrees Celsius. 
     The mass of a load, may be used as a basis for determining an amount of energy that is sufficient to warm the load to a desired temperature (e.g., −1 degrees Celsius). The energy required to defrost a load may be determined using Equation 1:
 
 Q=m*c*ΔT    (Equation 1)
 
where Q is an amount of required heat energy, m is a mass of a load to which the heat energy is applied, c is the specific heat of the load, and ΔT is the change in temperature desired to be effected to the load by the application of the heat energy. The specific heat of various types of food tends to be around 1-2 calories/(gram° C.), where one calorie is approximately 4.1868 joules. The change in temperature applied to a load of a defrosting system is generally from around −20° C. (degrees Celsius) to around 0° C., such that AT may be estimated at around 20° C. Thus, the amount of heat energy (in calories) required to defrost a given load may be estimated as around 30 times the mass of the load (in grams). It should be noted that, in some embodiments, a value for AT may be determined based on an initial temperature input by a user, rather than being assumed to be 20° C.
 
     It should be understood that, while the terms “mass” and “weight” may sometimes be used interchangeably herein, both terms are used to describe a measure of the quantity of matter that a given body (e.g., load) contains. An initial estimate of the mass of a load in a cavity of a defrosting system may be determined based on impedance matching conditions (e.g., variable component values, S11 parameter values, etc.) of the defrosting system after an initial best or acceptable impedance match has been established between an RF signal source (e.g., that provides RF energy for heating the load) and the cavity by the defrosting system. For example, the mass of the load may be estimated by comparing component values of variable components in a variable impedance matching network (upon establishing an initial match) to corresponding component values stored in a look-up table (LUT) that is stored within a memory that is accessible to a system controller, according to various embodiments. Alternatively, the mass of the load may be estimated by comparing a reflected power, a ratio of forward to reflected power (S11 parameter), or the voltage standing wave ratio (VSWR) at the RF signal source (upon establishing an initial match) to corresponding Si i parameter values or VSWR values stored in the LUT. The amount of energy sufficient to warm the load to a desired temperature (e.g., −1 degrees Celsius) may be used to determine RF signal parameters (e.g., RF signal power level) and heating time, as well as other applicable parameters. As described herein, the “RF signal power level” refers to the amplitude of the RF signal to be converted into electromagnetic energy that is applied to the load during a defrosting operation, and the RF signal power level may be varied throughout the operation. As described herein, “heating time” refers to the amount of time for which the electromagnetic energy corresponding to the RF signal is to be applied to the load during a defrosting operation. In this way, given the amount of energy sufficient to warm the load to the desired temperature, desired RF signal parameters (e.g., power level(s)) to be used throughout a defrosting operation may be determined by embodiments of the present system. Additionally, given the amount of energy sufficient to warm the load to the desired temperature and desired RF signal parameters, a total heating (defrosting) time may be determined by embodiments of the present system. 
     The initial temperature of a load, if unknown to the defrosting system, may be assumed by the system to be a predetermined value (e.g., −20° C.). However, this assumption may not always be accurate, which can affect the accuracy of mass estimation performed by the defrosting system based on the assumed temperature. In particular, a warmer load having a smaller mass may have similar impedance matching conditions to those of a colder load having a larger mass. However, as the load is heated by the defrosting system, the electrical impedance of the load (and, correspondingly, that of the cavity) changes. As a result, the variable impedance matching circuit of the defrosting system may be repeatedly reconfigured during defrosting operations to establish and re-establish an acceptable impedance match between the RF signal source and the cavity (plus load). 
     A load with a smaller mass may have a greater rate of change in electrical impedance when heated compared to that of load with a larger mass, independent of temperature. The S11 parameter value and the voltage standing wave ratio (VSWR) at the RF signal source are each generally indicative of the quality of the impedance match between the RF signal source and the cavity (plus load). The rate of change of either the S11 parameter or the VSWR as defrosting operations are performed is therefore indicative of the rate of change of the electrical impedance of the load. Thus, a more accurate estimate of the mass of a given load may be obtained by analyzing the rate of change of the S11 parameter or the VSWR at the RF signal source and comparing the S11 or VSWR rate of change to refine (i.e., update) the system&#39;s initial estimate of the mass of the load. 
     The S11 or VSWR rate of change may be determined by periodically measuring (e.g., by a system controller and power detection circuitry) the S11 parameter value or the VSWR value while defrosting operations are being performed following the establishment of an initial impedance match between the RF signal source and the cavity, then determining the slope of the S11 parameter or the VSWR as it changes over time with the changing impedance of the load. 
     The determined S11 rate of change or VSWR rate of change may then be compared to stored S11 or VSWR rates of change (sometimes referred to as stored parameter rates of change) and corresponding load masses that have been previously obtained through characterization of the defrosting system. For example, a LUT that is stored on a memory device of the defrosting system may include multiple entries, with each entry defining an S11 and/or a VSWR rate of change measured during a defrosting operation performed on a load, an RF power level supplied during the defrosting operation, and a corresponding load mass (e.g., verified during characterization of the defrosting system). After determining the S11 or VSWR rate of change of the defrosting system, the system controller may identify a corresponding entry of the LUT in order to determine the load mass associated with that entry. The system controller then refines the initial mass estimate to be the load mass of the identified LUT entry. 
     A refined defrost energy estimate (e.g., corresponding to the amount of RF energy estimated to be required to bring the load to a target completion temperature, such as a temperature of about −1° C.) may then be determined based on the refined mass estimate. Refined signal parameters (e.g., the amount of RF energy to be applied and/or the amount of time for which the RF energy is to be applied) may then be determined based on the refined defrost energy estimate. By refining the mass estimate of the load in this way, desired RF signal parameters, such as the amount of RF energy to apply to the cavity and the amount of time for which it should be applied, may be more accurately determined. Users may generally desire accuracy when being informed of the amount of time a defrosting operation is going to take. Additionally, accurate estimation of the amount of RF energy to be applied to a load may allow for more energy efficient operation of the defrosting system. 
       FIG. 1  is a perspective view of a defrosting system  100 , in accordance with an example embodiment. Defrosting system  100  includes a defrosting cavity  110  (e.g., cavity  360 ,  760 ,  1174 ,  FIGS. 3, 7, 11 ), a control panel  120 , one or more RF signal sources (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ), a power supply (e.g., power supply  326 ,  726 ,  FIGS. 3, 7 ), a first electrode  170  (e.g., electrode  340 ,  740 ,  1170 ,  FIGS. 3, 7, 11 ), a second electrode  172  (e.g., electrode  750 ,  1172 ,  FIGS. 7, 11 ), impedance matching circuitry (e.g., circuits  334 ,  370 ,  734 ,  772 ,  1160 ,  FIGS. 3, 7, 11 ), power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  730 ′,  730 ″,  1180 ,  FIGS. 3, 7, 11 ), and a system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ). The defrosting cavity  110  is defined by interior surfaces of top, bottom, side, and back cavity walls  111 ,  112 ,  113 ,  114 ,  115  and an interior surface of door  116 . With door  116  closed, the defrosting cavity  110  defines an enclosed air cavity. As used herein, the term “air cavity” may mean an enclosed area that contains air or other gasses (e.g., defrosting cavity  110 ). 
     According to an “unbalanced” embodiment, the first electrode  170  is arranged proximate to a cavity wall (e.g., top wall  111 ), the first electrode  170  is electrically isolated from the remaining cavity walls (e.g., walls  112 - 115  and door  116 ), and the remaining cavity walls are grounded. In such a configuration, the system may be simplistically modeled as a capacitor, where the first electrode  170  functions as one conductive plate (or electrode), the grounded cavity walls (e.g., walls  112 - 115 ) function as a second conductive plate (or electrode), and the air cavity (including any load contained therein) function as a dielectric medium between the first and second conductive plates. Although not shown in  FIG. 1 , a non-electrically conductive barrier (e.g., barrier  362 ,  762 ,  FIGS. 3, 7 ) also may be included in the system  100 , and the non-conductive barrier may function to electrically and physically isolate the load from the bottom cavity wall  112 . Although  FIG. 1  shows the first electrode  170  being proximate to the top wall  111 , the first electrode  170  alternatively may be proximate to any of the other walls  112 - 115 , as indicated by electrodes  172 - 175 . 
     According to a “balanced” embodiment, the first electrode  170  is arranged proximate to a first cavity wall (e.g., top wall  111 ), a second electrode  172  is arranged proximate to an opposite, second cavity wall (e.g., bottom wall  112 ), and the first and second electrodes  170 ,  172  are electrically isolated from the remaining cavity walls (e.g., walls  113 - 115  and door  116 ). In such a configuration, the system also may be simplistically modeled as a capacitor, where the first electrode  170  functions as one conductive plate (or electrode), the second electrode  172  functions as a second conductive plate (or electrode), and the air cavity (including any load contained therein) function as a dielectric medium between the first and second conductive plates. Although not shown in  FIG. 1 , a non-electrically conductive barrier (e.g., barrier  762 ,  1156 ,  FIGS. 7, 11 ) also may be included in the system  100 , and the non-conductive barrier may function to electrically and physically isolate the load from the second electrode  172  and the bottom cavity wall  112 . Although  FIG. 1  shows the first electrode  170  being proximate to the top wall  111 , and the second electrode  172  being proximate to the bottom wall  112 , the first and second electrodes  170 ,  172  alternatively may be proximate to other opposite walls (e.g., the first electrode may be electrode  173  proximate to wall  113 , and the second electrode may be electrode  174  proximate to wall  114 . 
     According to an embodiment, during operation of the defrosting system  100 , a user (not illustrated) may place one or more loads (e.g., food and/or liquids) into the defrosting cavity  110 , and optionally may provide inputs via the control panel  120  that specify characteristics of the load(s). For example, the specified characteristics may include an approximate mass of the load. In addition, the specified load characteristics may indicate the material(s) from which the load is formed (e.g., meat, bread, liquid). In alternate embodiments, the load characteristics may be obtained in some other way, such as by scanning a barcode on the load packaging or receiving a radio frequency identification (RFID) signal from an RFID tag on or embedded within the load. Either way, as will be described in more detail later, information regarding such load characteristics enables the system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ) to establish an initial state for the impedance matching network of the system at the beginning of the defrosting operation, where the initial state may be relatively close to an optimal state that enables maximum RF power transfer into the load. Alternatively, load characteristics may not be entered or received prior to commencement of a defrosting operation, and the system controller may establish a default initial state for the impedance matching network. 
     To begin the defrosting operation, the user may provide an input via the control panel  120 . In response, the system controller causes the RF signal source(s) (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ) to supply an RF signal to the first electrode  170  in an unbalanced embodiment, or to both the first and second electrodes  170 ,  172  in a balanced embodiment, and the electrode(s) responsively radiate electromagnetic energy into the defrosting cavity  110 . The electromagnetic energy increases the thermal energy of the load (i.e., the electromagnetic energy causes the load to warm up). 
     During the defrosting operation, the impedance of the load (and thus the total input impedance of the cavity  110  plus load) changes as the thermal energy of the load increases. The impedance changes alter the absorption of RF energy into the load, and thus alter the magnitude of reflected power. According to an embodiment, power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  1180 ,  FIGS. 3, 7, 11 ) continuously or periodically measures the reflected power along a transmission path (e.g., transmission path  328 ,  728 ,  1148 ,  FIGS. 3, 7, 11 ) between the RF signal source (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ) and the electrode(s)  170 ,  172 . Based on these measurements, the system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ) may detect completion of the defrosting operation, as will be described in detail below. According to a further embodiment, the impedance matching network is variable, and based on the reflected power measurements (or both the forward and reflected power measurements), the system controller may alter the state of the impedance matching network during the defrosting operation to increase the absorption of RF power by the load. 
     The defrosting system  100  of  FIG. 1  is embodied as a counter-top type of appliance. In a further embodiment, the defrosting system  100  also may include components and functionality for performing microwave cooking operations. Alternatively, components of a defrosting system may be incorporated into other types of systems or appliances. For example,  FIG. 2  is a perspective view of a refrigerator/freezer appliance  200  that includes other example embodiments of defrosting systems  210 ,  220 . More specifically, defrosting system  210  is shown to be incorporated within a freezer compartment  212  of the system  200 , and defrosting system  220  is shown to be incorporated within a refrigerator compartment  222  of the system. An actual refrigerator/freezer appliance likely would include only one of the defrosting systems  210 ,  220 , but both are shown in  FIG. 2  to concisely convey both embodiments. 
     Similar to the defrosting system  100 , each of defrosting systems  210 ,  220  includes a defrosting cavity, a control panel  214 ,  224 , one or more RF signal sources (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ), a power supply (e.g., power supply  326 ,  726 ,  FIGS. 3, 7 ), a first electrode (e.g., electrode  340 ,  740 ,  1170 ,  FIGS. 3, 7 ), a second electrode  172  (e.g., containment structure  366 , electrode  750 ,  FIGS. 3, 7, 11 ), impedance matching circuitry (e.g., circuits  334 ,  370 ,  734 ,  772 ,  1160 ,  FIGS. 3, 7, 11 ), power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  1180 ,  FIGS. 3, 7, 11 ), and a system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ). For example, the defrosting cavity may be defined by interior surfaces of bottom, side, front, and back walls of a drawer, and an interior top surface of a fixed shelf  216 ,  226  under which the drawer slides. With the drawer slid fully under the shelf, the drawer and shelf define the cavity as an enclosed air cavity. The components and functionalities of the defrosting systems  210 ,  220  may be substantially the same as the components and functionalities of defrosting system  100 , in various embodiments. 
     In addition, according to an embodiment, each of the defrosting systems  210 ,  220  may have sufficient thermal communication with the freezer or refrigerator compartment  212 ,  222 , respectively, in which the system  210 ,  220  is disposed. In such an embodiment, after completion of a defrosting operation, the load may be maintained at a safe temperature (i.e., a temperature at which food spoilage is retarded) until the load is removed from the system  210 ,  220 . More specifically, upon completion of a defrosting operation by the freezer-based defrosting system  210 , the cavity within which the defrosted load is contained may thermally communicate with the freezer compartment  212 , and if the load is not promptly removed from the cavity, the load may re-freeze. Similarly, upon completion of a defrosting operation by the refrigerator-based defrosting system  220 , the cavity within which the defrosted load is contained may thermally communicate with the refrigerator compartment  222 , and if the load is not promptly removed from the cavity, the load may be maintained in a defrosted state at the temperature within the refrigerator compartment  222 . 
     Those of skill in the art would understand, based on the description herein, that embodiments of defrosting systems may be incorporated into systems or appliances having other configurations, as well. Accordingly, the above-described implementations of defrosting systems in a stand-alone appliance, a microwave oven appliance, a freezer, and a refrigerator are not meant to limit use of the embodiments only to those types of systems. 
     Although defrosting systems  100 ,  200  are shown with their components in particular relative orientations with respect to one another, it should be understood that the various components may be oriented differently, as well. In addition, the physical configurations of the various components may be different. For example, control panels  120 ,  214 ,  224  may have more, fewer, or different user interface elements, and/or the user interface elements may be differently arranged. In addition, although a substantially cubic defrosting cavity  110  is illustrated in  FIG. 1 , it should be understood that a defrosting cavity may have a different shape, in other embodiments (e.g., cylindrical, and so on). Further, defrosting systems  100 ,  210 ,  220  may include additional components (e.g., a fan, a stationary or rotating plate, a tray, an electrical cord, and so on) that are not specifically depicted in  FIGS. 1, 2 . 
       FIG. 3  is a simplified block diagram of an unbalanced defrosting system  300  (e.g., defrosting system  100 ,  210 ,  220 ,  FIGS. 1, 2 ), in accordance with an example embodiment. Defrosting system  300  includes RF subsystem  310 , defrosting cavity  360 , user interface  380 , system controller  312 , RF signal source  320 , power supply and bias circuitry  326 , variable impedance matching network  370 , electrode  340 , containment structure  366 , and power detection circuitry  330 , in an embodiment. In addition, in other embodiments, defrosting system  300  may include temperature sensor(s), and/or infrared (IR) sensor(s)  390 , although some or all of these sensor components may be excluded. It should be understood that  FIG. 3  is a simplified representation of a defrosting system  300  for purposes of explanation and ease of description, and that practical embodiments may include other devices and components to provide additional functions and features, and/or the defrosting system  300  may be part of a larger electrical system. 
     User interface  380  may correspond to a control panel (e.g., control panel  120 ,  214 ,  224 ,  FIGS. 1, 2 ), for example, which enables a user to provide inputs to the system regarding parameters for a defrosting operation (e.g., characteristics of the load to be defrosted, and so on), start and cancel buttons, mechanical controls (e.g., a door/drawer open latch), and so on. In addition, the user interface may be configured to provide user-perceptible outputs indicating the status of a defrosting operation (e.g., a countdown timer, visible indicia indicating progress or completion of the defrosting operation, and/or audible tones indicating completion of the defrosting operation) and other information. 
     Some embodiments of defrosting system  300  may include temperature sensor(s), and/or IR sensor(s)  390 . The temperature sensor(s) and/or IR sensor(s) may be positioned in locations that enable the temperature of the load  364  to be sensed during the defrosting operation. When provided to the system controller  312 , the temperature information enables the system controller  312  to alter the power of the RF signal supplied by the RF signal source  320  (e.g., by controlling the bias and/or supply voltages provided by the power supply and bias circuitry  326 ), to adjust the state of the variable impedance matching network  370 , and/or to determine when the defrosting operation should be terminated. The system controller  312  may use this information, for example, to determine a desired power level for the RF signal supplied by the RF signal source  320 , to determine an initial setting for the variable impedance matching network  370 , and/or to determine an approximate duration for the defrosting operation. 
     The RF subsystem  310  includes a system controller  312 , an RF signal source  320 , first impedance matching circuit  334  (herein “first matching circuit”), power supply and bias circuitry  326 , and power detection circuitry  330 , in an embodiment. System controller  312  may include one or more general purpose or special purpose processors (e.g., a microprocessor, microcontroller, Application Specific Integrated Circuit (ASIC), and so on), volatile and/or non-volatile memory (e.g., Random Access Memory (RAM), Read Only Memory (ROM), flash, various registers, and so on), one or more communication busses, and other components. According to an embodiment, system controller  312  is coupled to user interface  380 , RF signal source  320 , variable impedance matching network  370 , power detection circuitry  330 , and sensors  390  (if included). System controller  312  is configured to receive signals indicating user inputs received via user interface  380 , and to receive signals indicating RF signal reflected power (and possibly RF signal forward power) from power detection circuitry  330 . Responsive to the received signals and measurements, and as will be described in more detail later, system controller  312  provides control signals to the power supply and bias circuitry  326  and to the RF signal generator  322  of the RF signal source  320 . In addition, system controller  312  provides control signals to the variable impedance matching network  370 , which cause the network  370  to change its state or configuration. 
     Defrosting cavity  360  includes a capacitive defrosting arrangement with first and second parallel plate electrodes that are separated by an air cavity within which a load  364  to be defrosted may be placed. For example, a first electrode  340  may be positioned above the air cavity, and a second electrode may be provided by a portion of a containment structure  366 . More specifically, the containment structure  366  may include bottom, top, and side walls, the interior surfaces of which define the cavity  360  (e.g., cavity  110 ,  FIG. 1 ). According to an embodiment, the cavity  360  may be sealed (e.g., with a door  116 ,  FIG. 1  or by sliding a drawer closed under a shelf  216 ,  226 ,  FIG. 2 ) to contain the electromagnetic energy that is introduced into the cavity  360  during a defrosting operation. The system  300  may include one or more interlock mechanisms that ensure that the seal is intact during a defrosting operation. If one or more of the interlock mechanisms indicates that the seal is breached, the system controller  312  may cease the defrosting operation. According to an embodiment, the containment structure  366  is at least partially formed from conductive material, and the conductive portion(s) of the containment structure may be grounded. Alternatively, at least the portion of the containment structure  366  that corresponds to the bottom surface of the cavity  360  may be formed from conductive material and grounded. Either way, the containment structure  366  (or at least the portion of the containment structure  366  that is parallel with the first electrode  340 ) functions as a second electrode of the capacitive defrosting arrangement. To avoid direct contact between the load  364  and the grounded bottom surface of the cavity  360 , a non-conductive barrier  362  may be positioned over the bottom surface of the cavity  360 . 
     Essentially, defrosting cavity  360  includes a capacitive defrosting arrangement with first and second parallel plate electrodes  340 ,  366  that are separated by an air cavity within which a load  364  to be defrosted may be placed. The first electrode  340  is positioned within containment structure  366  to define a distance  352  between the electrode  340  and an opposed surface of the containment structure  366  (e.g., the bottom surface, which functions as a second electrode), where the distance  352  renders the cavity  360  a sub-resonant cavity, in an embodiment. 
     In various embodiments, the distance  352  is in a range of about 0.10 meters to about 1.0 meter, although the distance may be smaller or larger, as well. According to an embodiment, distance  352  is less than one wavelength of the RF signal produced by the RF subsystem  310 . In other words, as mentioned above, the cavity  360  is a sub-resonant cavity. In some embodiments, the distance  352  is less than about half of one wavelength of the RF signal. In other embodiments, the distance  352  is less than about one quarter of one wavelength of the RF signal. In still other embodiments, the distance  352  is less than about one eighth of one wavelength of the RF signal. In still other embodiments, the distance  352  is less than about one 50th of one wavelength of the RF signal. In still other embodiments, the distance  352  is less than about one 100th of one wavelength of the RF signal. 
     In general, a system  300  designed for lower operational frequencies (e.g., frequencies between 10 megahertz (MHz) and 100 MHz) may be designed to have a distance  352  that is a smaller fraction of one wavelength. For example, when system  300  is designed to produce an RF signal with an operational frequency of about 10 MHz (corresponding to a wavelength of about 30 meters), and distance  352  is selected to be about 0.5 meters, the distance  352  is about one 60th of one wavelength of the RF signal. Conversely, when system  300  is designed for a higher operational frequency of about 300 MHz (corresponding to a wavelength of about 1 meter), and distance  352  is selected to be about 0.5 meters, the distance  352  is about one half of one wavelength of the RF signal. 
     With the operational frequency and the distance  352  between electrode  340  and containment structure  366  being selected to define a sub-resonant interior cavity  360 , the first electrode  340  and the containment structure  366  are capacitively coupled. More specifically, the first electrode  340  may be analogized to a first plate of a capacitor, the containment structure  366  may be analogized to a second plate of a capacitor, and the load  364 , barrier  362 , and air within the cavity  360  may be analogized to a capacitor dielectric. Accordingly, the first electrode  340  alternatively may be referred to herein as an “anode,” and the containment structure  366  may alternatively be referred to herein as a “cathode.” 
     Essentially, the voltage across the first electrode  340  and the containment structure  366  heats the load  364  within the cavity  360 . According to various embodiments, the RF subsystem  310  is configured to generate the RF signal to produce voltages between the electrode  340  and the containment structure  366  in a range of about 90 volts to about 3,000 volts, in one embodiment, or in a range of about 3000 volts to about 10,000 volts, in another embodiment, although the system may be configured to produce lower or higher voltages between the electrode  340  and the containment structure  366 , as well. 
     The first electrode  340  is electrically coupled to the RF signal source  320  through a first matching circuit  334 , a variable impedance matching network  370 , and a conductive transmission path, in an embodiment. The first matching circuit  334  is configured to perform an impedance transformation from an impedance of the RF signal source  320  (e.g., less than about  10  ohms) to an intermediate impedance (e.g.,  50  ohms,  75  ohms, or some other value). According to an embodiment, the conductive transmission path includes a plurality of conductors  328 - 1 ,  328 - 2 , and  328 - 3  connected in series, and referred to collectively as transmission path  328 . According to an embodiment, the conductive transmission path  328  is an “unbalanced” path, which is configured to carry an unbalanced RF signal (i.e., a single RF signal referenced against ground). In some embodiments, one or more connectors (not shown, but each having male and female connector portions) may be electrically coupled along the transmission path  328 , and the portion of the transmission path  328  between the connectors may comprise a coaxial cable or other suitable connector. Such a connection is shown in  FIG. 7  and described later (e.g., including connectors  736 ,  738  and a conductor  728 - 3  such as a coaxial cable between the connectors  736 ,  738 ). 
     As will be described in more detail later, the variable impedance matching circuit  370  is configured to perform an impedance transformation from the above-mentioned intermediate impedance to an input impedance of defrosting cavity  320  as modified by the load  364  (e.g., on the order of hundreds or thousands of ohms, such as about 1000 ohms to about 4000 ohms or more). In an embodiment, the variable impedance matching network  370  includes a network of passive components (e.g., inductors, capacitors, resistors). 
     According to one more specific embodiment, the variable impedance matching network  370  includes a plurality of fixed-value lumped inductors (e.g., inductors  412 - 414 ,  FIG. 4A ) that are positioned within the cavity  360  and which are electrically coupled to the first electrode  340 . In addition, the variable impedance matching network  370  includes a plurality of variable inductance networks (e.g., networks  410 ,  411 ,  500 ,  FIGS. 4A, 5A ), which may be located inside or outside of the cavity  360 . According to another more specific embodiment, the variable impedance matching network  370  includes a plurality of variable capacitance networks (e.g., networks  442 ,  446 ,  540 ,  FIG. 4B, 5B ), which may be located inside or outside of the cavity  360 . The inductance or capacitance value provided by each of the variable inductance or capacitance networks is established using control signals from the system controller  312 , as will be described in more detail later. In any event, by changing the state of the variable impedance matching network  370  over the course of a defrosting operation to dynamically match the ever-changing cavity plus load impedance, the amount of RF power that is absorbed by the load  364  may be maintained at a high level despite variations in the load impedance during the defrosting operation. 
     According to an embodiment, RF signal source  320  includes an RF signal generator  322  and a power amplifier (e.g., including one or more power amplifier stages  324 ,  325 ). In response to control signals provided by system controller  312  over connection  314 , RF signal generator  322  is configured to produce an oscillating electrical signal having a frequency in the ISM (industrial, scientific, and medical) band, although the system could be modified to support operations in other frequency bands, as well. The RF signal generator  322  may be controlled to produce oscillating signals of different power levels and/or different frequencies, in various embodiments. For example, the RF signal generator  322  may produce a signal that oscillates in a VHF (very high frequency) range (e.g., about 30 MHz to about 300 MHz), a signal that oscillates in a frequency range of about 10.0 MHz to about 100 MHz, and/or a signal that oscillates in a frequency range of about 100 MHz to about 3.0 gigahertz (GHz). Some desirable frequencies may be, for example, 13.56 MHz (+/−5 percent), 27.125 MHz (+/−5 percent), 40.68 MHz (+/−5 percent), and 2.45 GHz (+/−5 percent). In one particular embodiment, for example, the RF signal generator  322  may produce a signal that oscillates in a range of about 40.66 MHz to about 40.70 MHz and at a power level in a range of about 10 decibel-milliwatts (dBm) to about 15 dBm. Alternatively, the frequency of oscillation and/or the power level may be lower or higher. 
     In the embodiment of  FIG. 3 , the power amplifier includes a driver amplifier stage  324  and a final amplifier stage  325 . The power amplifier is configured to receive the oscillating signal from the RF signal generator  322 , and to amplify the signal to produce a significantly higher-power signal at an output of the power amplifier. For example, the output signal may have a power level in a range of about 100 watts to about 400 watts or more. The gain applied by the power amplifier may be controlled using gate bias voltages and/or drain supply voltages provided by the power supply and bias circuitry  326  to each amplifier stage  324 ,  325 . More specifically, power supply and bias circuitry  326  provides bias and supply voltages to each RF amplifier stage  324 ,  325  in accordance with control signals received from system controller  312 . 
     In an embodiment, each amplifier stage  324 ,  325  is implemented as a power transistor, such as a field effect transistor (FET), having an input terminal (e.g., a gate or control terminal) and two current carrying terminals (e.g., source and drain terminals). Impedance matching circuits (not illustrated) may be coupled to the input (e.g., gate) of the driver amplifier stage  324 , between the driver and final amplifier stages  325 , and/or to the output (e.g., drain terminal) of the final amplifier stage  325 , in various embodiments. In an embodiment, each transistor of the amplifier stages  324 ,  325  includes a laterally diffused metal oxide semiconductor FET (LDMOSFET) transistor. However, it should be noted that the transistors are not intended to be limited to any particular semiconductor technology, and in other embodiments, each transistor may be realized as a gallium nitride (GaN) transistor, another type of MOSFET transistor, a bipolar junction transistor (BJT), or a transistor utilizing another semiconductor technology. 
     In  FIG. 3 , the power amplifier arrangement is depicted to include two amplifier stages  324 ,  325  coupled in a particular manner to other circuit components. In other embodiments, the power amplifier arrangement may include other amplifier topologies and/or the amplifier arrangement may include only one amplifier stage (e.g., as shown in the embodiment of amplifier  724 ,  FIG. 7 ), or more than two amplifier stages. For example, the power amplifier arrangement may include various embodiments of a single-ended amplifier, a Doherty amplifier, a Switch Mode Power Amplifier (SMPA), or another type of amplifier. 
     Defrosting cavity  360  and any load  364  (e.g., food, liquids, and so on) positioned in the defrosting cavity  360  present a cumulative load for the electromagnetic energy (or RF power) that is radiated into the cavity  360  by the first electrode  340 . More specifically, the cavity  360  and the load  364  present an impedance to the system, referred to herein as a “cavity plus load impedance.” The cavity plus load impedance changes during a defrosting operation as the temperature of the load  364  increases. The cavity plus load impedance has a direct effect on the magnitude of reflected signal power along the conductive transmission path  328  between the RF signal source  320  and electrodes  340 . In most cases, it is desirable to maximize the magnitude of transferred signal power into the cavity  360 , and/or to minimize the reflected-to-forward signal power ratio along the conductive transmission path  328 . 
     In order to at least partially match the output impedance of the RF signal generator  320  to the cavity plus load impedance, a first matching circuit  334  is electrically coupled along the transmission path  328 , in an embodiment. The first matching circuit  334  may have any of a variety of configurations. According to an embodiment, the first matching circuit  334  includes fixed components (i.e., components with non-variable component values), although the first matching circuit  334  may include one or more variable components, in other embodiments. For example, the first matching circuit  334  may include any one or more circuits selected from an inductance/capacitance (LC) network, a series inductance network, a shunt inductance network, or a combination of bandpass, high-pass and low-pass circuits, in various embodiments. Essentially, the fixed matching circuit  334  is configured to raise the impedance to an intermediate level between the output impedance of the RF signal generator  320  and the cavity plus load impedance. 
     As will be described in conjunction with  FIG. 15  later, the impedance of many types of food loads changes with respect to temperature in a somewhat predictable manner as the food load transitions from a frozen state to a defrosted state. According to an embodiment, based on reflected power measurements (and forward power measurements, in some embodiments) from the power detection circuitry  330 , the system controller  312  is configured to identify a point in time during a defrosting operation when the rate of change of cavity plus load impedance indicates that the load  364  is approaching 0° Celsius, at which time the system controller  312  may terminate the defrosting operation. 
     According to an embodiment, power detection circuitry  330  is coupled along the transmission path  328  between the output of the RF signal source  320  and the electrode  340 . In a specific embodiment, the power detection circuitry  330  forms a portion of the RF subsystem  310 , and is coupled to the conductor  328 - 2  between the output of the first matching circuit  334  and the input to the variable impedance matching network  370 , in an embodiment. In alternate embodiments, the power detection circuitry  330  may be coupled to the portion  328 - 1  of the transmission path  328  between the output of the RF signal source  320  and the input to the first matching circuit  334 , or to the portion  328 - 3  of the transmission path  328  between the output of the variable impedance matching network  370  and the first electrode  340 . 
     Wherever it is coupled, power detection circuitry  330  is configured to monitor, measure, or otherwise detect the power of the reflected signals traveling along the transmission path  328  between the RF signal source  320  and electrode  340  (i.e., reflected RF signals traveling in a direction from electrode  340  toward RF signal source  320 ). In some embodiments, power detection circuitry  330  also is configured to detect the power of the forward signals traveling along the transmission path  328  between the RF signal source  320  and the electrode  340  (i.e., forward RF signals traveling in a direction from RF signal source  320  toward electrode  340 ). Over connection  332 , power detection circuitry  330  supplies signals to system controller  312  conveying the magnitudes of the reflected signal power (and the forward signal power, in some embodiments) to system controller  312 . In embodiments in which both the forward and reflected signal power magnitudes are conveyed, system controller  312  may calculate a reflected-to-forward signal power ratio, or the S11 parameter. In some embodiments the system controller  312  may also calculate the VSWR of the system based on the forward and reflected signal power magnitudes. As will be described in more detail below, when the reflected signal power magnitude exceeds a reflected signal power threshold, or when the reflected-to-forward signal power ratio exceeds an S11 parameter threshold, or when the VSWR exceeds a threshold, this indicates that the system  300  is not adequately matched to the cavity plus load impedance, and that energy absorption by the load  364  within the cavity  360  may be sub-optimal. In such a situation, system controller  312  orchestrates a process of altering the state of the variable matching network  370  to drive the reflected signal power, the S11 parameter, and/or the VSWR toward or below a desired level (e.g., below the reflected signal power threshold, and/or the reflected-to-forward signal power ratio threshold, and/or the VSWR threshold), thus re-establishing an acceptable match and facilitating more optimal energy absorption by the load  364 . 
     More specifically, the system controller  312  may provide control signals over control path  316  to the variable matching circuit  370 , which cause the variable matching circuit  370  to vary inductive, capacitive, and/or resistive values of one or more components within the circuit, thus adjusting the impedance transformation provided by the circuit  370 . Adjustment of the configuration of the variable matching circuit  370  desirably decreases the magnitude of reflected signal power, which corresponds to decreasing the magnitude of the S11 parameter or decreasing the magnitude of the VSWR, and increasing the power absorbed by the load  364 . 
     As discussed above, the variable impedance matching network  370  is used to match the input impedance of the defrosting cavity  360  plus load  364  to maximize, to the extent possible, the RF power transfer into the load  364 . The initial impedance of the defrosting cavity  360  and the load  364  may not be known with accuracy at the beginning of a defrosting operation. Further, the impedance of the load  364  changes during a defrosting operation as the load  364  warms up. According to an embodiment, the system controller  312  may provide control signals to the variable impedance matching network  370 , which cause modifications to the state of the variable impedance matching network  370 . This enables the system controller  312  to establish an initial state of the variable impedance matching network  370  at the beginning of the defrosting operation that has a relatively low reflected to forward power ratio, and thus a relatively high absorption of the RF power by the load  364 . In addition, this enables the system controller  312  to modify the state of the variable impedance matching network  370  so that an adequate match may be maintained throughout the defrosting operation, despite changes in the impedance of the load  364 . 
     Non-limiting examples of configurations for the variable matching network  370  are shown in  FIGS. 4A, 4B, 5A, and 5B . For example, the network  370  may include any one or more circuits selected from an inductance/capacitance (LC) network, an inductance-only network, a capacitance-only network, or a combination of bandpass, high-pass and low-pass circuits, in various embodiments. In an embodiment, the variable matching network  370  includes a single-ended network (e.g., network  400 ,  440 ,  FIG. 4A, 4B ). The inductance, capacitance, and/or resistance values provided by the variable matching network  370 , which in turn affect the impedance transformation provided by the network  370 , are established using control signals from the system controller  312 , as will be described in more detail later. In any event, by changing the state of the variable matching network  370  over the course of a defrosting operation to dynamically match the ever-changing impedance of the cavity  360  plus the load  364  within the cavity  360 , the system efficiency may be maintained at a high level throughout the defrosting operation. 
     The variable matching network  370  may have any of a wide variety of circuit configurations, and non-limiting examples of such configurations are shown in  FIGS. 4A, 4B, 5A , and  5 B. According to an embodiment, as exemplified in  FIGS. 4A and 5A , the variable impedance matching network  370  may include a single-ended network of passive components, and more specifically a network of fixed-value inductors (e.g., lumped inductive components) and variable inductors (or variable inductance networks). According to another embodiment, as exemplified in  FIGS. 4B and 5B , the variable impedance matching network  370  may include a single-ended network of passive components, and more specifically a network of variable capacitors (or variable capacitance networks). As used herein, the term “inductor” means a discrete inductor or a set of inductive components that are electrically coupled together without intervening components of other types (e.g., resistors or capacitors). Similarly, the term “capacitor” means a discrete capacitor or a set of capacitive components that are electrically coupled together without intervening components of other types (e.g., resistors or inductors). 
     Referring first to the variable-inductance impedance matching network embodiment,  FIG. 4A  is a schematic diagram of a single-ended variable impedance matching network  400  (e.g., variable impedance matching network  370 ,  FIG. 3 ), in accordance with an example embodiment. As will be explained in more detail below, the variable impedance matching network  370  essentially has two portions: one portion to match the RF signal source (or the final stage power amplifier); and another portion to match the cavity plus load. 
     Variable impedance matching network  400  includes an input node  402 , an output node  404 , first and second variable inductance networks  410 ,  411 , and a plurality of fixed-value inductors  412 - 415 , according to an embodiment. When incorporated into a defrosting system (e.g., system  300 ,  FIG. 3 ), the input node  402  is electrically coupled to an output of the RF signal source (e.g., RF signal source  320 ,  FIG. 3 ), and the output node  404  is electrically coupled to an electrode (e.g., first electrode  340 ,  FIG. 3 ) within the defrosting cavity (e.g., defrosting cavity  360 ,  FIG. 3 ). 
     Between the input and output nodes  402 ,  404 , the variable impedance matching network  400  includes first and second, series coupled lumped inductors  412 ,  414 , in an embodiment. The first and second lumped inductors  412 ,  414  are relatively large in both size and inductance value, in an embodiment, as they may be designed for relatively low frequency (e.g., about 40.66 MHz to about 40.70 MHz) and high power (e.g., about 50 watts (W) to about 500 W) operation. For example, inductors  412 ,  414  may have values in a range of about 200 nanohenries (nH) to about 600 nH, although their values may be lower and/or higher, in other embodiments. 
     The first variable inductance network  410  is a first shunt inductive network that is coupled between the input node  402  and a ground reference terminal (e.g., the grounded containment structure  366 ,  FIG. 3 ). According to an embodiment, the first variable inductance network  410  is configurable to match the impedance of the RF signal source (e.g., RF signal source  320 ,  FIG. 3 ) as modified by the first matching circuit (e.g., circuit  334 ,  FIG. 3 ), or more particularly to match the impedance of the final stage power amplifier (e.g., amplifier  325 ,  FIG. 3 ) as modified by the first matching circuit  334  (e.g., circuit  334 ,  FIG. 3 ). Accordingly, the first variable inductance network  410  may be referred to as the “RF signal source matching portion” of the variable impedance matching network  400 . According to an embodiment, and as will be described in more detail in conjunction with  FIG. 5 , the first variable inductance network  410  includes a network of inductive components that may be selectively coupled together to provide inductances in a range of about 10 nH to about 400 nH, although the range may extend to lower or higher inductance values, as well. 
     In contrast, the “cavity matching portion” of the variable impedance matching network  400  is provided by a second shunt inductive network  416  that is coupled between a node  422  between the first and second lumped inductors  412 ,  414  and the ground reference terminal. According to an embodiment, the second shunt inductive network  416  includes a third lumped inductor  413  and a second variable inductance network  411  coupled in series, with an intermediate node  422  between the third lumped inductor  413  and the second variable inductance network  411 . Because the state of the second variable inductance network  411  may be changed to provide multiple inductance values, the second shunt inductive network  416  is configurable to optimally match the impedance of the cavity plus load (e.g., cavity  360  plus load  364 ,  FIG. 3 ). For example, inductor  413  may have a value in a range of about 400 nH to about 800 nH, although its value may be lower and/or higher, in other embodiments. According to an embodiment, and as will be described in more detail in conjunction with  FIG. 5 , the second variable inductance network  411  includes a network of inductive components that may be selectively coupled together to provide inductances in a range of about 50 nH to about 800 nH, although the range may extend to lower or higher inductance values, as well. 
     Finally, the variable impedance matching network  400  includes a fourth lumped inductor  415  coupled between the output node  404  and the ground reference terminal. For example, inductor  415  may have a value in a range of about 400 nH to about 800 nH, although its value may be lower and/or higher, in other embodiments. 
     As will be described in more detail in conjunction with  FIG. 12A , the set  430  of lumped inductors  412 - 415  may form a portion of a module that is at least partially physically located within the cavity (e.g., cavity  360 ,  FIG. 3 ), or at least within the confines of the containment structure (e.g., containment structure  366 ,  FIG. 3 ). This enables the radiation produced by the lumped inductors  412 - 415  to be safely contained within the system, rather than being radiated out into the surrounding environment. In contrast, the variable inductance networks  410 ,  411  may or may not be contained within the cavity or the containment structure, in various embodiments. 
     According to an embodiment, the variable impedance matching network  400  embodiment of  FIG. 4A  includes “only inductors” to provide a match for the input impedance of the defrosting cavity  360  plus load  364 . Thus, the network  400  may be considered an “inductor-only” matching network. As used herein, the phrases “only inductors” or “inductor-only” when describing the components of the variable impedance matching network means that the network does not include discrete resistors with significant resistance values or discrete capacitors with significant capacitance values. In some cases, conductive transmission lines between components of the matching network may have minimal resistances, and/or minimal parasitic capacitances may be present within the network. Such minimal resistances and/or minimal parasitic capacitances are not to be construed as converting embodiments of the “inductor-only” network into a matching network that also includes resistors and/or capacitors. Those of skill in the art would understand, however, that other embodiments of variable impedance matching networks may include differently configured inductor-only matching networks, and matching networks that include combinations of discrete inductors, discrete capacitors, and/or discrete resistors. As will be described in more detail in conjunction with  FIG. 6 , an “inductor-only” matching network alternatively may be defined as a matching network that enables impedance matching of a capacitive load using solely or primarily inductive components. 
       FIG. 5A  is a schematic diagram of a variable inductance network  500  that may be incorporated into a variable impedance matching network (e.g., as variable inductance networks  410  and/or  411 ,  FIG. 4A ), in accordance with an example embodiment. Network  500  includes an input node  530 , an output node  532 , and a plurality, N, of discrete inductors  501 - 504  coupled in series with each other between the input and output nodes  530 ,  532 , where N may be an integer between 2 and 10, or more. In addition, network  500  includes a plurality, N, of bypass switches  511 - 514 , where each switch  511 - 514  is coupled in parallel across the terminals of one of the inductors  501 - 504 . Switches  511 - 514  may be implemented as transistors, mechanical relays or mechanical switches, for example. The electrically conductive state of each switch  511 - 514  (i.e., open or closed) is controlled through control signals  521 - 524  from the system controller (e.g., system controller  312 ,  FIG. 3 ). 
     For each parallel inductor/switch combination, substantially all current flows through the inductor when its corresponding switch is in an open or non-conductive state, and substantially all current flows through the switch when the switch is in a closed or conductive state. For example, when all switches  511 - 514  are open, as illustrated in  FIG. 5A , substantially all current flowing between input and output nodes  530 ,  532  flows through the series of inductors  501 - 504 . This configuration represents the maximum inductance state of the network  500  (i.e., the state of network  500  in which a maximum inductance value is present between input and output nodes  530 ,  532 ). Conversely, when all switches  511 - 514  are closed, substantially all current flowing between input and output nodes  530 ,  532  bypasses the inductors  501 - 504  and flows instead through the switches  511 - 514  and the conductive interconnections between nodes  530 ,  532  and switches  511 - 514 . This configuration represents the minimum inductance state of the network  500  (i.e., the state of network  500  in which a minimum inductance value is present between input and output nodes  530 ,  532 ). Ideally, the minimum inductance value would be near zero inductance. However, in practice a “trace” inductance is present in the minimum inductance state due to the cumulative inductances of the switches  511 - 514  and the conductive interconnections between nodes  530 ,  532  and the switches  511 - 514 . For example, in the minimum inductance state, the trace inductance for the variable inductance network  500  may be in a range of about 10 nH to about 50 nH, although the trace inductance may be smaller or larger, as well. Larger, smaller, or substantially similar trace inductances also may be inherent in each of the other network states, as well, where the trace inductance for any given network state is a summation of the inductances of the sequence of conductors and switches through which the current primarily is carried through the network  500 . 
     Starting from the maximum inductance state in which all switches  511 - 514  are open, the system controller may provide control signals  521 - 524  that result in the closure of any combination of switches  511 - 514  in order to reduce the inductance of the network  500  by bypassing corresponding combinations of inductors  501 - 504 . In one embodiment, each inductor  501 - 504  has substantially the same inductance value, referred to herein as a normalized value of I. For example, each inductor  501 - 504  may have a value in a range of about 10 nH to about 200 nH, or some other value. In such an embodiment, the maximum inductance value for the network  500  (i.e., when all switches  511 - 514  are in an open state) would be about N×J, plus any trace inductance that may be present in the network  500  when it is in the maximum inductance state. When any n switches are in a closed state, the inductance value for the network  500  would be about (N−n)×I (plus trace inductance). In such an embodiment, the state of the network  500  may be configured to have any of N+1 values of inductance. 
     In an alternate embodiment, the inductors  501 - 504  may have different values from each other. For example, moving from the input node  530  toward the output node  532 , the first inductor  501  may have a normalized inductance value of I, and each subsequent inductor  502 - 504  in the series may have a larger or smaller inductance value. For example, each subsequent inductor  502 - 504  may have an inductance value that is a multiple (e.g., about twice) the inductance value of the nearest downstream inductor  501 - 503 , although the difference may not necessarily be an integer multiple. In such an embodiment, the state of the network  500  may be configured to have any of 2 N  values of inductance. For example, when N=4 and each inductor  501 - 504  has a different value, the network  500  may be configured to have any of 16 values of inductance. For example, but not by way of limitation, assuming that inductor  501  has a value of I, inductor  502  has a value of 2×I, inductor  503  has a value of 4×I, and inductor  504  has a value of 8×I, Table 1, below indicates the total inductance value for all 16 possible states of the network  500  (not accounting for trace inductances): 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Total inductance values for all possible variable inductance network states 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Switch 511 
                 Switch 512 
                 Switch 513 
                 Switch 514 
                 Total network 
               
               
                 Network 
                 state (501 
                 state (502 
                 state (503 
                 state (504 
                 inductance (w/o 
               
               
                 state 
                 value = I) 
                 value = 2 × I) 
                 value = 4 × I) 
                 value = 8 × I) 
                 trace inductance) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 closed 
                 closed 
                 closed 
                 closed 
                 0 
               
               
                 1 
                 open 
                 closed 
                 closed 
                 closed 
                 I 
               
               
                 2 
                 closed 
                 open 
                 closed 
                 closed 
                 2 × I 
               
               
                 3 
                 open 
                 open 
                 closed 
                 closed 
                 3 × I 
               
               
                 4 
                 closed 
                 closed 
                 open 
                 closed 
                 4 × I 
               
               
                 5 
                 open 
                 closed 
                 open 
                 closed 
                 5 × I 
               
               
                 6 
                 closed 
                 open 
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     Referring again to  FIG. 4A , an embodiment of variable inductance network  410  may be implemented in the form of variable inductance network  500  with the above-described example characteristics (i.e., N=4 and each successive inductor is about twice the inductance of the preceding inductor). Assuming that the trace inductance in the minimum inductance state is about 10 nH, and the range of inductance values achievable by network  410  is about 10 nH (trace inductance) to about 400 nH, the values of inductors  501 - 504  may be, for example, about 30 nH, about 50 nH, about 100 nH, and about 200 nH, respectively. Similarly, if an embodiment of variable inductance network  411  is implemented in the same manner, and assuming that the trace inductance is about 50 nH and the range of inductance values achievable by network  411  is about 50 nH (trace inductance) to about 800 nH, the values of inductors  501 - 504  may be, for example, about 50 nH, about 100 nH, about 200 nH, and about 400 nH, respectively. Of course, more or fewer than four inductors  501 - 504  may be included in either variable inductance network  410 ,  411 , and the inductors within each network  410 ,  411  may have different values. 
     Although the above example embodiment specifies that the number of switched inductances in the network  500  equals four, and that each inductor  501 - 504  has a value that is some multiple of a value of I, alternate embodiments of variable inductance networks may have more or fewer than four inductors, different relative values for the inductors, a different number of possible network states, and/or a different configuration of inductors (e.g., differently connected sets of parallel and/or series coupled inductors). Either way, by providing a variable inductance network in an impedance matching network of a defrosting system, the system may be better able to match the ever-changing cavity plus load impedance that is present during a defrosting operation. 
       FIG. 4B  is a schematic diagram of a single-ended variable capacitive matching network  440  (e.g., variable impedance matching network  370 ,  FIG. 3 ), which may be implemented instead of the variable-inductance impedance matching network  400  ( FIG. 4A ), in accordance with an example embodiment. Variable impedance matching network  440  includes an input node  402 , an output node  404 , first and second variable capacitance networks  442 ,  446 , and at least one inductor  454 , according to an embodiment. When incorporated into a defrosting system (e.g., system  300 ,  FIG. 3 ), the input node  402  is electrically coupled to an output of the RF signal source (e.g., RF signal source  320 ,  FIG. 3 ), and the output node  404  is electrically coupled to an electrode (e.g., first electrode  340 ,  FIG. 3 ) within the defrosting cavity (e.g., defrosting cavity  360 ,  FIG. 3 ). 
     Between the input and output nodes  402 ,  404 , the variable impedance matching network  440  includes a first variable capacitance network  442  coupled in series with an inductor  454 , and a second variable capacitance network  446  coupled between an intermediate node  451  and a ground reference terminal (e.g., the grounded containment structure  366 ,  FIG. 3 ), in an embodiment. The inductor  454  may be designed for relatively low frequency (e.g., about 40.66 MHz to about 40.70 MHz) and high power (e.g., about 50 W to about 500 W) operation, in an embodiment. For example, inductor  454  may have a value in a range of about 200 nH to about 600 nH, although its value may be lower and/or higher, in other embodiments. According to an embodiment, inductor  454  is a fixed-value, lumped inductor (e.g., a coil). In other embodiments, the inductance value of inductor  454  may be variable. 
     The first variable capacitance network  442  is coupled between the input node  402  and the intermediate node  451 , and the first variable capacitance network  442  may be referred to as a “series matching portion” of the variable impedance matching network  440 . According to an embodiment, the first variable capacitance network  442  includes a first fixed-value capacitor  443  coupled in parallel with a first variable capacitor  444 . The first fixed-value capacitor  443  may have a capacitance value in a range of about 1 picofarad (pF) to about 100 pF, in an embodiment. As will be described in more detail in conjunction with  FIG. 5B , the first variable capacitor  444  may include a network of capacitive components that may be selectively coupled together to provide capacitances in a range of 0 pF to about 100 pF. Accordingly, the total capacitance value provided by the first variable capacitance network  442  may be in a range of about 1 pF to about 200 pF, although the range may extend to lower or higher capacitance values, as well. 
     A “shunt matching portion” of the variable impedance matching network  440  is provided by the second variable capacitance network  446 , which is coupled between node  451  (located between the first variable capacitance network  442  and lumped inductor  454 ) and the ground reference terminal. According to an embodiment, the second variable capacitance network  446  includes a second fixed-value capacitor  447  coupled in parallel with a second variable capacitor  448 . The second fixed-value capacitor  447  may have a capacitance value in a range of about 1 pF to about 100 pF, in an embodiment. As will be described in more detail in conjunction with  FIG. 5B , the second variable capacitor  448  may include a network of capacitive components that may be selectively coupled together to provide capacitances in a range of 0 pF to about 100 pF. Accordingly, the total capacitance value provided by the second variable capacitance network  446  may be in a range of about 1 pF to about 200 pF, although the range may extend to lower or higher capacitance values, as well. The states of the first and second variable capacitance networks  442 ,  446  may be changed to provide multiple capacitance values, and thus may be configurable to optimally match the impedance of the cavity plus load (e.g., cavity  360  plus load  364 ,  FIG. 3 ) to the RF signal source (e.g., RF signal source  320 ,  FIG. 3 ). 
       FIG. 5B  is a schematic diagram of a single-ended variable capacitive network  540  that may be incorporated into a variable impedance matching network (e.g., for each instance of variable capacitors  444 ,  448 ,  FIG. 4B ), in accordance with an example embodiment. Network  540  includes an input node  531 , an output node  533 , and a plurality, N, of discrete capacitors  541 - 544  coupled in parallel with each other between the input and output nodes  531 ,  533 , where N may be an integer between 2 and 10, or more. In addition, network  540  includes a plurality, N, of bypass switches  551 - 554 , where each switch  551 - 554  is coupled in series with one of the terminals of one of the capacitors  541 - 544 . Switches  551 - 554  may be implemented as transistors, mechanical relays or mechanical switches, for example. The electrically conductive state of each switch  551 - 554  (i.e., open or closed) is controlled through control signals  561 - 564  from the system controller (e.g., system controller  312 ,  FIG. 3 ). In the embodiment illustrated in  FIG. 5B , in each parallel-coupled branch, a single switch is connected to one of the terminals of each capacitor, and the terminal to which the switch is coupled alternates between a bottom terminal (e.g., for capacitors  541  and  543 ) and a top terminal (e.g., for capacitors  542  and  544 ) across the series of parallel-coupled capacitors  541 - 544 . In alternate embodiments, the terminal to which the switch is coupled may be the same across the network (e.g., each switch is coupled to a top terminal or to a bottom terminal in each parallel-coupled branch, but not both), or two switches may be coupled to both the top and bottom terminals of each capacitor in each parallel-coupled branch. In the latter embodiment, the two switches coupled to each capacitor may be controlled to open and close in a synchronized manner. 
     In the illustrated embodiment, for each series capacitor/switch combination in each parallel-coupled branch, substantially all current flows through the capacitor when its corresponding switch is in a closed or conductive state, and substantially zero current flows through the capacitor when the switch is in an open or non-conductive state. For example, when all switches  551 - 554  are closed, as illustrated in  FIG. 5B , substantially all current flowing between input and output nodes  531 ,  533  flows through the parallel combination of capacitors  541 - 544 . This configuration represents the maximum capacitance state of the network  540  (i.e., the state of network  540  in which a maximum capacitance value is present between input and output nodes  531 ,  533 ). Conversely, when all switches  551 - 554  are open, substantially zero current flows between input and output nodes  531 ,  533 . This configuration represents the minimum capacitance state of the network  540  (i.e., the state of network  540  in which a minimum capacitance value is present between input and output nodes  531 ,  533 ). 
     Starting from the maximum capacitance state in which all switches  551 - 554  are closed, the system controller may provide control signals  561 - 564  that result in the opening of any combination of switches  551 - 554  in order to reduce the capacitance of the network  540  by switching out corresponding combinations of capacitors  541 - 544 . In one embodiment, each capacitor  541 - 544  has substantially the same capacitance value, referred to herein as a normalized value of J. For example, each capacitor  541 - 544  may have a value in a range of about 1 pF to about 25 pF, or some other value. In such an embodiment, the maximum capacitance value for the network  540  (i.e., when all switches  551 - 554  are in a closed state) would be about N×J. When any n switches are in an open state, the capacitance value for the network  540  would be about (N−n)×f. In such an embodiment, the state of the network  540  may be configured to have any of N+1 values of capacitance. 
     In an alternate embodiment, the capacitors  541 - 544  may have different values from each other. For example, moving from the input node  531  toward the output node  533 , the first capacitor  541  may have a normalized capacitance value of J, and each subsequent capacitor  542 - 544  in the series may have a larger or smaller capacitance value. For example, each subsequent capacitor  542 - 544  may have a capacitance value that is a multiple (e.g., about twice) the capacitance value of the nearest downstream capacitor  541 - 543 , although the difference may not necessarily be an integer multiple. In such an embodiment, the state of the network  540  may be configured to have any of 2 N  values of capacitance. For example, when N=4 and each capacitor  541 - 544  has a different value, the network  540  may be configured to have any of 16 values of capacitance. For example, but not by way of limitation, assuming that capacitor  541  has a value of J, capacitor  542  has a value of 2×J, capacitor  543  has a value of 4×J, and capacitor  544  has a value of 8×J, the total capacitance value for all 16 possible states of the network  540  may be represented by a table similar to Table 1, above (except switching the value of I for J, and reversing the “open” and “closed” designations). 
       FIG. 6  is an example of a Smith chart  600  depicting how the plurality of inductances in an embodiment of a variable impedance matching network (e.g., network  370 ,  400 ,  FIGS. 3, 4A ) may match the cavity plus load impedance to the RF signal source. Although not illustrated, a plurality of capacitances in an embodiment of a variable impedance matching network (e.g., network  370 ,  440 ,  FIGS. 3, 4B ) may similarly match the cavity plus load impedance to the RF signal source. The example Smith chart  600  assumes that the system is a 50 Ohm system, and that the output of the RF signal source is 50 Ohms. Those of skill in the art would understand, based on the description herein, how the Smith chart could be modified for a system and/or RF signal source with different characteristic impedances. 
     In Smith chart  600 , point  601  corresponds to the point at which the load (e.g., the cavity  360  plus load  364 ,  FIG. 3 ) would locate (e.g., at the beginning of a defrosting operation) absent the matching provided by the variable impedance matching network (e.g., network  370 ,  400 ,  FIGS. 3, 4A ). As indicated by the position of the load point  601  in the lower right quadrant of the Smith chart  600 , the load is a capacitive load. According to an embodiment, the shunt and series inductances of the variable impedance matching network sequentially move the substantially-capacitive load impedance toward an optimal matching point  606  (e.g., 50 Ohms) at which RF energy transfer to the load may occur with minimal losses. More specifically, and referring also to  FIG. 4A , shunt inductance  415  moves the impedance to point  602 , series inductance  414  moves the impedance to point  603 , shunt inductance  416  moves the impedance to point  604 , series inductance  412  moves the impedance to point  605 , and shunt inductance  410  moves the impedance to the optimal matching point  606 . 
     It should be noted that the combination of impedance transformations provided by embodiments of the variable impedance matching network keep the impedance at any point within or very close to the lower right quadrant of the Smith chart  600 . As this quadrant of the Smith chart  600  is characterized by relatively high impedances and relatively low currents, the impedance transformation is achieved without exposing components of the circuit to relatively high and potentially damaging currents. Accordingly, an alternate definition of an “inductor-only” matching network, as used herein, may be a matching network that enables impedance matching of a capacitive load using solely or primarily inductive components, where the impedance matching network performs the transformation substantially within the lower right quadrant of the Smith chart. 
     As discussed previously, the impedance of the load changes during the defrosting operation. Accordingly, point  601  correspondingly moves during the defrosting operation. Movement of load point  601  is compensated for, according to the previously-described embodiments, by varying the impedance of the first and second shunt inductances  410 ,  411  so that the final match provided by the variable impedance matching network still may arrive at or near the optimal matching point  606 . Although a specific variable impedance matching network has been illustrated and described herein, those of skill in the art would understand, based on the description herein, that differently-configured variable impedance matching networks may achieve the same or similar results to those conveyed by Smith chart  600 . For example, alternative embodiments of a variable impedance matching network may have more or fewer shunt and/or series inductances, and or different ones of the inductances may be configured as variable inductance networks (e.g., including one or more of the series inductances). Accordingly, although a particular variable inductance matching network has been illustrated and described herein, the inventive subject matter is not limited to the illustrated and described embodiment. 
     The description associated with  FIGS. 3-6  discuss, in detail, an “unbalanced” defrosting apparatus, in which an RF signal is applied to one electrode (e.g., electrode  340 ,  FIG. 3 ), and the other “electrode” (e.g., the containment structure  366 ,  FIG. 3 ) is grounded. As mentioned above, an alternate embodiment of a defrosting apparatus comprises a “balanced” defrosting apparatus. In such an apparatus, balanced RF signals are provided to both electrodes. 
     For example,  FIG. 7  is a simplified block diagram of a balanced defrosting system  700  (e.g., defrosting system  100 ,  210 ,  220 ,  FIGS. 1, 2 ), in accordance with an example embodiment. Defrosting system  700  includes RF subsystem  710 , defrosting cavity  760 , user interface  780 , system controller  712 , RF signal source  720 , power supply and bias circuitry  726 , variable impedance matching network  770 , two electrodes  740 ,  750 , and power detection circuitry  730 , in an embodiment. In addition, in other embodiments, defrosting system  700  may include temperature sensor(s), and/or infrared (IR) sensor(s)  790 , although some or all of these sensor components may be excluded. It should be understood that  FIG. 7  is a simplified representation of a defrosting system  700  for purposes of explanation and ease of description, and that practical embodiments may include other devices and components to provide additional functions and features, and/or the defrosting system  700  may be part of a larger electrical system. 
     User interface  780  may correspond to a control panel (e.g., control panel  120 ,  214 ,  224 ,  FIGS. 1, 2 ), for example, which enables a user to provide inputs to the system regarding parameters for a defrosting operation (e.g., characteristics of the load to be defrosted, and so on), start and cancel buttons, mechanical controls (e.g., a door/drawer open latch), and so on. In addition, the user interface may be configured to provide user-perceptible outputs indicating the status of a defrosting operation (e.g., a countdown timer, visible indicia indicating progress or completion of the defrosting operation, and/or audible tones indicating completion of the defrosting operation) and other information. 
     The RF subsystem  710  includes a system controller  712 , an RF signal source  720 , a first impedance matching circuit  734  (herein “first matching circuit”), power supply and bias circuitry  726 , and power detection circuitry  730 , in an embodiment. System controller  712  may include one or more general purpose or special purpose processors (e.g., a microprocessor, microcontroller, ASIC, and so on), volatile and/or non-volatile memory (e.g., RAM, ROM, flash, various registers, and so on), one or more communication busses, and other components. According to an embodiment, system controller  712  is operatively and communicatively coupled to user interface  780 , RF signal source  720 , power supply and bias circuitry  726 , power detection circuitry  730  (or  730 ′ or  730 ″), variable matching subsystem  770 , sensor(s)  790  (if included), and sensors  792  (if included). System controller  712  is configured to receive signals indicating user inputs received via user interface  780 , to receive signals indicating RF signal reflected power (and possibly RF signal forward power) from power detection circuitry  730  (or  730 ′ or  730 ″), and to receive sensor signals from sensor(s)  790 . Responsive to the received signals and measurements, and as will be described in more detail later, system controller  712  provides control signals to the power supply and bias circuitry  726  and/or to the RF signal generator  722  of the RF signal source  720 . In addition, system controller  712  provides control signals to the variable matching subsystem  770  (over path  716 ), which cause the subsystem  770  to change the state or configuration of a variable impedance matching circuit  772  of the subsystem  770  (herein “variable matching circuit”). 
     Defrosting cavity  760  includes a capacitive defrosting arrangement with first and second parallel plate electrodes  740 ,  750  that are separated by an air cavity within which a load  764  to be defrosted may be placed. Within a containment structure  766 , first and second electrodes  740 ,  750  (e.g., electrodes  140 ,  150 ,  FIG. 1 ) are positioned in a fixed physical relationship with respect to each other on either side of an interior defrosting cavity  760  (e.g., interior cavity  260 ,  FIG. 2 ). According to an embodiment, a distance  752  between the electrodes  740 ,  750  renders the cavity  760  a sub-resonant cavity, in an embodiment. 
     The first and second electrodes  740 ,  750  are separated across the cavity  760  by a distance  752 . In various embodiments, the distance  752  is in a range of about  0 . 10  meters to about 1.0 meter, although the distance may be smaller or larger, as well. According to an embodiment, distance  752  is less than one wavelength of the RF signal produced by the RF subsystem  710 . In other words, as mentioned above, the cavity  760  is a sub-resonant cavity. In some embodiments, the distance  752  is less than about half of one wavelength of the RF signal. In other embodiments, the distance  752  is less than about one quarter of one wavelength of the RF signal. In still other embodiments, the distance  752  is less than about one eighth of one wavelength of the RF signal. In still other embodiments, the distance  752  is less than about one 50th of one wavelength of the RF signal. In still other embodiments, the distance  752  is less than about one 100th of one wavelength of the RF signal. 
     In general, a system  700  designed for lower operational frequencies (e.g., frequencies between 10 MHz and 100 MHz) may be designed to have a distance  752  that is a smaller fraction of one wavelength. For example, when system  700  is designed to produce an RF signal with an operational frequency of about 10 MHz (corresponding to a wavelength of about 30 meters), and distance  752  is selected to be about 0.5 meters, the distance  752  is about one 60th of one wavelength of the RF signal. Conversely, when system  700  is designed for a higher operational frequency of about 300 MHz (corresponding to a wavelength of about 1 meter), and distance  752  is selected to be about 0.5 meters, the distance  752  is about one half of one wavelength of the RF signal. 
     With the operational frequency and the distance  752  between electrodes  740 ,  750  being selected to define a sub-resonant interior cavity  760 , the first and second electrodes  740 ,  750  are capacitively coupled. More specifically, the first electrode  740  may be analogized to a first plate of a capacitor, the second electrode  750  may be analogized to a second plate of a capacitor, and the load  764 , barrier  762 , and air within the cavity  760  may be analogized to a capacitor dielectric. Accordingly, the first electrode  740  alternatively may be referred to herein as an “anode,” and the second electrode  750  may alternatively be referred to herein as a “cathode.” 
     Essentially, the voltage across the first and second electrodes  740 ,  750  heats the load  764  within the cavity  760 . According to various embodiments, the RF subsystem  710  is configured to generate the RF signal to produce voltages across the electrodes  740 ,  750  in a range of about 70 volts to about 3000 volts, in one embodiment, or in a range of about 3000 volts to about 10,000 volts, in another embodiment, although the system may be configured to produce lower or higher voltages across electrodes  740 ,  750 , as well. 
     An output of the RF subsystem  710 , and more particularly an output of RF signal source  720 , is electrically coupled to the variable matching subsystem  770  through a conductive transmission path, which includes a plurality of conductors  728 - 1 ,  728 - 2 ,  728 - 3 ,  728 - 4 , and  728 - 5  connected in series, and referred to collectively as transmission path  728 . According to an embodiment, the conductive transmission path  728  includes an “unbalanced” portion and a “balanced” portion, where the “unbalanced” portion is configured to carry an unbalanced RF signal (i.e., a single RF signal referenced against ground), and the “balanced” portion is configured to carry a balanced RF signal (i.e., two signals referenced against each other). The “unbalanced” portion of the transmission path  728  may include unbalanced first and second conductors  728 - 1 ,  728 - 2  within the RF subsystem  710 , one or more connectors  736 ,  738  (each having male and female connector portions), and an unbalanced third conductor  728 - 3  electrically coupled between the connectors  736 ,  738 . According to an embodiment, the third conductor  728 - 3  comprises a coaxial cable, although the electrical length may be shorter or longer, as well. In an alternate embodiment, the variable matching subsystem  770  may be housed with the RF subsystem  710 , and in such an embodiment, the conductive transmission path  728  may exclude the connectors  736 ,  738  and the third conductor  728 - 3 . Either way, the “balanced” portion of the conductive transmission path  728  includes a balanced fourth conductor  728 - 4  within the variable matching subsystem  770 , and a balanced fifth conductor  728 - 5  electrically coupled between the variable matching subsystem  770  and electrodes  740 ,  750 , in an embodiment. 
     As indicated in  FIG. 7 , the variable matching subsystem  770  houses an apparatus configured to receive, at an input of the apparatus, the unbalanced RF signal from the RF signal source  720  over the unbalanced portion of the transmission path (i.e., the portion that includes unbalanced conductors  728 - 1 ,  728 - 2 , and  728 - 3 ), to convert the unbalanced RF signal into two balanced RF signals (e.g., two RF signals having a phase difference between 120 and 240 degrees, such as about 180 degrees), and to produce the two balanced RF signals at two outputs of the apparatus. For example, the conversion apparatus may be a balun  774 , in an embodiment. The balanced RF signals are conveyed over balanced conductors  728 - 4  to the variable matching circuit  772  and, ultimately, over balanced conductors  728 - 5  to the electrodes  740 ,  750 . 
     In an alternate embodiment, as indicated in a dashed box in the center of  FIG. 7 , and as will be discussed in more detail below, an alternate RF signal generator  720 ′ may produce balanced RF signals on balanced conductors  728 - 1 ′, which may be directly coupled to the variable matching circuit  772  (or coupled through various intermediate conductors and connectors). In such an embodiment, the balun  774  may be excluded from the system  700 . Either way, as will be described in more detail below, a double-ended variable matching circuit  772  (e.g., variable matching circuit  800 ,  900 ,  1000 ,  FIGS. 8-10 ) is configured to receive the balanced RF signals (e.g., over connections  728 - 4  or  728 - 1 ′), to perform an impedance transformation corresponding to a then-current configuration of the double-ended variable matching circuit  772 , and to provide the balanced RF signals to the first and second electrodes  740 ,  750  over connections  728 - 5 . 
     According to an embodiment, RF signal source  720  includes an RF signal generator  722  and a power amplifier  724  (e.g., including one or more power amplifier stages). In response to control signals provided by system controller  712  over connection  714 , RF signal generator  722  is configured to produce an oscillating electrical signal having a frequency in an ISM (industrial, scientific, and medical) band, although the system could be modified to support operations in other frequency bands, as well. The RF signal generator  722  may be controlled to produce oscillating signals of different power levels and/or different frequencies, in various embodiments. For example, the RF signal generator  722  may produce a signal that oscillates in a VHF (very high frequency) range (e.g., about 30 MHz to about 300 MHz), a signal that oscillates in a frequency range of about 10.0 MHz to about 100 MHz, and/or or a signal that oscillates in a frequency range of about 100 MHz to about 3.0 GHz. Some desirable frequencies may be, for example, 13.56 MHz (+/−5 percent), 27.125 MHz (+/−5 percent), 40.68 MHz (+/−5 percent), and 2.45 GHz (+/−5 percent). In one particular embodiment, for example, the RF signal generator  722  may produce a signal that oscillates in a range of about 40.66 MHz to about 40.70 MHz and at a power level in a range of about 10 dBm to about 15 dBm. Alternatively, the frequency of oscillation and/or the power level may be lower or higher than the above-given ranges or values. 
     The power amplifier  724  is configured to receive the oscillating signal from the RF signal generator  722 , and to amplify the signal to produce a significantly higher-power signal at an output of the power amplifier  724 . For example, the output signal may have a power level in a range of about 100 watts to about 400 watts or more, although the power level may be lower or higher, as well. The gain applied by the power amplifier  724  may be controlled using gate bias voltages and/or drain bias voltages provided by the power supply and bias circuitry  726  to one or more stages of amplifier  724 . More specifically, power supply and bias circuitry  726  provides bias and supply voltages to the inputs and/or outputs (e.g., gates and/or drains) of each RF amplifier stage in accordance with control signals received from system controller  712 . 
     The power amplifier may include one or more amplification stages. In an embodiment, each stage of amplifier  724  is implemented as a power transistor, such as a FET, having an input terminal (e.g., a gate or control terminal) and two current carrying terminals (e.g., source and drain terminals). Impedance matching circuits (not illustrated) may be coupled to the input (e.g., gate) and/or output (e.g., drain terminal) of some or all of the amplifier stages, in various embodiments. In an embodiment, each transistor of the amplifier stages includes an LDMOS FET. However, it should be noted that the transistors are not intended to be limited to any particular semiconductor technology, and in other embodiments, each transistor may be realized as a GaN transistor, another type of MOS FET transistor, a BJT, or a transistor utilizing another semiconductor technology. 
     In  FIG. 7 , the power amplifier arrangement  724  is depicted to include one amplifier stage coupled in a particular manner to other circuit components. In other embodiments, the power amplifier arrangement  724  may include other amplifier topologies and/or the amplifier arrangement may include two or more amplifier stages (e.g., as shown in the embodiment of amplifier  324 / 325 ,  FIG. 3 ). For example, the power amplifier arrangement may include various embodiments of a single-ended amplifier, a double-ended (balanced) amplifier, a push-pull amplifier, a Doherty amplifier, a Switch Mode Power Amplifier (SMPA), or another type of amplifier. 
     For example, as indicated in the dashed box in the center of  FIG. 7 , an alternate RF signal generator  720 ′ may include a push-pull or balanced amplifier  724 ′, which is configured to receive, at an input, an unbalanced RF signal from the RF signal generator  722 , to amplify the unbalanced RF signal, and to produce two balanced RF signals at two outputs of the amplifier  724 ′, where the two balanced RF signals are thereafter conveyed over conductors  728 - 1 ′ to the electrodes  740 ,  750 . In such an embodiment, the balun  774  may be excluded from the system  700 , and the conductors  728 - 1 ′ may be directly connected to the variable matching circuit  772  (or connected through multiple coaxial cables and connectors or other multi-conductor structures). 
     Defrosting cavity  760  and any load  764  (e.g., food, liquids, and so on) positioned in the defrosting cavity  760  present a cumulative load for the electromagnetic energy (or RF power) that is radiated into the interior chamber  762  by the electrodes  740 ,  750 . More specifically, and as described previously, the defrosting cavity  760  and the load  764  present an impedance to the system, referred to herein as a “cavity plus load impedance.” The cavity plus load impedance changes during a defrosting operation as the temperature of the load  764  increases. The cavity plus load impedance has a direct effect on the magnitude of reflected signal power along the conductive transmission path  728  between the RF signal source  720  and the electrodes  740 ,  750 . In most cases, it is desirable to maximize the magnitude of transferred signal power into the cavity  760 , and/or to minimize the reflected-to-forward signal power ratio along the conductive transmission path  728 . 
     In order to at least partially match the output impedance of the RF signal generator  720  to the cavity plus load impedance, a first matching circuit  734  is electrically coupled along the transmission path  728 , in an embodiment. The first matching circuit  734  is configured to perform an impedance transformation from an impedance of the RF signal source  720  (e.g., less than about 10 ohms) to an intermediate impedance (e.g., 50 ohms, 75 ohms, or some other value). The first matching circuit  734  may have any of a variety of configurations. According to an embodiment, the first matching circuit  734  includes fixed components (i.e., components with non-variable component values), although the first matching circuit  734  may include one or more variable components, in other embodiments. For example, the first matching circuit  734  may include any one or more circuits selected from an inductance/capacitance (LC) network, a series inductance network, a shunt inductance network, or a combination of bandpass, high-pass and low-pass circuits, in various embodiments. Essentially, the first matching circuit  734  is configured to raise the impedance to an intermediate level between the output impedance of the RF signal generator  720  and the cavity plus load impedance. 
     According to an embodiment, and as mentioned above, power detection circuitry  730  is coupled along the transmission path  728  between the output of the RF signal source  720  and the electrodes  740 ,  750 . In a specific embodiment, the power detection circuitry  730  forms a portion of the RF subsystem  710 , and is coupled to the conductor  728 - 2  between the RF signal source  720  and connector  736 . In alternate embodiments, the power detection circuitry  730  may be coupled to any other portion of the transmission path  728 , such as to conductor  728 - 1 , to conductor  728 - 3 , to conductor  728 - 4  between the RF signal source  720  (or balun  774 ) and the variable matching circuit  772  (i.e., as indicated with power detection circuitry  730 ′), or to conductor  728 - 5  between the variable matching circuit  772  and the electrode(s)  740 ,  750  (i.e., as indicated with power detection circuitry  730 ″). For purposes of brevity, the power detection circuitry is referred to herein with reference number  730 , although the circuitry may be positioned in other locations, as indicated by reference numbers  730 ′ and  730 ″. 
     Wherever it is coupled, power detection circuitry  730  is configured to monitor, measure, or otherwise detect the power of the reflected signals traveling along the transmission path  728  between the RF signal source  720  and one or both of the electrode(s)  740 ,  750  (i.e., reflected RF signals traveling in a direction from electrode(s)  740 ,  750  toward RF signal source  720 ). In some embodiments, power detection circuitry  730  also is configured to detect the power of the forward signals traveling along the transmission path  728  between the RF signal source  720  and the electrode(s)  740 ,  750  (i.e., forward RF signals traveling in a direction from RF signal source  720  toward electrode(s)  740 ,  750 ). 
     Over connection  732 , power detection circuitry  730  supplies signals to system controller  712  conveying the measured magnitudes of the reflected signal power, and in some embodiments, also the measured magnitude of the forward signal power. In embodiments in which both the forward and reflected signal power magnitudes are conveyed, system controller  712  may calculate a reflected-to-forward signal power ratio, or the S11 parameter. In some embodiments the system controller  712  may also calculate the VSWR of the system based on the forward and reflected signal power magnitudes. As will be described in more detail below, when the reflected signal power magnitude exceeds a reflected signal power threshold, or when the reflected-to-forward signal power ratio exceeds an S11 parameter threshold, or when the VSWR exceeds a VSWR threshold, this indicates that the system  700  is not adequately matched to the cavity plus load impedance, and that energy absorption by the load  764  within the cavity  760  may be sub-optimal. In such a situation, system controller  712  orchestrates a process of altering the state of the variable matching circuit  772  to drive the reflected signal power or the S11 parameter or the VSWR toward or below a desired level (e.g., below the reflected signal power threshold and/or the reflected-to-forward signal power ratio threshold), thus re-establishing an acceptable match and facilitating more optimal energy absorption by the load  764 . 
     More specifically, the system controller  712  may provide control signals over control path  716  to the variable matching circuit  772 , which cause the variable matching circuit  772  to vary inductive, capacitive, and/or resistive values of one or more components within the circuit, thus adjusting the impedance transformation provided by the circuit  772 . Adjustment of the configuration of the variable matching circuit  772  desirably decreases the magnitude of reflected signal power, which corresponds to decreasing the magnitude of the S11 parameter or the VSWR and increasing the power absorbed by the load  764 . 
     As discussed above, the variable matching circuit  772  is used to match the input impedance of the defrosting cavity  760  plus load  764  to maximize, to the extent possible, the RF power transfer into the load  764 . The initial impedance of the defrosting cavity  760  and the load  764  may not be known with accuracy at the beginning of a defrosting operation. Further, the impedance of the load  764  changes during a defrosting operation as the load  764  warms up. According to an embodiment, the system controller  712  may provide control signals to the variable matching circuit  772 , which cause modifications to the state of the variable matching circuit  772 . This enables the system controller  712  to establish an initial state of the variable matching circuit  772  at the beginning of the defrosting operation that has a relatively low reflected to forward power ratio, and thus a relatively high absorption of the RF power by the load  764 . In addition, this enables the system controller  712  to modify the state of the variable matching circuit  772  so that an adequate match may be maintained throughout the defrosting operation, despite changes in the impedance of the load  764 . 
     The variable matching circuit  772  may have any of a variety of configurations. For example, the circuit  772  may include any one or more circuits selected from an inductance/capacitance (LC) network, an inductance-only network, a capacitance-only network, or a combination of bandpass, high-pass and low-pass circuits, in various embodiments. In an embodiment in which the variable matching circuit  772  is implemented in a balanced portion of the transmission path  728 , the variable matching circuit  772  is a double-ended circuit with two inputs and two outputs. In an alternate embodiment in which the variable matching circuit is implemented in an unbalanced portion of the transmission path  728 , the variable matching circuit may be a single-ended circuit with a single input and a single output (e.g., similar to matching circuit  400  or  440 ,  FIGS. 4A, 4B ). According to a more specific embodiment, the variable matching circuit  772  includes a variable inductance network (e.g., double-ended network  800 ,  900 ,  FIGS. 8, 9 ). According to another more specific embodiment, the variable matching circuit  772  includes a variable capacitance network (e.g., double-ended network  1000 ,  FIG. 10 ). In still other embodiments, the variable matching circuit  772  may include both variable inductance and variable capacitance elements. The inductance, capacitance, and/or resistance values provided by the variable matching circuit  772 , which in turn affect the impedance transformation provided by the circuit  772 , are established through control signals from the system controller  712 , as will be described in more detail later. In any event, by changing the state of the variable matching circuit  772  over the course of a treatment operation to dynamically match the ever-changing impedance of the cavity  760  plus the load  764  within the cavity  760 , the system efficiency may be maintained at a high level throughout the defrosting operation. 
     The variable matching circuit  772  may have any of a wide variety of circuit configurations, and non-limiting examples of such configurations are shown in  FIGS. 8-10 . For example,  FIG. 8  is a schematic diagram of a double-ended variable impedance matching circuit  800  that may be incorporated into a defrosting system (e.g., system  100 ,  200 ,  700 ,  FIGS. 1, 2, 7 ), in accordance with an example embodiment. According to an embodiment, the variable matching circuit  800  includes a network of fixed-value and variable passive components. 
     Circuit  800  includes a double-ended input  801 - 1 ,  801 - 2  (referred to as input  801 ), a double-ended output  802 - 1 ,  802 - 2  (referred to as output  802 ), and a network of passive components connected in a ladder arrangement between the input  801  and output  802 . For example, when connected into system  700 , the first input  801 - 1  may be connected to a first conductor of balanced conductor  728 - 4 , and the second input  801 - 2  may be connected to a second conductor of balanced conductor  728 - 4 . Similarly, the first output  802 - 1  may be connected to a first conductor of balanced conductor  728 - 5 , and the second output  802 - 2  may be connected to a second conductor of balanced conductor  728 - 5 . 
     In the specific embodiment illustrated in  FIG. 8 , circuit  800  includes a first variable inductor  811  and a first fixed inductor  815  connected in series between input  801 - 1  and output  802 - 1 , a second variable inductor  816  and a second fixed inductor  820  connected in series between input  801 - 2  and output  802 - 2 , a third variable inductor  821  connected between inputs  801 - 1  and  801 - 2 , and a third fixed inductor  824  connected between nodes  825  and  826 . 
     According to an embodiment, the third variable inductor  821  corresponds to an “RF signal source matching portion”, which is configurable to match the impedance of the RF signal source (e.g., RF signal source  720 ,  FIG. 7 ) as modified by the first matching circuit (e.g., circuit  734 ,  FIG. 7 ), or more particularly to match the impedance of the final stage power amplifier (e.g., amplifier  724 ,  FIG. 7 ) as modified by the first matching circuit (e.g., circuit  734 ,  FIG. 7 ). According to an embodiment, the third variable inductor  821  includes a network of inductive components that may be selectively coupled together to provide inductances in a range of about 5 nH to about 200 nH, although the range may extend to lower or higher inductance values, as well. 
     In contrast, the “cavity matching portion” of the variable impedance matching network  800  is provided by the first and second variable inductors  811 ,  816 , and fixed inductors  815 ,  820 , and  824 . Because the states of the first and second variable inductors  811 ,  816  may be changed to provide multiple inductance values, the first and second variable inductors  811 ,  816  are configurable to optimally match the impedance of the cavity plus load (e.g., cavity  760  plus load  764 ,  FIG. 7 ). For example, inductors  811 ,  816  each may have a value in a range of about 10 nH to about 200 nH, although their values may be lower and/or higher, in other embodiments. 
     The fixed inductors  815 ,  820 ,  824  also may have inductance values in a range of about 50 nH to about 800 nH, although the inductance values may be lower or higher, as well. Inductors  811 ,  815 ,  816 ,  820 ,  821 ,  824  may include discrete inductors, distributed inductors (e.g., printed coils), wirebonds, transmission lines, and/or other inductive components, in various embodiments. In an embodiment, variable inductors  811  and  816  are operated in a paired manner, meaning that their inductance values during operation are controlled to be equal to each other, at any given time, in order to ensure that the RF signals conveyed to outputs  802 - 1  and  802 - 2  are balanced. 
     As discussed above, variable matching circuit  800  is a double-ended circuit that is configured to be connected along a balanced portion of the transmission path  728  (e.g., between connectors  728 - 4  and  728 - 5 ), and other embodiments may include a single-ended (i.e., one input and one output) variable matching circuit that is configured to be connected along the unbalanced portion of the transmission path  728 . 
     By varying the inductance values of inductors  811 ,  816 ,  821  in circuit  800 , the system controller  712  may increase or decrease the impedance transformation provided by circuit  800 . Desirably, the inductance value changes improve the overall impedance match between the RF signal source  720  and the cavity plus load impedance, which should result in a reduction of the reflected signal power and/or the reflected-to-forward signal power ratio. In most cases, the system controller  712  may strive to configure the circuit  800  in a state in which a maximum electromagnetic field intensity is achieved in the cavity  760 , and/or a maximum quantity of power is absorbed by the load  764 , and/or a minimum quantity of power is reflected by the load  764 . 
       FIG. 9  is a schematic diagram of a double-ended variable impedance matching network  900 , in accordance with another example embodiment. Network  900  includes a double-ended input  901 - 1 ,  901 - 2  (referred to as input  901 ), a double-ended output  902 - 1 ,  902 - 2  (referred to as output  902 ), and a network of passive components connected in a ladder arrangement between the input  901  and output  902 . The ladder arrangement includes a first plurality, N, of discrete inductors  911 - 914  coupled in series with each other between input  901 - 1  and output  902 - 1 , where N may be an integer between  2  and  10 , or more. The ladder arrangement also includes a second plurality, N, of discrete inductors  916 - 919  coupled in series with each other between input  901 - 2  and output  902 - 2 . Additional discrete inductors  915  and  920  may be coupled between intermediate nodes  925 ,  926  and the output nodes  902 - 1 ,  902 - 2 . Further still, the ladder arrangement includes a third plurality of discrete inductors  921 - 923  coupled in series with each other between inputs  901 - 1  and  901 - 2 , and an additional discrete inductor  924  coupled between nodes  925  and  926 . For example, the fixed inductors  915 ,  920 ,  924  each may have inductance values in a range of about 50 nH to about 800 nH, although the inductance values may be lower or higher, as well. 
     The series arrangement of inductors  911 - 914  may be considered a first variable inductor (e.g., inductor  811 ,  FIG. 8 ), the series arrangement of inductors  916 - 919  may be considered a second variable inductor (e.g., inductor  816 ,  FIG. 8 ), and series arrangement of inductors  921 - 923  may be considered a third variable inductor (e.g., inductor  821 ,  FIG. 8 ). To control the variability of the “variable inductors”, network  900  includes a plurality of bypass switches  931 - 934 ,  936 - 939 ,  941 , and  943 , where each switch  931 - 934 ,  936 - 939 ,  941 , and  943  is coupled in parallel across the terminals of one of inductors  911 - 914 ,  916 - 919 ,  921 , and  923 . Switches  931 - 934 ,  936 - 939 ,  941 , and  943  may be implemented as transistors, mechanical relays or mechanical switches, for example. The electrically conductive state of each switch  931 - 934 ,  936 - 939 ,  941 , and  943  (i.e., open or closed) is controlled using control signals  951 - 954 ,  956 - 959 ,  961 ,  963  from the system controller (e.g., control signals from system controller  712  provided over connection  716 ,  FIG. 7 ). 
     In an embodiment, sets of corresponding inductors in the two paths between input  901  and output  902  have substantially equal values, and the conductive state of the switches for each set of corresponding inductors is operated in a paired manner, meaning that the switch states during operation are controlled to be the same as each other, at any given time, in order to ensure that the RF signals conveyed to outputs  902 - 1  and  902 - 2  are balanced. For example, inductors  911  and  916  may constitute a first “set of corresponding inductors” or “paired inductors” with substantially equal values, and during operation, the states of switches  931  and  936  are controlled to be the same (e.g., either both open or both closed), at any given time. Similarly, inductors  912  and  917  may constitute a second set of corresponding inductors with equal inductance values that are operated in a paired manner, inductors  913  and  918  may constitute a third set of corresponding inductors with equal inductance values that are operated in a paired manner, and inductors  914  and  919  may constitute a fourth set of corresponding inductors with equal inductance values that are operated in a paired manner. 
     For each parallel inductor/switch combination, substantially all current flows through the inductor when its corresponding switch is in an open or non-conductive state, and substantially all current flows through the switch when the switch is in a closed or conductive state. For example, when all switches  931 - 934 ,  936 - 939 ,  941 , and  943  are open, as illustrated in  FIG. 9 , substantially all current flowing between input and output nodes  901 - 1 ,  902 - 1  flows through the series of inductors  911 - 915 , and substantially all current flowing between input and output nodes  901 - 2 ,  902 - 2  flows through the series of inductors  916 - 920  (as modified by any current flowing through inductors  921 - 923  or  924 ). This configuration represents the maximum inductance state of the network  900  (i.e., the state of network  900  in which a maximum inductance value is present between input and output nodes  901 ,  902 ). Conversely, when all switches  931 - 934 ,  936 - 939 ,  941 , and  943  are closed, substantially all current flowing between input and output nodes  901 ,  902  bypasses the inductors  911 - 914  and  916 - 919  and flows instead through the switches  931 - 934  or  936 - 939 , inductors  915  or  920 , and the conductive interconnections between the input and output nodes  901 ,  902  and switches  931 - 934 ,  936 - 939 . This configuration represents the minimum inductance state of the network  900  (i.e., the state of network  900  in which a minimum inductance value is present between input and output nodes  901 ,  902 ). Ideally, the minimum inductance value would be near zero inductance. However, in practice a relatively small inductance is present in the minimum inductance state due to the cumulative inductances of the switches  931 - 934  or  936 - 939 , inductors  915  or  920 , and the conductive interconnections between nodes  901 ,  902  and the switches  931 - 934  or  936 - 939 . For example, in the minimum inductance state, a trace inductance for the series combination of switches  931 - 934  or  936 - 939  may be in a range of about 10 nH to about 400 nH, although the trace inductance may be smaller or larger, as well. Larger, smaller, or substantially similar trace inductances also may be inherent in each of the other network states, as well, where the trace inductance for any given network state is a summation of the inductances of the sequence of conductors and switches through which the current primarily is carried through the network  900 . 
     Starting from the maximum inductance state in which all switches  931 - 934 ,  936 - 939  are open, the system controller may provide control signals  951 - 954 ,  956 - 959  that result in the closure of any combination of switches  931 - 934 ,  936 - 939  in order to reduce the inductance of the network  900  by bypassing corresponding combinations of inductors  911 - 914 ,  916 - 919 . 
     Similar to the embodiment of  FIG. 8 , in circuit  900 , the first and second pluralities of discrete inductors  911 - 914 ,  916 - 919  and fixed inductor  924  correspond to a “cavity matching portion” of the circuit. Similar to the embodiment described above in conjunction with  FIG. 5A , in one embodiment, each inductor  911 - 914 ,  916 - 919  has substantially the same inductance value, referred to herein as a normalized value of I. For example, each inductor  911 - 914 ,  916 - 919  may have a value in a range of about 1 nH to about 400 nH, or some other value. In such an embodiment, the maximum inductance value between input node  901 - 1  and  902 - 2 , and the maximum inductance value between input node  901 - 2  and  902 - 2  (i.e., when all switches  931 - 934 ,  936 - 939  are in an open state) would be about N×I, plus any trace inductance that may be present in the network  900  when it is in the maximum inductance state. When any n switches are in a closed state, the inductance value between corresponding input and output nodes would be about (N-n)xl (plus trace inductance). 
     As also explained in conjunction with  FIG. 5A , above, in an alternate embodiment, the inductors  911 - 914 ,  916 - 919  may have different values from each other. For example, moving from the input node  901 - 1  toward the output node  902 - 1 , the first inductor  911  may have a normalized inductance value of I, and each subsequent inductor  912 - 914  in the series may have a larger or smaller inductance value. Similarly, moving from the input node  901 - 2  toward the output node  902 - 2 , the first inductor  916  may have a normalized inductance value of I, and each subsequent inductor  917 - 919  in the series may have a larger or smaller inductance value. For example, each subsequent inductor  912 - 914  or  917 - 919  may have an inductance value that is a multiple (e.g., about twice or half) the inductance value of the nearest downstream inductor  911 - 914  or  916 - 918 . The example of Table 1, above, applies also to the first series inductance path between input and output nodes  901 - 1  and  902 - 1 , and the second series inductance path between input and output nodes  901 - 2  and  902 - 1 . More specifically, inductor/switch combinations  911 / 931  and  916 / 956  each are analogous to inductor/switch combination  501 / 511 , inductor/switch combinations  912 / 932  and  917 / 957  each are analogous to inductor/switch combination  502 / 512 , inductor/switch combinations  913 / 933  and  918 / 958  each are analogous to inductor/switch combination  503 / 513 , and inductor/switch combinations  914 / 934  and  919 / 959  each are analogous to inductor/switch combination  504 / 514 . 
     Assuming that the trace inductance through series inductors  911 - 914  in the minimum inductance state is about 10 nH, and the range of inductance values achievable by the series inductors  911 - 914  is about 10 nH (trace inductance) to about 400 nH, the values of inductors  911 - 914  may be, for example, about 10 nH, about 20 nH, about 40 nH, about 80 nH, and about 160 nH, respectively. The combination of series inductors  916 - 919  may be similarly or identically configured. Of course, more or fewer than four inductors  911 - 914  or  916 - 919  may be included in either series combination between input and output nodes  901 - 1 / 902 - 1  or  901 - 2 / 902 - 2 , and the inductors within each series combination may have different values from the example values given above. 
     Although the above example embodiment specifies that the number of switched inductances in each series combination between corresponding input and output nodes equals four, and that each inductor  911 - 914 ,  916 - 919  has a value that is some multiple of a value of I, alternate embodiments of variable series inductance networks may have more or fewer than four inductors, different relative values for the inductors, and/or a different configuration of inductors (e.g., differently connected sets of parallel and/or series coupled inductors). Either way, by providing a variable inductance network in an impedance matching network of a defrosting system, the system may be better able to match the ever-changing cavity plus load impedance that is present during a defrosting operation. 
     As with the embodiment of  FIG. 8 , the third plurality of discrete inductors  921 - 923  corresponds to an “RF signal source matching portion” of the circuit. The third variable inductor comprises the series arrangement of inductors  921 - 923 , where bypass switches  941  and  943  enable inductors  921  and  923  selectively to be connected into the series arrangement or bypassed based on control signals  961  and  963 . In an embodiment, each of inductors  921 - 923  may have equal values (e.g., values in a range of about 1 nH to about 100 nH. In an alternate embodiment, the inductors  921 - 923  may have different values from each other. Inductor  922  is electrically connected between input terminals  901 - 1  and  901 - 2  regardless of the state of bypass switches  941  and  943 . Accordingly, the inductance value of inductor  922  serves as a baseline (i.e., minimum) inductance between input terminals  901 - 1  and  901 - 2 . According to an embodiment, the first and third inductors  921 ,  923  may have inductance values that are a ratio of each other. For example, when the first inductor  921  has a normalized inductance value of J, inductor  923  may have a value of 2*J, 3*J, 4*J, or some other ratio, in various embodiments. 
       FIG. 10  is a schematic diagram of a double-ended variable impedance matching circuit  1000  that may be incorporated into a defrosting system (e.g., system  100 ,  200 ,  700 ,  FIGS. 1, 2, 7 ), in accordance with another example embodiment. As with the matching circuits  800 ,  900  ( FIGS. 8 and 9 ), according to an embodiment, the variable matching circuit  1000  includes a network of fixed-value and variable passive components. 
     Circuit  1000  includes a double-ended input  1001 - 1 ,  1001 - 2  (referred to as input  1001 ), a double-ended output  1002 - 1 ,  1002 - 2  (referred to as output  1002 ), and a network of passive components connected between the input  1001  and output  1002 . For example, when connected into system  700 , the first input  1001 - 1  may be connected to a first conductor of balanced conductor  728 - 4 , and the second input  1001 - 2  may be connected to a second conductor of balanced conductor  728 - 4 . Similarly, the first output  1002 - 1  may be connected to a first conductor of balanced conductor  728 - 5 , and the second output  1002 - 2  may be connected to a second conductor of balanced conductor  728 - 5 . 
     In the specific embodiment illustrated in  FIG. 10 , circuit  1000  includes a first variable capacitance network  1011  and a first inductor  1015  connected in series between input  1001 - 1  and output  1002 - 1 , a second variable capacitance network  1016  and a second inductor  1020  connected in series between input  1001 - 2  and output  1002 - 2 , and a third variable capacitance network  1021  connected between nodes  1025  and  1026 . The inductors  1015 ,  1020  are relatively large in both size and inductance value, in an embodiment, as they may be designed for relatively low frequency (e.g., about 40.66 MHz to about 40.70 MHz) and high power (e.g., about 50 W to about 500 W) operation. For example, inductors  1015 ,  1020  each may have a value in a range of about 100 nH to about 1000 nH (e.g., in a range of about 200 nH to about 600 nH), although their values may be lower and/or higher, in other embodiments. According to an embodiment, inductors  1015 ,  1020  are fixed-value, lumped inductors (e.g., coils, discrete inductors, distributed inductors (e.g., printed coils), wirebonds, transmission lines, and/or other inductive components, in various embodiments). In other embodiments, the inductance value of inductors  1015 ,  1020  may be variable. In any event, the inductance values of inductors  1015 ,  1020  are substantially the same either permanently (when inductors  1015 ,  1020  are fixed-value) or at any given time (when inductors  1015 ,  1020  are variable, they are operated in a paired manner), in an embodiment. 
     The first and second variable capacitance networks  1011 ,  1016  correspond to “series matching portions” of the circuit  1000 . According to an embodiment, the first variable capacitance network  1011  includes a first fixed-value capacitor  1012  coupled in parallel with a first variable capacitor  1013 . The first fixed-value capacitor  1012  may have a capacitance value in a range of about 1 pF to about 100 pF, in an embodiment. As was described previously in conjunction with  FIG. 5B , the first variable capacitor  1013  may include a network of capacitive components that may be selectively coupled together to provide capacitances in a range of 0 pF to about 100 pF. Accordingly, the total capacitance value provided by the first variable capacitance network  1011  may be in a range of about 1 pF to about 200 pF, although the range may extend to lower or higher capacitance values, as well. 
     Similarly, the second variable capacitance network  1016  includes a second fixed-value capacitor  1017  coupled in parallel with a second variable capacitor  1018 . The second fixed-value capacitor  1017  may have a capacitance value in a range of about 1 pF to about 100 pF, in an embodiment. As was described previously in conjunction with  FIG. 5B , the second variable capacitor  1018  may include a network of capacitive components that may be selectively coupled together to provide capacitances in a range of 0 pF to about 100 pF. Accordingly, the total capacitance value provided by the second variable capacitance network  1016  may be in a range of about 1 pF to about 200 pF, although the range may extend to lower or higher capacitance values, as well. 
     In any event, to ensure the balance of the signals provided to outputs  1002 - 1  and  1002 - 2 , the capacitance values of the first and second variable capacitance networks  1011 ,  1016  are controlled to be substantially the same at any given time, in an embodiment. For example, the capacitance values of the first and second variable capacitors  1013 ,  1018  may be controlled so that the capacitance values of the first and second variable capacitance networks  1011 ,  1016  are substantially the same at any given time. The first and second variable capacitors  1013 ,  1018  are operated in a paired manner, meaning that their capacitance values during operation are controlled, at any given time, to ensure that the RF signals conveyed to outputs  1002 - 1  and  1002 - 2  are balanced. The capacitance values of the first and second fixed-value capacitors  1012 ,  1017  may be substantially the same, in some embodiments, although they may be different, in others. 
     The “shunt matching portion” of the variable impedance matching network  1000  is provided by the third variable capacitance network  1021  and fixed inductors  1015 ,  1020 . According to an embodiment, the third variable capacitance network  1021  includes a third fixed-value capacitor  1023  coupled in parallel with a third variable capacitor  1024 . The third fixed-value capacitor  1023  may have a capacitance value in a range of about 1 pF to about 500 pF, in an embodiment. As was described previously in conjunction with  FIG. 5B , the third variable capacitor  1024  may include a network of capacitive components that may be selectively coupled together to provide capacitances in a range of 0 pF to about 200 pF. Accordingly, the total capacitance value provided by the third variable capacitance network  1021  may be in a range of about 1 pF to about 700 pF, although the range may extend to lower or higher capacitance values, as well. 
     Because the states of the variable capacitance networks  1011 ,  1016 ,  1021  may be changed to provide multiple capacitance values, the variable capacitance networks  1011 ,  1016 ,  1021  are configurable to optimally match the impedance of the cavity plus load (e.g., cavity  760  plus load  764 ,  FIG. 7 ) to the RF signal source (e.g., RF signal source  720 ,  720 ′,  FIG. 7 ). By varying the capacitance values of capacitors  1013 ,  1018 ,  1024  in circuit  1000 , the system controller (e.g., system controller  712 ,  FIG. 7 ) may increase or decrease the impedance transformation provided by circuit  1000 . Desirably, the capacitance value changes improve the overall impedance match between the RF signal source  720  and the impedance of the cavity plus load, which should result in a reduction of the reflected signal power and/or the reflected-to-forward signal power ratio. In most cases, the system controller  712  may strive to configure the circuit  1000  in a state in which a maximum electromagnetic field intensity is achieved in the cavity  760 , and/or a maximum quantity of power is absorbed by the load  764 , and/or a minimum quantity of power is reflected by the load  764 . 
     It should be understood that the variable impedance matching circuits  800 ,  900 ,  100  illustrated in  FIGS. 8-10  are but three possible circuit configurations that may perform the desired double-ended variable impedance transformations. Other embodiments of double-ended variable impedance matching circuits may include differently arranged inductive or capacitive networks, or may include passive networks that include various combinations of inductors, capacitors, and/or resistors, where some of the passive components may be fixed-value components, and some of the passive components may be variable-value components (e.g., variable inductors, variable capacitors, and/or variable resistors). Further, the double-ended variable impedance matching circuits may include active devices (e.g., transistors) that switch passive components into and out of the network to alter the overall impedance transformation provided by the circuit. 
     A particular physical configuration of a defrosting system will now be described in conjunction with  FIG. 11 . More particularly,  FIG. 11  is a cross-sectional, side view of a defrosting system  1100 , in accordance with an example embodiment. The defrosting system  1100  generally includes a defrosting cavity  1174 , a user interface (not shown), a system controller  1130 , an RF signal source  1120 , power supply and bias circuitry (not shown), power detection circuitry  1180 , a variable impedance matching network  1160 , a first electrode  1170 , and a second electrode  1172 , in an embodiment. According to an embodiment, the system controller  1130 , RF signal source  1120 , power supply and bias circuitry, and power detection circuitry  1180 , may form portions of a first module (e.g., RF module  1300 ,  FIG. 13 ), and the variable impedance matching network  1160  may form portions of a second module (e.g., either module  1200  or  1240 ,  FIGS. 12A, 12B ). In addition, in some embodiments, defrosting system  1100  may include temperature sensor(s), and/or IR sensor(s)  1192 . 
     The defrosting system  1100  is contained within a containment structure  1150 , in an embodiment. According to an embodiment, the containment structure  1150  may define two or more interior areas, such as the defrosting cavity  1174  and a circuit housing area  1178 . The containment structure  1150  includes bottom, top, and side walls. Portions of the interior surfaces of some of the walls of the containment structure  1150  may define the defrosting cavity  1174 . The defrosting cavity  1174  includes a capacitive defrosting arrangement with first and second parallel plate electrodes  1170 ,  1172  that are separated by an air cavity within which a load  1164  to be defrosted may be placed. For example, the first electrode  1170  may be positioned above the air cavity, and a second electrode  1172  may be, in the single-ended system embodiment, provided by a conductive portion of the containment structure  1150  (e.g., a portion of the bottom wall of the containment structure  1150 ). Alternatively, in the single- or double-ended system embodiments, the second electrode  1172  may be formed from a conductive plate, as shown, that is distinct from the containment structure  1150 . According to an embodiment, non-electrically conductive support structure(s)  1154  may be employed to suspend the first electrode  1170  above the air cavity, to electrically isolate the first electrode  1170  from the containment structure  1150 , and to hold the first electrode  1170  in a fixed physical orientation with respect to the air cavity. In addition, to avoid direct contact between the load  1164  and the second electrode  1172 , a non-conductive support and barrier structure  1156  may be positioned over the bottom surface of the containment structure  1150 . 
     According to an embodiment, the containment structure  1150  is at least partially formed from conductive material, and the conductive portion(s) of the containment structure may be grounded to provide a ground reference for various electrical components of the system. Alternatively, at least the portion of the containment structure  1150  that corresponds to the second electrode  1172  may be formed from conductive material and grounded. 
     The temperature sensor(s) and/or IR sensor(s)  1192  may be positioned in locations that enable the temperature of the load  1164  to be sensed both before, during, and after a defrosting operation. According to an embodiment, the temperature sensor(s) and/or IR sensor(s)  1192  are configured to provide load temperature estimates to the system controller  1130 . 
     Some or all of the various components of the system controller  1130 , the RF signal source  1120 , the power supply and bias circuitry (not shown), the power detection circuitry  1180 , and the variable impedance matching network  1160 , may be coupled to one or more common substrates (e.g., substrate  1152 ) within the circuit housing area  1178  of the containment structure  1150 , in an embodiment. For example, some of all of the above-listed components may be included in an RF module (e.g., RF module  1300 ,  FIG. 13 ) and a variable impedance matching circuit module (e.g., a variation of module  1200  or  1240 ,  FIGS. 12A, 12B ), which are housed within the circuit housing area  1178  of the containment structure  1150 . According to an embodiment, the system controller  1130  is coupled to the user interface, RF signal source  1120 , variable impedance matching network  1160 , and power detection circuitry  1180  through various conductive interconnects on or within the common substrate  1152 , and/or through various cables (e.g., coaxial cables), not shown. In addition, the power detection circuitry  1180  is coupled along the transmission path  1148  between the output of the RF signal source  1120  and the input to the variable impedance matching network  1160 , in an embodiment. For example, the substrate  1152  (or the substrates defining an RF module  1300  or variable impedance matching network module  1200 ,  1240 ) may include a microwave or RF laminate, a polytetrafluorethylene (PTFE) substrate, a printed circuit board (PCB) material substrate (e.g., FR-4), an alumina substrate, a ceramic tile, or another type of substrate. In various alternate embodiments, various ones of the components may be coupled to different substrates with electrical interconnections between the substrates and components. In still other alternate embodiments, some or all of the components may be coupled to a cavity wall, rather than being coupled to a distinct substrate. 
     In either a single-ended or double-ended embodiment, the first electrode  1170  is electrically coupled to the RF signal source  1120  through a variable impedance matching network  1160  and a transmission path  1148 , in an embodiment. In a double-ended embodiment, the second electrode  1172  also is electrically coupled to the RF signal source  1120  through a variable impedance matching network  1160  and a transmission path  1148 . As discussed previously, single-ended embodiments of the variable impedance matching network  1160  may include a single-ended variable inductance network (e.g., network  400 ,  FIG. 4A ) or a single-ended variable capacitance network (e.g., network  440 ,  FIG. 4B ). Alternatively, double-ended embodiments of the variable impedance matching network  1160  may include a double-ended variable inductance network (e.g., network  800 ,  900 ,  FIGS. 8, 9 ) or a double-ended variable capacitance network (e.g., network  1000 ,  FIG. 10 ). In an embodiment, the variable impedance matching network  1160  is implemented as a module (e.g., one of modules  1200 ,  1240 ,  FIGS. 12A, 12B ), or is coupled to the common substrate  1152  and located within the circuit housing area  1178 . Conductive structures (e.g., conductive vias, traces, cables, wires, and other structures) may provide for electrical communication between the circuitry within the circuit housing area  1178  and electrodes  1170 ,  1172 . 
     According to various embodiments, the circuitry associated with the single-ended or double-ended variable impedance matching networks discussed herein may be implemented in the form of one or more modules, where a “module” is defined herein as an assembly of electrical components coupled to a common substrate. For example,  FIGS. 12A and 12B  are a perspective views of examples of modules  1200 ,  1240  that include a double-ended variable impedance matching network (e.g., networks  800 ,  900 ,  1000 ,  FIGS. 8-10 ), in accordance with two example embodiments. More specifically,  FIG. 12A  illustrates a module  1200  that houses a variable inductance impedance matching network (e.g., networks  800 ,  900 ,  FIGS. 8, 9 ), and  FIG. 12B  illustrates a module  1240  that houses a variable capacitance impedance matching network (e.g., network  1000 ,  FIG. 10 ). 
     Each of the modules  1200 ,  1240  includes a printed circuit board (PCB)  1204 ,  1244  with a front side  1206 ,  1246  and an opposite back side  1208 ,  1248 . The PCB  1204 ,  1244  is formed from one or more dielectric layers, and two or more printed conductive layers. Conductive vias (not visible in  FIGS. 12A, 12B ) may provide for electrical connections between the multiple conductive layers. At the front side  1206 ,  1246 , a plurality of printed conductive traces formed from a first printed conductive layer provides for electrical connectivity between the various components that are coupled to the front side  1206 ,  1246  of the PCB  1204 ,  1244 . Similarly, at the back side  1208 ,  1248 , a plurality of printed conductive traces formed from a second printed conductive layer provides for electrical connectivity between the various components that are coupled to the back side  1208 ,  1248  of the PCB  1204 ,  1244 . 
     According to an embodiment, each PCB  1204 ,  1244  houses an RF input connector  1238 ,  1278  (e.g., coupled to back side  1208 ,  1248  and thus not visible in the views of  FIGS. 12A, 12B , but corresponding to connector  738 ,  FIG. 7 ) and a balun  1274 ,  1284  (e.g., coupled to back side  1208 ,  1248  and thus not visible in the view of  FIGS. 12A, 12B , but corresponding to balun  774 ,  FIG. 7 ). The input connector  1238 ,  1278  is configured to be electrically connected to an RF subsystem (e.g., subsystem  310 ,  710 ,  FIGS. 3, 7 ) with a connection (e.g., connection  728 - 3 ,  FIG. 7 ) such as a coaxial cable or other type of conductor. In such an embodiment, an unbalanced RF signal received by the balun  1274 ,  1284  from the RF input connector  1238 ,  1278  is converted to a balanced signal, which is provided over a pair of balanced conductors (e.g., connections  728 - 4 ,  FIG. 7 ) to a double-ended input that includes first and second inputs  1201 - 1 ,  1201 - 2  or  1241 - 1 ,  1242 - 2 . The connection between the input connector  1238 ,  1278  and the balun  1274 ,  1284 , and the connections between the balun  1274 ,  1284  and the inputs  1201 - 1 ,  1201 - 2 ,  1241 - 1 ,  1241 - 2  each may be implemented using conductive traces and vias formed on and in the PCB  1204 ,  1244 . In an alternate embodiment, as discussed above, an alternate embodiment may include a balanced amplifier (e.g., balanced amplifier  724 ′,  FIG. 7 ), which produces a balanced signal on connections (e.g., conductors  728 - 1 ′,  FIG. 7 ) that can be directly coupled to the inputs  1201 - 1 ,  1201 - 2 ,  1241 - 1 ,  1241 - 2 . In such an embodiment, the balun  1274 ,  1284  may be excluded from the module  1200 ,  1240 . 
     In addition, each PCB  1204 ,  1244  houses circuitry associated with a double-ended variable impedance matching network (e.g., network  772 ,  800 ,  900 ,  1000 ,  FIGS. 7-10 ). Referring first to  FIG. 12A , which corresponds to a module  1200  that houses a variable inductance impedance matching network (e.g., networks  800 ,  900 ,  FIGS. 8, 9 ), the circuitry housed by the PCB  1204  includes the double-ended input  1201 - 1 ,  1201 - 2  (e.g., inputs  901 - 1 ,  901 - 2 ,  FIG. 9 ), a double-ended output  1202 - 1 ,  1202 - 2  (e.g., outputs  902 - 1 ,  902 - 2 ,  FIG. 9 ), a first plurality of inductors  1211 ,  1212 ,  1213 ,  1214 ,  1215  (e.g., inductors  911 - 915 ,  FIG. 9 ) coupled in series between a first input  1201 - 1  of the double-ended input and a first output  1202 - 1  of the double-ended output, a second plurality of inductors  1216 ,  1217 ,  1218 ,  1219 ,  1220  (e.g., inductors  916 - 920 ,  FIG. 9 ) coupled in series between a second input  1201 - 2  of the double-ended input and a second output  1202 - 2  of the double-ended output, a third plurality of inductors (not visible in the view of  FIG. 12 , but corresponding to inductors  921 - 923 ,  FIG. 9 , for example) coupled in series between the first and second inputs  1201 - 1 ,  1201 - 2 , and one or more additional inductors  1224  (e.g., inductor  924 ,  FIG. 9 ) coupled between nodes  1225  and  1226  (e.g., nodes  925 ,  926 ). 
     A plurality of switches or relays (e.g., not visible in the view of  FIG. 12 , but corresponding to switches  931 - 934 ,  936 - 939 ,  941 ,  943 ,  FIG. 9 , for example) also are coupled to the PCB  1204 . For example, the plurality of switches or relays may be coupled to the front side  1206  or to the back side  1208  of the PCB  1204 . Each of the switches or relays is electrically connected in parallel across one of the inductors  1211 - 1214 ,  1216 - 1219 , or one of the inductors (e.g., inductors  921 ,  923 ,  FIG. 9 ) between inputs  1202 - 1  and  1202 - 2 , in an embodiment. A control connector  1230  is coupled to the PCB  1204 , and conductors of the control connector  1230  are electrically coupled to conductive traces  1232  to provide control signals to the switches (e.g., control signals  951 - 954 ,  956 - 959 ,  961 ,  963 ,  FIG. 9 ), and thus to switch the inductors into or out of the circuit, as described previously. As shown in  FIG. 12A , fixed-value inductors  1215 ,  1220  (e.g., inductors  915 ,  920 ,  FIG. 9 ) may be formed from relatively large coils, although they may be implemented using other structures as well. Further, as shown in the embodiment of  FIG. 12A , the conductive features corresponding to outputs  1202 - 1 ,  1202 - 2  may be relatively large, and may be elongated for direct attachment to the electrodes (e.g., electrodes  740 ,  750 ,  FIG. 7 ) of the system. 
     Referring now to  FIG. 12B , which corresponds to a module  1240  that houses a variable capacitance impedance matching network (e.g., network  1000 ,  FIG. 10 ), the circuitry housed by the PCB  1244  includes a double-ended input  1241 - 1 ,  1241 - 2  (e.g., inputs  1001 - 1 ,  1001 - 2 ,  FIG. 10 ), a double-ended output  1242 - 1 ,  1242 - 2  (e.g., outputs  1002 - 1 ,  1002 - 2 ,  FIG. 10 ), a first plurality of capacitors  1251 ,  1252  (e.g., capacitors  1012 ,  1013 ,  FIG. 10 ) that comprise a first variable capacitance network (e.g., network  1011 ,  FIG. 10 ) coupled between a first input  1241 - 1  of the double-ended input and a first intermediate node  1265  (e.g., node  1025 ,  FIG. 10 ), a second plurality of capacitors  1256 ,  1257  (e.g., capacitors  1017 ,  1018 ,  FIG. 10 ) that comprise a second variable capacitance network (e.g., network  1016 ,  FIG. 10 ) coupled between a second input  1241 - 2  of the double-ended input and a second intermediate node  1266  (e.g., node  1026 ,  FIG. 10 ), a third plurality of capacitors  1258 ,  1259  (e.g., capacitors  1023 ,  1024 ,  FIG. 10 ) coupled between nodes  1265 ,  1266  (e.g., nodes  1025 ,  1026 ), and one or more additional inductors  1255 ,  1260  (e.g., inductors  1015 ,  1020 ,  FIG. 10 ) coupled between nodes  1265  and  1266  and outputs  1242 - 1 ,  1242 - 2 . 
     The first, second, and third pluralities of capacitors each include a fixed capacitor  1251 ,  1256 ,  1258  (e.g., capacitors  1012 ,  1017 ,  1023 ,  FIG. 10 ), and a set of one or more capacitors  1252 ,  1257 ,  1259  that make up a variable capacitor (e.g., variable capacitors  1013 ,  1018 ,  1024 ). Each set of variable capacitors  1252 ,  1257 ,  1259  may be implemented using a capacitive network, such as network  500 ,  FIG. 5 . A plurality of switches or relays (e.g., not visible in the view of  FIG. 12B , but corresponding to switches  551 - 554 ,  FIG. 5 , for example) also are coupled to the PCB  1244 . For example, the plurality of switches or relays may be coupled to the front side  1246  or to the back side  1248  of the PCB  1244 . Each of the switches or relays is electrically connected in series with a terminal of a different one of the capacitors associated with the variable capacitors  1252 ,  1257 ,  1259 . A control connector  1290  is coupled to the PCB  1244 , and conductors of the control connector (not shown in  FIG. 12B ) are electrically coupled to conductive traces within PCB  1244  to provide control signals to the switches (e.g., control signals  561 - 564 ,  FIG. 5 ), and thus to switch the capacitors into or out of the circuit, as described previously. 
     As shown in  FIG. 12B , fixed-value inductors  1255 ,  1260  (e.g., inductors  1015 ,  1020 ,  FIG. 10 ) are electrically coupled between intermediate nodes  1265  and  1266  and outputs  1242 - 1 ,  1242 - 2 . The inductors  1255 ,  1260  may be formed from relatively large coils, although they may be implemented using other structures as well. Further, as shown in the embodiment of  FIG. 12B , the conductive features corresponding to outputs  1242 - 1 ,  1242 - 2  may be relatively large, and may be elongated for direct attachment to the electrodes (e.g., electrodes  740 ,  750 ,  FIG. 7 ) of the system. According to an embodiment, and as illustrated in  FIG. 12B , the inductors  1255 ,  1260  are arranged so that their primary axes are perpendicular to each other (i.e., the axes extending through the centers of the inductors  1255 ,  1260  are at about 90 degree angles). This may result in significantly reduced electromagnetic coupling between the inductors  1255 ,  1260 . In other embodiments, the inductors  1255 ,  1260  may be arranged so that their primary axes are parallel, or may be arranged with other angular offsets. 
     In various embodiments, the circuitry associated with the RF subsystem (e.g., RF subsystem  310 ,  710 ,  FIGS. 3, 7 ) also may be implemented in the form of one or more modules. For example,  FIG. 13  is a perspective view of an RF module  1300  that includes an RF subsystem (e.g., RF subsystem  310 ,  710 ,  FIGS. 3, 7 ), in accordance with an example embodiment. The RF module  1300  includes a PCB  1302  coupled to a ground substrate  1304 . The ground substrate  1304  provides structural support for the PCB  1302 , and also provides an electrical ground reference and heat sink functionality for the various electrical components coupled to the PCB  1302 . 
     According to an embodiment, the PCB  1302  houses the circuitry associated with the RF subsystem (e.g., subsystem  310  or  710 ,  FIGS. 3, 7 ). Accordingly, the circuitry housed by the PCB  1302  includes system controller circuitry  1312  (e.g., corresponding to system controller  312 ,  712 ,  FIGS. 3, 7 ), RF signal source circuitry  1320  (e.g., corresponding to RF signal source  320 ,  720 ,  FIGS. 3, 7 , including an RF signal generator  322 ,  722  and power amplifier  324 ,  325 ,  724 ), power detection circuitry  1330  (e.g., corresponding to power detection circuitry  330 ,  730 ,  FIGS. 3, 7 ), and impedance matching circuitry  1334  (e.g., corresponding to first matching circuitry  334 ,  734 ,  FIGS. 3, 7 ). 
     In the embodiment of  FIG. 13 , the system controller circuitry  1312  includes a processor IC and a memory IC, the RF signal source circuitry  1320  includes a signal generator IC and one or more power amplifier devices, the power detection circuitry  1330  includes a power coupler device, and the impedance matching circuitry  1334  includes a plurality of passive components (e.g., inductors  1335 ,  1336  and capacitors  1337 ) connected together to form an impedance matching network. The circuitry  1312 ,  1320 ,  1330 ,  1334  and the various sub-components may be electrically coupled together through conductive traces on the PCB  1302  as discussed previously in reference to the various conductors and connections discussed in conjunction with  FIGS. 3, 7 . 
     RF module  1300  also includes a plurality of connectors  1316 ,  1326 ,  1338 ,  1380 , in an embodiment. For example, connector  1380  may be configured to connect with a host system that includes a user interface (e.g., user interface  380 ,  780 ,  FIGS. 3, 7 ) and other functionality. Connector  1316  may be configured to connect with a variable matching circuit (e.g., circuit  372 ,  772 ,  FIGS. 3, 7 ) to provide control signals to the circuit, as previously described. Connector  1326  may be configured to connect to a power supply to receive system power. Finally, connector  1338  (e.g., connector  336 ,  736 ,  FIGS. 3, 7 ) may be configured to connect to a coaxial cable or other transmission line, which enables the RF module  1300  to be electrically connected (e.g., through a coaxial cable implementation of conductor  328 - 2 ,  728 - 3 ,  FIGS. 3, 7 ) to a variable matching subsystem (e.g., subsystem  370 ,  770 ,  FIGS. 3, 7 ). In an alternate embodiment, components of the variable matching subsystem (e.g., variable matching network  370 , balun  774 , and/or variable matching circuit  772 ,  FIGS. 3, 7 ) also may be integrated onto the PCB  1302 , in which case connector  1338  may be excluded from the module  1300 . Other variations in the layout, subsystems, and components of RF module  1300  may be made, as well. 
     Embodiments of an RF module (e.g., module  1300 ,  FIG. 13 ) and a variable impedance matching network module (e.g., module  1200 ,  1240 ,  FIGS. 12A, 12B ) may be electrically connected together, and connected with other components, to form a defrosting apparatus or system (e.g., apparatus  100 ,  200 ,  300 ,  700 ,  1100 ,  FIGS. 1-3, 7, 11 ). For example, an RF signal connection may be made through a connection (e.g., conductor  728 - 3 ,  FIG. 7 ), such as a coaxial cable, between the RF connector  1338  ( FIG. 13 ) and the RF connector  1238  ( FIG. 12A ) or RF connector  1278  ( FIG. 12B ), and control connections may be made through connections (e.g., conductors  716 ,  FIG. 7 ), such as a multi-conductor cable, between the connector  1316  ( FIG. 13 ) and the connector  1230  ( FIG. 12A ) or connector  1290  ( FIG. 12B ). To further assemble the system, a host system or user interface may be connected to the RF module  1300  through connector  1380 , a power supply may be connected to the RF module  1300  through connector  1326 , and electrodes (e.g., electrodes  740 ,  750 ,  FIG. 7 ) may be connected to the outputs  1202 - 1 ,  1202 - 2  ( FIG. 12A ) or  1242 - 1 ,  1242 - 2  ( FIG. 12B ). Of course, the above-described assembly also would be physically connected to various support structures and other system components so that the electrodes are held in a fixed relationship to each other across a defrosting cavity (e.g., cavity  110 ,  360 ,  760 ,  FIGS. 1, 3, 7 ), and the defrosting apparatus may be integrated within a larger system (e.g., systems  100 ,  200 ,  FIGS. 1, 2 ). 
     Now that embodiments of the electrical and physical aspects of defrosting systems have been described, various embodiments of methods for operating such defrosting systems will now be described in conjunction with  FIGS. 14A, 14B, 14C, 14D, and 15 . More specifically,  FIG. 14A  is a flowchart of a method of operating a defrosting system (e.g., system  100 ,  210 ,  220 ,  300 ,  700 ,  1100 ,  FIGS. 1-3, 7, 11 ) with dynamic load matching, in accordance with an example embodiment, and  FIG. 14B  is a flowchart of a method for performing one of the steps of the flowchart of  FIG. 14A , and more specifically the step for determining desired RF signal parameters based on load mass, in accordance with an embodiment.  FIG. 14C  is a flowchart of an alternative method to  FIG. 14B  for determining desired RF signal parameters based on load mass, in accordance with an embodiment.  FIG. 14D  is a flowchart of a method for periodically (e.g., each time a new match is determined for the defrosting system) refining a mass estimate and determining new desired RF signal parameters based on the refined mass estimate, in accordance with an embodiment. 
     Referring first to  FIG. 14A , the method may begin, in block  1402 , when the system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ) receives an indication that a defrosting operation should start. Such an indication may be received, for example, after a user has place a load (e.g., load  364 ,  764 ,  1164 ,  FIGS. 3, 7, 11 ) into the system&#39;s defrosting cavity (e.g., cavity  360 ,  760 ,  1174 ,  FIGS. 3, 7, 11 ), has sealed the cavity (e.g., by closing a door or drawer), and has pressed a start button (e.g., of the user interface  380 ,  780 ,  FIGS. 3, 7 ). In an embodiment, sealing of the cavity may engage one or more safety interlock mechanisms, which when engaged, indicate that RF power supplied to the cavity will not substantially leak into the environment outside of the cavity. As will be described later, disengagement of a safety interlock mechanism may cause the system controller immediately to pause or terminate the defrosting operation. 
     According to various embodiments, the system controller optionally may receive additional inputs indicating the load type (e.g., meats, liquids, or other materials), the initial load temperature, and/or the load mass. For example, information regarding the load type may be received from the user through interaction with the user interface (e.g., by the user selecting from a list of recognized load types). Alternatively, the system may be configured to scan a barcode visible on the exterior of the load, or to receive an electronic signal from an RFID device on or embedded within the load. Information regarding the initial load temperature may be received, for example, from one or more temperature sensors and/or IR sensors (e.g., sensors  390 ,  792 ,  790 ,  1192 ,  FIGS. 3, 7, 11 ) of the system. Information regarding the initial load temperature may be received from the user through interaction with the user interface, or from one or more temperature sensors and/or IR sensors (e.g., sensor  390 ,  790 ,  1192 ,  FIGS. 3, 7, 11 ) of the system. As indicated above, receipt of inputs indicating the load type, initial load temperature, and/or load mass is optional, and the system alternatively may not receive some or all of these inputs. It should be noted that, for embodiments in which load mass is input by the user, the automatic mass determination methods described in connection with block  1416 , below, may be skipped, and the user-input mass may be used for determining one or more desired signal parameters for the RF signal that is supplied by the RF signal source. 
     Upper and lower thresholds may be placed on these user-inputs. For example, if a user accidentally enters a mass that is too high (e.g., above a predefined threshold), a user interface (e.g., a user interface of the control panels  120 ,  214 ,  224 ,  FIGS. 1, 2 ) may provide an indication that the input is invalid. Alternatively, the system may automatically reduce the RF power to a level within the bounds of the upper and lower thresholds and/or may reduce the run time of the defrosting operation. 
     In block  1404 , the system controller provides control signals to the variable matching network (e.g., network  370 ,  400 ,  440 ,  772 ,  800 ,  900 ,  1000 ,  1160 ,  FIGS. 3, 4A, 4B, 7-11 ) to establish an initial configuration or state for the variable matching network. As described in detail in conjunction with  FIGS. 4A, 4B, 5A, 5B, and 8-10 , the control signals affect the values of various inductances and/or capacitances (e.g., inductances  410 ,  411 ,  414 ,  811 ,  816 ,  821 ,  FIGS. 4A, 8 , and capacitances  444 ,  448 ,  1013 ,  1018 ,  1024 ,  FIGS. 4B, 10 ) within the variable matching network. For example, the control signals may affect the states of bypass switches (e.g., switches  511 - 514 ,  551 - 554 ,  931 - 934 ,  936 - 939 ,  941 ,  943 ,  FIGS. 5A, 5B, 9 ), which are responsive to the control signals from the system controller (e.g., control signals  521 - 524 ,  561 - 564 ,  951 - 954 ,  956 - 959 ,  961 ,  963 ,  FIGS. 5A, 5B, 9 ). 
     As also discussed previously, a first portion of the variable matching network may be configured to provide a match for the RF signal source (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ) or the final stage power amplifier (e.g., power amplifier  325 ,  724 ,  FIGS. 3, 7 ), and a second portion of the variable matching network may be configured to provide a match for the cavity (e.g., cavity  360 ,  760 ,  1174 ,  FIGS. 3, 7, 11 ) plus the load (e.g., load  364 ,  764 ,  1164 ,  FIGS. 3, 7, 11 ). For example, referring to  FIG. 4A , a first shunt, variable inductance network  410  may be configured to provide the RF signal source match, and a second shunt, variable inductance network  416  may be configured to provide the cavity plus load match. Referring to  FIG. 4B , a first variable capacitance network  442 , in conjunction with a second variable capacitance network  446 , may be both configured to provide an optimum match between the RF signal source and the cavity plus load. 
     It has been observed that a best initial overall match for a frozen load (i.e., a match at which a maximum amount of RF power is absorbed by the load) typically has a relatively high inductance for the cavity matching portion of the matching network, and a relatively low inductance for the RF signal source matching portion of the matching network. For example,  FIG. 15  is a chart plotting optimal cavity match setting versus RF signal source match setting through a defrost operation for two different loads, where trace  1510  corresponds to a first load (e.g., having a first type, mass, and so on), and trace  1520  corresponds to a second load (e.g., having a second type, mass, and so on). In  FIG. 15 , the optimal initial match settings for the two loads at the beginning of a defrost operation (e.g., when the loads are frozen) are indicated by points  1512  and  1522 , respectively. As can be seen, both points  1512  and  1522  indicate relatively high cavity match settings in comparison to relatively low RF source match settings. Referring to the embodiment of  FIG. 4A , this translates to a relatively high inductance for variable inductance network  416 , and a relatively low inductance for variable inductance network  410 . Referring to the embodiment of  FIG. 8 , this translates to a relatively high inductance for variable inductance networks  811  and  816 , and a relatively low inductance for variable inductance network  821 . 
     According to an embodiment, to establish the initial configuration or state for the variable matching network in block  1404 , the system controller sends control signals to the first and second variable inductance networks (e.g., networks  410 ,  411 ,  FIG. 4A ) to cause the variable inductance network for the RF signal source match (e.g., network  410 ) to have a relatively low inductance, and to cause the variable inductance network for the cavity match (e.g., network  411 ) to have a relatively high inductance. The system controller may determine how low or how high the inductances are set based on load type/ mass /temperature information known to the system controller a priori. If no a priori load type/ mass /temperature information is available to the system controller, the system controller may select a relatively low default inductance for the RF signal source match and a relatively high default inductance for the cavity match. 
     Assuming, however, that the system controller does have a priori information regarding the load characteristics, the system controller may attempt to establish an initial configuration near the optimal initial matching point. For example, and referring again to  FIG. 15 , the optimal initial matching point  1512  for the first type of load has a cavity match (e.g., implemented by network  411 ,  FIG. 4A or 811 / 816 ,  FIG. 8 ) of about 80 percent of the network&#39;s maximum value, and has an RF signal source match (e.g., implemented by network  410  or  821 ,  FIGS. 4A, 8 ) of about 10 percent of the network&#39;s maximum value. Assuming each of the variable inductance networks has a structure similar to network  500  of  FIG. 5 , for example, and assuming that the states from Table 1, above, apply, then for the first type of load, system controller may initialize the variable inductance network so that the cavity match network (e.g., network  411  or  811 / 816 ) has state  12  (i.e., about 80 percent of the maximum possible inductance of network  411  or  811 / 816 ), and the RF signal source match network (e.g., network  410  or  821 ) has state  2  (i.e., about 10 percent of the maximum possible inductance of network  410 ). Conversely, the optimal initial matching point  1522  for the second type of load has a cavity match (e.g., implemented by network  411  or  811 / 816 ) of about 40 percent of the network&#39;s maximum value, and has an RF signal source match (e.g., implemented by network  410  or  821 ) of about 10 percent of the network&#39;s maximum value. Accordingly, for the second type of load, system controller may initialize the variable inductance network so that the cavity match network (e.g., network  411  or  811 / 816 ) has state  6  (i.e., about 40 percent of the maximum possible inductance of network  411  or  811 / 816 ), and the RF signal source match network (e.g., network  410  or  821 ) has state  2  (i.e., about 10 percent of the maximum possible inductance of network  410  or  821 ). Generally, during a defrosting operation, adjustments to the impedance values of the RF signal source match network and the cavity match network are made in an inverse manner. In other words, when the impedance value of the RF signal source match network is decreased, the impedance value of the cavity match network is increased, and vice versa. Although not described in detail herein, a similar adjustment process may be performed to control the matching provided by a variable capacitance network embodiment (e.g., networks  440 ,  FIG. 4B and 1000 ,  FIG. 10 ). 
     Referring again to  FIG. 14A , once the initial variable matching network configuration is established, the system controller may perform a process, at block  1410 , of adjusting, when appropriate, the configuration of the variable impedance matching network to find an acceptable or best match based on actual measurements that are indicative of the quality of the match. Additionally, at block  1410 , the system controller may estimate the mass of the load or may refine a previously determined mass estimate of the load (e.g., based on the rate of change of the S11 parameter or of the VSWR or based on the elapsed time between determining two consecutive matches via reconfiguration of the variable impedance matching network). The S11 parameter and the VSWR may be generally referred to herein as “RF signal ratios”, as the S11 parameter is a ratio of the reflected power and the forward power measured between the RF signal source and the cavity, and the VSWR is a ratio of the maximum voltage magnitude of the RF signal and the minimum voltage magnitude of the RF signal. 
     Based on the mass of the load, the system controller may also determine specific values for a set of parameters of the RF signal to be provided by the RF signal source (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ). This set of RF signal parameters, as determined by the system controller based on the estimated load mass, is referred to below as a set of “desired signal parameters” for the RF signal, and the RF signal produced with the set of desired signal parameters is referred to below as a “mass-estimate-based RF signal.” The desired signal parameters may be updated one or more times during defrosting operations as the estimated load mass is refined. 
       FIG. 14B  shows tasks  1410 - 1  that may be performed at block  1410  of the method shown in  FIG. 14A , according to an embodiment. At block  1411 , the system controller causes the RF signal source to supply a relatively low power RF signal through the variable impedance matching network to the electrode(s) (e.g., first electrode  340  or both electrodes  740 ,  750 ,  1170 ,  1172 ,  FIGS. 3, 7, 11 ). The system controller may control the RF signal power level through control signals to the power supply and bias circuitry (e.g., circuitry  326 ,  726 ,  FIGS. 3, 7 ), where the control signals cause the power supply and bias circuitry to provide supply and bias voltages to the amplifiers (e.g., amplifier stages  324 ,  325 ,  724 ,  FIGS. 3, 7 ) that are consistent with a desired signal power level. For example, the relatively low power RF signal may be a signal having a power level in a range of about 10 W to about 20 W, although different power levels alternatively may be used. Supplying a relatively low power level signal during block  1411  may be desirable to reduce the risk of damaging the cavity and/or load (e.g., if the initial match causes high reflected power), and to reduce the risk of damaging the switching components of the variable inductance or capacitance networks (e.g., due to arcing across the switch contacts). 
     In block  1412 , at an “evaluation time”, power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  1180 ,  FIGS. 3, 7, 11 ) then measures the reflected and (in some embodiments) forward power along the transmission path (e.g., path  328 ,  728 ,  1148 ,  FIGS. 3, 7, 11 ) between the RF signal source and the electrode(s), and provides those measurements to the system controller. The system controller may then determine a ratio between the reflected and forward signal powers, and may determine the S11 parameter (e.g., corresponding to return loss) and/or the VSWR for the system based on the ratio. The system controller may store the received power measurements (e.g., the received reflected power measurements, the received forward power measurement, or both), and/or the calculated ratios, and/or S11 parameters, and/or VSWR for future evaluation or comparison, in an embodiment. 
     At block  1413 , the system controller may determine, based on the reflected power measurements, and/or the reflected-to-forward signal power ratio, and/or the S11 parameter, and/or the VSWR, whether or not the match provided by the variable impedance matching network at the evaluation time is acceptable (e.g., the reflected power is below a threshold, or the reflected-to-forward signal power ratio is 10 percent or less (or below some other threshold), or the measurements or values compare favorably with some other criteria). Alternatively, the system controller may be configured to determine whether the match is the “best” match. A “best” match may be determined, for example, by iteratively measuring the reflected RF power (and in some embodiments the forward reflected RF power) for all possible impedance matching network configurations (or at least for a defined subset of impedance matching network configurations), and determining which configuration results in the lowest reflected RF power and/or the lowest reflected-to-forward power ratio. In some embodiments, a binary search algorithm or a regional search algorithm may instead be used to identify the “best match” configuration that results in the lowest reflected RF power and/or the lowest reflected-to-forward power ratio, which may reduce the amount of time needed to find the best match configuration. 
     When the system controller determines that the match is not acceptable or is not the best match, the system controller may adjust the match, in block  1414 , by reconfiguring the variable impedance matching network. For example, this reconfiguration may be achieved by sending control signals to the variable impedance matching network, which cause the network to increase and/or decrease the variable inductances and/or variable capacitances within the network (e.g., by causing the variable inductance networks  410 ,  411 ,  415 ,  811 ,  816 ,  821  ( FIGS. 4A, 8 ) or variable capacitance networks  422 ,  444 ,  446 ,  448 ,  1011 ,  1013 ,  1016 ,  1018 ,  1021 ,  1024  ( FIGS. 4B, 10 ) to have different inductance or capacitance states, or by switching inductors  501 - 504 ,  911 - 914 ,  916 - 919 ,  921 ,  923 , ( FIGS. 5A, 9 ) or capacitors  541 - 544  ( FIG. 5B ) into or out of the circuit). Then-current inductance values or states of variable inductance networks (e.g., inductance values of inductors  410 ,  411 ,  415 ,  811 ,  816 ,  821 ,  FIGS. 4A, 8 ) or capacitance values or states of variable capacitance networks (e.g., capacitance values of capacitors  442 ,  444 ,  446 ,  448 ,  1011 ,  1013 ,  1016 ,  1018 ,  1021 ,  1024 ,  FIG. 4B, 10 ) in the variable impedance matching network may be stored in a memory of the system controller. For each of the variable inductors and variable capacitors, the inductance value and capacitance value associated with a particular evaluation time may be referred to herein as a “current variable component value,” and a set of current variable component values for the one or more variable components in the variable inductance or capacitance networks at a particular evaluation time may be referred to herein as a “current variable component value set.” After reconfiguring (or adjusting) the variable impedance network, blocks  1411 ,  1412 , and  1413  may be iteratively performed until an acceptable or best match is determined in block  1413 . 
     When the variable impedance network is configured in a state in which an acceptable or best match is achieved (e.g., as indicated by the reflected power, reflected-to-forward signal power ratio, and/or S11 parameter, and/or VSWR being below corresponding thresholds), the current variable component value set includes the then-current values of the one or more variable components in the variable impedance matching network. For the variable inductance matching networks  400  or  800  of  FIG. 4A or 8 , for example, the current variable component set may include the inductance values of variable inductances  410 ,  411 ,  811 ,  816 , and  821  at the evaluation time (referred to herein as “current variable inductance values”), and for the variable capacitance matching networks  440  or  1000  of  FIG. 4B or 10 , for example, the current variable component set may include the capacitance values of variable capacitances  442  or  444 ,  446  or  448 ,  1011  or  1013 ,  1016  or  1018 , and  1021  or  1024  at the evaluation time (referred to herein as “current variable capacitance values”). According to an embodiment, the current variable component value set then may be used to estimate the mass of the load using one or more look-up tables (LUTs), as will be described below. In some embodiments, estimation of the mass may be performed only once during a defrosting operation. Alternatively, the mass estimation process may be performed more than once. 
     Once an acceptable or best match is determined at block  1413 , the system controller may determine, at block  1415 , whether the match determined at block  1413  was an “initial match” corresponding to the first match performed during the present defrosting operation. For example, an “initial flag” may be asserted (or set) in memory at block  1413  when the initial match of the defrosting operation is determined, and the system controller may check the status of the initial flag at block  1415 . If the initial flag is asserted, the system controller proceeds to block  1416  and de-asserts (or clears) the initial flag. If the initial flag is not asserted, the system controller proceeds to block  1419  for embodiments in which a refined mass estimate is determined only once, or proceeds directly to block  1430  via path  1435  for embodiments in which refined mass estimates are determined periodically throughout the defrosting operation. 
     If the system controller determines that the match that has just been determined at block  1415  is an initial match, at block  1416  the current variable component values of some or all of the variable components in the variable inductance networks and/or variable capacitance networks of the variable impedance matching network may be compared to entries within one or more LUTs, which may be stored in the memory of the system controller and/or memory otherwise accessible to the system controller, in order to estimate the mass of the load in the cavity. In some embodiments, the S11 parameter value of the initial match (sometimes referred to herein as the “initial S11 parameter value) may instead be compared to entries within the one or more LUTs as a basis for estimating the mass of the load. For example, an LUT may include a plurality of entries, where each entry includes a field for each variable component, a field for the S11 parameter value for the initial match, a field for an associated load mass, and/or a field for an associated load temperature. It should be noted that knowing the initial temperature of the load (e.g., via inputs received through the user interface or a temperature sensor in the cavity) may allow the system controller to more accurately estimate the mass of the load, but when no initial temperature of the load is provided or detectable, the system may automatically assume the load to be at a default initial temperature (e.g., −20° C. or some other temperature). 
     The configuration of the LUT (e.g., the fields in each LUT entry) depends at least in part on the configuration of the variable impedance matching network utilized in the system, and how many variable components are included in the variable impedance matching network. For example,  FIG. 16A  shows an illustrative example of a portion of an LUT  1600  associated with the variable inductance network  800  of  FIG. 8 , which includes three variable inductances  821 ,  811 ,  816 , referred to below as “L 1 ”, “L 2 ”, and “L 3 ,” respectively. In some embodiments, the inductance  811  may have the same value as inductance  816 , regardless of the value of the inductance  821 . LUT  1600  includes a plurality of columns  1602 ,  1604 ,  1606 ,  1608 ,  1610 ,  1612 ,  1614 ,  1616 ,  1618 , and a plurality of rows or entries  1622 ,  1624 ,  1626 ,  1628 ,  1630 ,  1632 , where only a subset of the rows/entries are illustrated in  FIG. 16A . Inductance values L 1 , L 2 , and L 3  stored in the LUT  1600  may be referred to as “stored inductance values,” and for a given row of LUT  1600 , the inductance values L 1 , L 2 , and L 3  in that row may be referred to as a “subset” of the stored inductance values. The intersection of each column and row is referred to herein as a “cell” of the LUT  1600 . 
     In the present example, the cells in the column  1602 , which is optional, includes various characterizations of the contents of the cavity of the system (“empty” or “ground beef” in the present example). The cells in the column  1604  include stored inductance values L 1  for a first variable inductance network (e.g., variable inductance  821 ,  FIG. 8 ). The cells in the column  1606  include stored inductance values L 2  for a second variable inductance network (e.g., variable inductance  811 ,  FIG. 8 ). The cells in the column  1608  include stored inductance values L 3  for a third variable inductance network (e.g., variable inductance  816 ,  FIG. 8 ). For embodiments in which the value of inductance  811  is the same as that of inductance  816 , columns  1606  and  1608  could be combined into a single column to simplify the table, as the inductance values in the column  1606  would be the same as those of column  1608 . It should be noted that the inductance values L 1 , shown in the column  1604 , are normalized to 50 nH while the inductance values L 2  and L 3 , respectively shown in the columns  1606  and  1608 , are normalized to 100 nH. In other embodiments, the stored inductance values may not be normalized, or the stored inductance values may be normalized to other values. 
     The cells of column  1610  of LUT  1600  include stored S11 parameter values for the system, shown in decibels (dB), representing the input return loss for the system upon the determination of an initial match, which may be affected by the quality of the impedance match between the RF signal source and the cavity, which is affected by the mass of the load. In some embodiments, the stored S11 parameters may be used instead of or in combination with the values of L 1 , L 2 , and L 3  in estimating the mass of the load in the cavity of the system. 
     The cells in the column  1612  may include the mass of the contents of the cavity in grams (g). As shown, the stored S11 parameter value decreases as the mass of the load increases (for the same initial temperature and load type) indicating that a better impedance match may be achieved for larger loads. The cells in the column  1614  may include the initial temperature of the contents of the cavity (e.g., of the load) in degrees Celsius (° C.). 
     The cells in the column  1616  may include different levels of RF power to be applied to a load based on the mass of the load and on the amount of time RF power is to be applied to the load. As shown, the amount of RF power applied to the load increases as the mass of the load increases, up to an illustrative maximum threshold of 300 W. It should be understood that the maximum threshold for RF power may vary depending on the operating parameters of the defrosting system. 
     The cells in the column  1618  may include different amounts of time for which RF power may be applied to a load based on the mass of the load and the amount of RF power to be applied to the load. As shown, even when the applied RF power has reached its maximum threshold in rows  1628 ,  1630 , and  1632 , loads having larger mass may be defrosted by increasing the amount of time for which the RF power is applied to the load. 
     The cells in the row  1622  correspond to an empty cavity. The cells in the row  1624  correspond to a cavity containing 200 g of ground beef at −20° C. The cells in the row  1626  correspond to a cavity containing 500 g of ground beef at −20° C. The cells in the row  1628  correspond to a cavity containing 1000 g of ground beef at −20° C. The cells in the row  1630  correspond to a cavity containing 1500 g of ground beef at −20° C. The cells in the row  1632  correspond to a cavity containing 2000 g of ground beef at −20° C. 
     LUT  1600  is stored in memory accessible to the system controller in accordance with an example embodiment. The system controller may compare or correlate current inductance values of variable inductance networks (e.g., the current inductance values corresponding to those stored in the memory of the system controller at block  1414  of  FIG. 14B ) in the variable impedance matching network to corresponding inductance values in the columns  1604 ,  1606 , and  1608  of the LUT in order to estimate mass of the load. 
     As another example,  FIG. 16B  shows an illustrative example of a portion of a LUT  1700  associated with the variable capacitance network  1000  of  FIG. 10 , which includes three variable capacitances  1011 ,  1016 ,  1023 , referred to below as “C 1 ”, “C 2 ”, and “C 3 ,” respectively. In some embodiments, the capacitance  1011  may have the same value as the capacitance  1016 , regardless of the value of the capacitance  1023 . LUT  1700  includes a plurality of columns  1702 ,  1704 ,  1706 ,  1708 ,  1710 ,  1712 ,  1714 ,  1716 ,  1718  and a plurality of rows or entries  1722 ,  1724 ,  1726 ,  1728 ,  1730 ,  1732 , where only a subset of the rows/entries are illustrated in  FIG. 16B . Capacitance values C 1 , C 2 , and C 3  stored in the LUT  1700  may be referred to as “stored inductance values,” and for a given row of the LUT  1700 , the capacitance values C 1 , C 2 , and C 3  in that row may be referred to as a “subset” of the stored capacitance values. The intersection of each column and row is referred to herein as a “cell” of the LUT  1700 . 
     In the present example, the cells in the column  1702 , which is optional, includes various characterizations of the contents of the cavity of the system (“empty” or “ground beef” in the present example). The cells in the column  1704  include stored capacitance values C 1  for a first variable inductance network (e.g., variable capacitance  1011 ,  FIG. 10 ). The cells in the column  1706  include stored capacitance values C 2  for a second variable capacitance network (e.g., variable capacitance  1016 ,  FIG. 10 ). For embodiments in which the value of the capacitance  1011  is the same as that of the capacitance  1016 , the columns  1704  and  1706  could be combined into a single column to simplify the table, as the capacitance values in the column  1704  would be the same as those of the column  1706 . The cells in the column  1708  include stored inductance values C 3  for a third variable inductance network (e.g., variable inductance  1023 ,  FIG. 8 ). It should be noted that the capacitance values C 1  and C 2 , respectively shown in the columns  1704  and  1706 , are normalized to  200  nF while the capacitance values C 3 , shown in the column  1708 , are normalized to  500  nF. In other embodiments, the stored capacitance values may not be normalized, or the stored capacitance values may be normalized to other values. 
     The cells of the column  1710  of the LUT  1700  include stored S11 parameter values for the system, shown in decibels (dB), representing the input return loss for the system upon the determination of an initial match, which may be affected by the quality of the impedance match between the RF signal source and the cavity which is affected by the mass of the load. In some embodiments, the stored S11 parameters may be used instead of or in combination with the values of C 1 , C 2 , and C 3  in estimating the mass of the load in the cavity of the system. 
     The cells in the column  1712  may include the mass of the contents of the cavity in grams (g). As shown, the stored S11 parameter value decreases as the mass of the load increases (for the same initial temperature and load type) indicating that a better impedance match may be achieved for larger loads. The cells in the column  1714  may include the initial temperature of the contents of the cavity (e.g., of the load) in degrees Celsius (° C.). 
     The cells in the column  1716  may include different levels of RF power to be applied to a load based on the mass of the load and on the amount of time RF power is to be applied to the load. As shown, the amount of RF power applied to the load increases as the mass of the load increases, up to an illustrative maximum threshold of 300 W. It should be understood that the maximum threshold for RF power may vary depending on the operating parameters of the defrosting system. 
     The cells in the column  1718  may include different amounts of time for which RF power may be applied to a load based on the mass of the load and the amount of RF power to be applied to the load. As shown, even when the applied RF power has reached its maximum threshold in the rows  1728 ,  1730 , and  1732 , loads having larger mass may be defrosted by increasing the amount of time for which the RF power is applied to the load. 
     The cells in the row  1722  correspond to an empty cavity. The cells in the row  1724  correspond to a cavity containing 200 g of ground beef at −20° C. The cells in the row  1726  correspond to a cavity containing 500 g of ground beef at −20° C. The cells in the row  1728  correspond to a cavity containing 1000 g of ground beef at −20° C. The cells in the row  1730  correspond to a cavity containing 1500 g of ground beef at −20° C. The cells in the row  1732  correspond to a cavity containing 2000 g of ground beef at −20° C. 
     The LUT  1700  is stored in memory accessible to the system controller in accordance with an example embodiment. The system controller may compare or correlate current capacitance values of variable capacitance networks (e.g., the current capacitance values corresponding to those stored in the memory of the system controller at block  1414  of  FIG. 14B ) in the variable impedance matching network to corresponding capacitance values in the columns  1704 ,  1706 , and  1708  of LUT  1700  in order to estimate mass of the load. 
     It should be understood that the LUTs associated with variable inductance networks and variable capacitance networks described in connection with  FIGS. 16A and 16B  are meant to be illustrative and not limiting. Other variable impedance networks (e.g., including variable impedance networks for unbalanced (e.g., single-ended) systems such as the networks  400 ,  440 ,  500 ,  540 ,  FIGS. 4A, 4B, 5A, and 5B , differently-configured variable inductance networks, differently-configured variable capacitance networks, and networks that include both variable inductors and variable capacitors) could alternatively be used in the system, and variable component values of these networks may populate the entries of one or more differently-configured LUTs stored in the memory of the system controller. In addition, it should be noted that a “variable network” may include fixed components, as well as variable components, and may also may include variable or fixed resistors. It should further be noted that, a “variable capacitor” or “variable inductor” may include switching elements (e.g., transistors or mechanical relays, as reflected in  FIGS. 5A, 5B, and 9 ) that cause the capacitance or inductance between input and output nodes to be variable. Additional switching elements may be included that may switch some or all of the passive components into or out of the variable impedance network(s). Alternatively, such a variable component may itself be physically modifiable to provide a variable value (e.g., by tapping into different locations on an inductor coil or moving plates of a capacitor closer or further apart). 
     Given knowledge of the set of current variable component values that correspond to the acceptable/best match (e.g., determined in block  1413 ), the system controller may compare or correlate each of the one or more variable component values within the current component value set to the corresponding stored component value(s) within each entry (e.g., row) listed in the LUT(s) stored in the memory of the system controller. For example, referring again to the example LUT  1600  in  FIG. 16A , the current variable component value for inductance value L 1  (e.g., inductance  821 ,  FIG. 8 ) may be compared with the corresponding stored values for L 1  in the column  1604  to determine first differences between the current variable component value for inductor L 1  and each stored L 1  value. Similarly, the current variable component value for inductor L 2  may be compared with the corresponding stored values for L 2  in the column  1606  to determine second differences between the current variable component value for inductor L 2  and each stored L 2  value, and so on. In the context of the comparison process, the current variable component values may be normalized (assuming that the corresponding stored values in LUT  1600  also are normalized). 
     Based on this comparison process, the controller may determine which entry of the LUT corresponds to the best match (e.g., an identical match or a closest match) having stored variable component values that most closely correlate with the current variable component values. The row or entry corresponding to the “best match” is referred to herein as a “correlated entry.” An example implementation of determining the best match involves iteratively adjusting the impedance values of the variable impedance matching network and measuring the S11 parameter value as low RF power is applied at each iteration to identify the lowest S11 parameter value achievable. The variable impedance matching network configuration corresponding the lowest S11 parameter value achievable would then be identified by the defrosting system (e.g., by the system controller) as providing the best match. 
     Alternate methods of identifying the best match may instead be applied, which, rather than testing all possible configurations of the variable impedance matching network, only test configurations within a predetermined range of the current configuration. Some methods may predict which variable impedance matching network configurations to test based on historical configuration data stored in the memory of the defrosting system (e.g., collected during previously performed defrosting/heating operations). In some embodiments, the best match may be identified as any variable impedance matching network configuration determined to allow more than a predetermined threshold percentage (e.g., 95%-99%) of the applied RF energy is absorbed by the load. 
     In some embodiments, the accuracy of the determination of the correlated entry may be enhanced by comparing an initial temperature of the load to stored temperature values listed in the LUT (e.g., in the column  1614  of LUT  1600 ). In such embodiments, the controller may determine which entry of the LUT is the correlated entry based on comparisons between not only the current and stored variable component values, but also between the initial temperature of the load and the stored temperature values. Otherwise, the initial temperature of the load may be assumed by the system controller to be a default temperature (e.g., −20° C. or some other temperature). For example, the controller may determine that the entry of the LUT having stored variable component values and/or S11 parameter value that most closely correlate with the current variable component values and/or initial S11 parameter value, and having a stored temperature value that most closely correlates with the initial temperature value is the correlated entry. The system controller may then estimate the mass of the load as the mass included in the correlated entry of the LUT. 
     In some embodiments, multiple entries in the LUT may have identical stored variable component values, but different stored temperature values and/or stored load type specifiers. Accordingly, multiple correlated entries may be determined in the above-described process, where the multiple correlated entries have identical stored component values but different temperature and/or load types. In such an embodiment, given a user-provided or sensed temperature of the load and/or a user-provided or sensed load type (e.g., ground beef at −20 degrees Celsius, in a present example), one of the multiple correlated entries may be selected as a final correlated entry (e.g., the correlated entry with a stored mass or stored temperature value that most closely matches the user-provided or sensed load type or temperature). After determining the correlated entry, the mass of the load may be estimated (e.g., by the system controller) as the mass value listed in the column  1612  of the correlated entry. Again, in an embodiment, the correlated entry is an entry for which the corresponding subset of inductance values L 1 , L 2 , and L 3  stored in the columns  1604 ,  1606 , and  1608  most closely match or correlate with the current variable component value set (e.g., more specifically the current inductance values of variable inductance network(s) of the variable impedance matching circuit). 
     To summarize, given knowledge of the set of component values that correspond to the acceptable/best match (e.g., the current component values determined in block  1413 ), the system controller may compare the set of current component values to the component values listed in a LUT stored in the memory of the system controller, and then may determine which entry/row of the LUT corresponds to the best match (e.g., an identical match or a closest match). 
     As indicated previously, there may be instances for which the current inductance values (e.g., current L 1 , L 2 , and L 3  values of the variable inductance network) do not exactly match any subset of inductance values in an entry of LUT  1600 . In such instances, the system controller may identify two (or multiple) correlated entries, and may interpolate between (e.g., using linear interpolation), mathematically average, or otherwise mathematically manipulate the two (or multiple) corresponding mass values in the two (or multiple) correlated entries to determine an initial estimated mass value. 
     For example, referring to the example stored values shown in the LUT  1600 , when the current inductance values L 1 , L 2 , and L 3  are 1.3, 2.55, and 2.55, respectively, the system controller may identify entries  1626  and  1628  as potential correlated entries, and may interpolate the two mass values of 500 and 1000 (in the column  1612 ), since entries  1626  and  1628  correspond to the two most closely matching subsets of stored inductance values L 1 , L 2 , and L 3  in the LUT  1600 . Assuming the interpolation corresponds to an average between the two values, this may result in an initial mass estimate of the load of 750 grams. 
     While the present example values in the LUT  1600  includes data corresponding to ground beef at −20° C., this intended to be illustrative and not limiting. It should be understood that other LUTs including data corresponding to loads of varying mass, temperature, and type may be stored in the memory of the system controller. A given LUT may, for example, be characterized in advance, with loads of various /masses, temperatures, and types being tested and corresponding variable component values (e.g., inductance values L 1 , L 2 , and L 3 ) and initial S11 parameters being collected and stored in the LUT. It should be understood that while S11 parameters are described in connection with  FIGS. 16A and 16B  as being a basis for determining an estimated mass of a load, other RF signal parameters, such as VSWR or reflected power of the RF signal may instead or additionally be included in the LUT  1600  or the LUT  1700  and used as a basis for determining the estimated mass of the load. 
     Returning to  FIG. 14B , once the system controller has determined an initial mass estimate for the load in the cavity, the system controller may estimate an amount of energy (sometimes referred to herein as an initial energy estimate) required to warm the load to the desired temperature in the cavity, at block  1417 , based on the initial mass estimate (e.g., using Equation 1 (or other applicable equation) or a LUT derived from Equation 1 (or other applicable equation) and stored in the memory of the system) in combination with the known (e.g., provided as an input at block  1402  or measured via a temperature sensor in the cavity) or assumed temperature of the load (e.g., a default starting temperature stored in the memory of the system controller; such as about −20° C. or some other temperature). 
     The RF signal provided by the RF signal source may be characterized by multiple signal parameters. For example, RF signal parameters may include, but are not limited to, a frequency, an amplitude, and a power level, and each of these parameters have a particular value at any given time. At block  1418 , the system controller may determine one or more “desired signal parameters” for the RF signal produced by the RF signal source based on the initial energy estimate (e.g., according to a LUT stored in the memory of the system). For example, the desired signal parameter(s) may include, but are not limited to, a desired frequency, a desired amplitude, and a desired power level (e.g., a desired RF power level) of the RF signal. Since the desired signal parameters may be determined based on initial energy estimate, and the initial energy estimate is determined based on the initial mass estimate for the load, a “initial-mass-estimate-based RF signal,” as used herein, refers to an RF signal that is characterized by the one or more desired RF signal parameters following the initial match and prior to a subsequent match. The system controller may further determine the amount of time needed to apply the initial-mass-estimate-based RF signal in order to deliver the initially estimated amount of energy to the load. 
     Block  1419  is performed in embodiments where a refined mass estimate is determined only once subsequent to the determination of the initial mass estimate by the system controller. For other embodiments (e.g., in which multiple refined mass estimates are determined periodically, and/or at multiple times throughout the defrosting operation), block  1419  may be bypassed, and the system controller may proceed directly to block  1430  through via path  1435 . 
     At block  1419 , the system controller may determine whether the match determined at block  1413  was a “subsequent match” corresponding to a non-initial match performed during the present defrosting operation. For example, a “second flag” may be asserted (e.g., set) in memory at block  1413  when the subsequent match of the defrosting operation is determined, and the system controller may check the status of the second flag at block  1419 . If the second flag is asserted, the system controller proceeds to block  1430  and de-asserts (e.g., clears) the second flag. Otherwise, if the second flag is not asserted, the system controller may identify that a refined mass estimate has already been determined in a previous iteration of block  1410 , bypass blocks  1430 ,  1432 , and  1434 , and proceed to block  1420 . 
     Generally, there is a negative correlation between the rate at which the impedance of a cavity containing a load changes during defrosting operations (e.g., as the temperature of the load increases) and the mass of the load. For example, the impedance of a load having a smaller mass may change more quickly as RF energy is applied to the load (e.g., to heat the load) compared to the impedance change rate of a load having a larger mass to which the same amount of RF energy is applied. The change in impedance of the load is reflected in the change in the quality of the impedance match between the cavity and the RF signal source, which correlates with the rate of change of the S11 parameter or the VSWR, for example. Thus, by monitoring the rate of change of the S11 parameter or the rate of change of the VSWR during defrosting operations, the mass of a load can be determined. As the rate of change of the S11 parameter and/or the VSWR are unknown at the outset of the defrosting process, an initial mass estimate for the load may be made using an alternative method (e.g., at block  1416 ), and may then be refined based on the rate of change of the S11 parameter or the rate of change of the VSWR. 
     At block  1430 , a refined mass estimate is determined based on the initial mass estimate and the rate of change of a system parameter, such as the Si i parameter or the VSWR (e.g., monitored at block  1420 ) between the RF signal source and the electrode(s) at the cavity. For example, the refined mass estimate may be determined by comparing the rate of change of the S11 parameter or the rate of change of the VSWR and the supplied RF power level to entries of a LUT stored in memory accessible to the system controller. The LUT may include a set of stored load mass values, a set of stored RF power levels, and a set of stored S11 and/or VSWR rates of change (sometimes referred to as stored parameter rates of change), all organized into multiple correlated entries. Each entry of the correlated entries may include a stored load mass value, a stored RF power level (e.g., the amount of RF energy applied to the load), and a stored S11 and/or VSWR rate of change (e.g., observed when the stored RF power level is applied to a load corresponding to the stored load mass value), in accordance with an embodiment. The system controller may search the LUT to identify an entry corresponding (or most closely corresponding) to the S11 parameter or VSWR rate of change measured for the defrosting system and the RF power level of the RF signal being supplied to the electrodes at the cavity containing the load. If the identified entry includes a stored load mass value that does not match the initial mass estimate (or the most recently made mass estimate, according to some embodiments), the system controller may determine a refined mass estimate that is equal to the load mass listed in the identified entry. 
     Once the system controller has determined a refined mass estimate for the load in the cavity, the system controller may determine a refined energy estimate by estimating an amount of energy required to warm the load to the desired temperature in the cavity, at block  1432 , based on the refined mass estimate (e.g., using Equation 1 (or another suitable equation) or a LUT derived from Equation 1 (or another suitable equation) and stored in the memory of the system) in combination with the known (e.g., provided as an input at block  1402  or measured via a temperature sensor in the cavity) or assumed temperature of the load (e.g., a default starting temperature stored in the memory of the system controller; such as about −20° C. or some other temperature). 
     At block  1434 , the system controller may update or “refine” the desired signal parameters for the RF signal produced by the RF signal source based on the refined energy estimate. These updated desired signal parameters may sometimes be referred to herein as “refined signal parameters” or “refined desired signal parameters”. Since the desired signal parameters may be determined based on a refined energy estimate, and the refined energy estimate is determined based on the refined mass estimate for the load, a “refined-mass-estimate-based RF signal,” as used herein, refers to an RF signal that is characterized by the one or more desired RF signal parameters following the second (or subsequent) match. The system controller may further determine the amount of time needed to apply the refined-mass-estimate-based RF signal in order to deliver the refined estimated amount of energy to the load. 
       FIG. 14C  shows tasks  1410 - 2  that may be performed at block  1410  of the method shown in  FIG. 14A , according to an alternate embodiment. At block  1811 , the system controller causes the RF signal source to supply a relatively low power RF signal through the variable impedance matching network to the electrode(s) (e.g., first electrode  340  or both electrodes  740 ,  750 ,  1170 ,  1172 ,  FIGS. 3, 7, 11 ). The system controller may control the RF signal power level through control signals to the power supply and bias circuitry (e.g., circuitry  326 ,  726 ,  FIGS. 3, 7 ), where the control signals cause the power supply and bias circuitry to provide supply and bias voltages to the amplifiers (e.g., amplifier stages  324 ,  325 ,  724 ,  FIGS. 3, 7 ) that are consistent with a desired signal power level. 
     In block  1812 , at an “evaluation time”, power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  1180 ,  FIGS. 3, 7, 11 ) then measures the reflected and (in some embodiments) forward power along the transmission path (e.g., path  328 ,  728 ,  1148 ,  FIGS. 3, 7, 11 ) between the RF signal source and the electrode(s), and provides those measurements to the system controller. The system controller may then determine a ratio between the reflected and forward signal powers, and may determine the S11 parameter (e.g., corresponding to return loss) and/or the VSWR for the system based on the ratio. The system controller may store the received power measurements (e.g., the received reflected power measurements, the received forward power measurement, or both), and/or the calculated ratios, and/or S11 parameters, and/or the VSWR for future evaluation or comparison, in an embodiment. 
     At block  1813 , the system controller may determine, based on the reflected power measurements, and/or the reflected-to-forward signal power ratio, and/or the S11 parameter, and/or the VSWR whether or not the match provided by the variable impedance matching network at the evaluation time is acceptable (e.g., the reflected power is below a threshold, or the reflected-to-forward signal power ratio is 10 percent or less (or below some other threshold), or the measurements or values compare favorably with some other criteria). Alternatively, the system controller may be configured to determine whether the match is the best match, as defined previously. In some embodiments, a binary search algorithm or a regional search algorithm may instead be used to identify the “best match” configuration that results in the lowest reflected RF power and/or the lowest reflected-to-forward power ratio, which may reduce the amount of time needed to find the best match configuration. 
     When the system controller determines that the match is not acceptable or is not the best match, the system controller may adjust the match, in block  1814 , by reconfiguring the variable impedance matching network. For example, this reconfiguration may be achieved by sending control signals to the variable impedance matching network, which cause the network to increase and/or decrease the variable inductances and/or variable capacitances within the network (e.g., by causing the variable inductance networks  410 ,  411 ,  415 ,  811 ,  816 ,  821  ( FIGS. 4A, 8 ) or variable capacitance networks  422 ,  444 ,  446 ,  448 ,  1011 ,  1013 ,  1016 ,  1018 ,  1021 ,  1024  ( FIGS. 4B, 10 ) to have different inductance or capacitance states, or by switching inductors  501 - 504 ,  911 - 914 ,  916 - 919 ,  921 ,  923 , ( FIGS. 5A, 9 ) or capacitors  541 - 544  ( FIG. 5B ) into or out of the circuit). Then-current inductance values or states of variable inductance networks (e.g., inductance values of inductors  410 ,  411 ,  415 ,  811 ,  816 ,  821 ,  FIGS. 4A, 8 ) or capacitance values or states of variable capacitance networks (e.g., capacitance values of capacitors  442 ,  444 ,  446 ,  448 ,  1011 ,  1013 ,  1016 ,  1018 ,  1021 ,  1024 ,  FIG. 4B, 10 ) in the variable impedance matching network may be stored in a memory of the system controller. After reconfiguring (or adjusting) the variable impedance network, blocks  1811 ,  1812 , and  1813  may be iteratively performed until an acceptable or best match is determined in block  1813 . 
     When the variable impedance network is configured in a state in which an acceptable or best match is achieved (e.g., as indicated by the reflected power, reflected-to-forward signal power ratio, VSWR, and/or S11 parameter being below corresponding thresholds), the current variable component value set includes the then-current values of the one or more variable components in the variable impedance matching network. For the variable inductance matching networks  400  or  800  of  FIG. 4A or 8 , for example, the current variable component set may include the inductance values of variable inductances  410 ,  411 ,  811 ,  816 , and  821  at the evaluation time, and for the variable capacitance matching networks  440  or  1000  of  FIG. 4B or 10 , for example, the current variable component set may include the capacitance values of variable capacitances  442  or  444 ,  446  or  448 ,  1011  or  1013 ,  1016  or  1018 , and  1021  or  1024  at the evaluation time. According to an embodiment, the current variable component value set then may be used to estimate the mass of the load using one or more LUTs. In some embodiments, estimation of the mass may be performed only once during a defrosting operation. Alternatively, the mass estimation process may be performed more than once.  
     Once an acceptable or best match is determined at block  1813 , the system controller may determine, at block  1815 , whether the match determined at block  1813  was an “initial match” corresponding to the first match performed during the present defrosting operation. For example, an “initial flag” may be asserted (e.g., set) in memory at block  1813  when the initial match of the defrosting operation is determined (e.g., at a first evaluation time), and the system controller may check the status of the initial flag at block  1815 . If the initial flag is asserted, the system controller proceeds to block  1816  and de-asserts (e.g., clears) the initial flag. If the initial flag is not asserted, the system controller proceeds to block  1819 . 
     If the system controller determines that the match that has just been determined at block  1815  is an initial match, at block  1816  the current variable component values of some or all of the variable components in the variable inductance networks and/or variable capacitance networks of the variable impedance matching network may be compared to entries within one or more LUTs, which may be stored in the memory of the system controller and/or memory otherwise accessible to the system controller, in order to estimate the mass or mass of the load in the cavity. In some embodiments, the S11 parameter value of the initial match may instead be compared to entries within the one or more LUTs as a basis for estimating the mass of the load. For example, a LUT may include a plurality of entries, where each entry includes a field for each variable component, a field for the S11 parameter value for the initial match, a field for an associated load mass, and/or a field for an associated load temperature. It should be noted that knowing the initial temperature of the load (e.g., via inputs received through the user interface or a temperature sensor in the cavity) may allow the system controller to more accurately estimate the mass of the load, but when no initial temperature of the load is provided or detectable, the system may automatically assume the load to be at a default initial temperature (e.g., −20° C.). Examples of LUTs that may be accessed by the system controller in block  1816  are shown in  FIGS. 16A and 16B , described above. 
     Once the system controller has determined an initial mass estimate for the load in the cavity, the system controller may estimate an amount of energy (sometimes referred to herein as an initial energy estimate) required to warm the load to the desired temperature in the cavity, at block  1817 , based on the initial mass estimate (e.g., using Equation 1 (or another suitable equation) or a LUT derived from Equation 1 (or another suitable equation) and stored in the memory of the system) in combination with the known (e.g., provided as an input at block  1802  or measured via a temperature sensor in the cavity) or assumed temperature of the load (e.g., a default starting temperature stored in the memory of the system controller; such as about −20° C. or another temperature). 
     The RF signal provided by the RF signal source may be characterized by multiple signal parameters. For example, RF signal parameters may include, but are not limited to, a frequency, an amplitude, and a power level, and each of these parameters have a particular value at any given time. At block  1818 , the system controller may determine one or more “desired signal parameters” for the RF signal produced by the RF signal source based on the initial energy estimate (e.g., according to a LUT stored in the memory of the system). For example, the desired signal parameter(s) may include, but are not limited to, a desired frequency, a desired amplitude, and a desired power level of the RF signal. The system controller may further determine the amount of time needed to apply the initial-mass-estimate-based RF signal in order to deliver the initial estimated amount of energy to the load. 
     Block  1819  is performed in embodiments where a refined mass estimate is determined only once subsequent to the determination of the initial mass estimate by the system controller. At block  1819 , the system controller may determine whether the match determined at block  1813  was a “subsequent match” corresponding to a non-initial match performed during the present defrosting operation. For example, a “second flag” may be asserted (e.g., set) in memory at block  1813  when the subsequent match of the defrosting operation is determined (e.g., at a second evaluation time), and the system controller may check the status of the second flag at block  1819 . If the second flag is asserted, the system controller proceeds to block  1830  and de-asserts (e.g., clears) the second flag. Otherwise, if the second flag is not asserted, the system controller may identify that a refined mass estimate has already been determined in a previous iteration of block  1410 - 2 , bypass blocks  1830 ,  1832 , and  1834 , and proceed to block  1820 . 
     At block  1830 , a refined mass estimate is determined based on the elapsed time between the determination of the initial match and the determination of the subsequent match. For example, the system controller may determine the elapsed time to be the difference between the first evaluation time and the second evaluation time. The refined mass estimate may be determined by comparing the elapsed time between the initial match and the subsequent match and the supplied RF power level to entries of a LUT stored in memory accessible to the system controller. The LUT may include a set of stored load mass values, a set of stored RF power levels, and a set of elapsed time values, all organized into multiple correlated entry entries. Each entry of the correlated entries may include a stored load mass value, an RF power level (e.g., the amount of RF energy applied to the load), and a stored elapsed time (e.g., between the initial match and the subsequent match), in accordance with an embodiment. The system controller may search the LUT to identify an entry corresponding (or most closely corresponding) to the time elapsed between matches and the RF power level of the RF signal being supplied to the electrodes at the cavity containing the load. If the identified entry includes a stored load mass value that does not match the initial mass estimate (or the most recently made mass estimate, according to some embodiments), the system controller may determine a refined mass estimate that is equal to the load mass listed in the identified entry. 
     Once the system controller has determined a refined mass estimate for the load in the cavity, the system controller may determine a refined energy estimate by estimating an amount of energy required to warm the load to the desired temperature in the cavity, at block  1832 , based on the refined mass estimate (e.g., using Equation 1 (or another suitable equation) or a LUT derived from Equation 1 (or another suitable equation) and stored in the memory of the system) in combination with the known (e.g., provided as an input at block  1802  or measured via a temperature sensor in the cavity) or assumed temperature of the load (e.g., a default starting temperature stored in the memory of the system controller; such as about −20° C. or another temperature). 
     At block  1834 , the system controller may update or “refine” the desired signal parameters for the RF signal produced by the RF signal source based on the refined energy estimate. Since the desired signal parameters may be determined based on refined energy estimate, and the refined energy estimate is determined based on the refined mass estimate for the load, a “refined-mass-estimate-based RF signal,” as used herein, refers to an RF signal that is characterized by the one or more desired RF signal parameters following the subsequent match. The system controller may further determine the amount of time needed to apply the refined-mass-estimate-based RF signal in order to deliver the refined estimated amount of energy to the load. 
       FIG. 14D  shows a flowchart corresponding to a method that may be performed by a system controller (e.g., system controller  312 ,  712 ,  1130 ,  FIGS. 3, 7, 11 ) of a defrosting system (e.g., system  100 ,  210 ,  220 ,  300 ,  700 ,  1100 ,  FIGS. 1-3, 7, 11 ) with dynamic load matching to generate a refined mass estimate for a load being defrosted. 
     At block  1902 , the system controller determines an initial mass estimate for a load to be defrosted by the defrosting system, an initial energy estimate of the amount of RF energy needed to warm the load to the desired temperature, and desired signal parameters for an RF signal applied to heat the load based on initial match conditions. The system controller may determine the initial mass estimate by comparing the initial match conditions to entries of a LUT stored in a memory accessible by the system controller. For example, the initial match conditions may include S11 parameter values and/or variable component values of a variable impedance matching network (e.g., network  772 ,  800 ,  900 ,  1000 ,  FIGS. 7-10 ) upon the determination of a “best match” for the system (e.g., as described in connection with blocks  1416 ,  1816 ,  FIGS. 14B, 14C ) between an RF signal source (e.g., RF signal source (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ) and a defrosting cavity (e.g., cavity  360 ,  760 ,  1174 ,  FIGS. 3, 7, 11 ). The system controller may calculate or otherwise determine an initial energy estimate corresponding to the amount of RF energy needed to be applied to the load in order to bring the load to a predetermined temperature (e.g., −1° C. or another temperature) based on the initial mass estimate (e.g., using Equation 1 (or another suitable equation) or a LUT derived from Equation 1 (or another suitable equation) and stored in the memory of the system) in combination with the known (e.g., provided as an input at block  1402  or measured via a temperature sensor in the cavity) or assumed initial temperature of the load (e.g., a default starting temperature stored in the memory of the system controller; such as about −20° C. or another temperature). The system controller may then determine one or more desired signal parameters for the RF signal produced by the RF signal source based on the initial energy estimate (e.g., according to a LUT stored in the memory of the system). For example, the desired signal parameter(s) may include, but are not limited to, a desired frequency, a desired amplitude, and a desired power level of the RF signal. 
     At block  1904 , the system controller may periodically measure the S11 parameter or the VSWR derived from measurements of the forward and reflected power between the RF signal source and the defrosting cavity. The system controller may calculate and store the rate of change of the S11 parameter or of the VSWR based on these measurements. 
     At block  1906 , if the system controller identifies that a new match has been determined via the reconfiguration of the variable impedance matching network, it proceeds to block  1908  to refine the mass estimate of the load and the desired signal parameters for the RF signal. Otherwise, the system controller returns to block  1904  to continue measuring the S11 parameter and/or the VSWR and calculating rates of change thereof. 
     At block  1908 , the system controller may determine a refined mass estimate, a refined energy estimate, and refined, desired signal parameters based on the rate of change of the S11 parameter or the VSWR periodically determined at block  1904 . For example, the system controller may compare the rate of change of the S11 parameter or the VSWR and an RF power level supplied to heat the load during defrosting operations to entries of a LUT stored in a memory accessible by the system controller to determine the refined mass estimate. The system controller may calculate or otherwise determine a refined energy estimate based on the refined mass estimate (e.g., using Equation 1 (or another suitable equation) or a LUT derived from Equation 1 (or another suitable equation) and stored in the memory of the system) in combination with the known or assumed initial temperature of the load. The system controller may then determine one or more refined, desired signal parameters for the RF signal produced by the RF signal source based on the refined energy estimate (e.g., according to a LUT stored in the memory of the system). For example, the desired signal parameter(s) may include, but are not limited to, a desired frequency, a desired amplitude, and a desired power level of the RF signal. 
     Returning to  FIG. 14A , once an acceptable or best match and the one or more desired signal parameters are determined, the defrosting operation may commence or continue. Commencement or continuation of the defrosting operation includes, in block  1420 , causing the RF signal source (e.g., RF signal source  320 ,  720 ,  1120 ,  FIGS. 3, 7, 11 ) to produce the RF signal (or the mass-estimate-based RF signal) with the desired signal parameters (e.g., with a desired RF power level) that were determined in block  1418 ,  1818 ,  1434 , or  1834 , which corresponds to a relatively high power RF signal. Other RF signal parameters (e.g., frequency) also may be included as a “desired signal parameter”, as indicated previously. Once again, the system controller may control the RF signal parameters, including the RF signal power level, through control signals to the RF signal source and to the power supply and bias circuitry (e.g., circuitry  326 ,  726 ,  FIGS. 3, 7 ). The control signals to the RF signal source may control the frequency of the RF signal, for example, and the control signals to the power supply and bias circuitry may cause the power supply and bias circuitry to provide supply and bias voltages to the amplifiers (e.g., amplifier stages  324 ,  325 ,  724 ,  FIGS. 3, 7 ) that are consistent with the desired signal power level. For example, the mass-estimate-based RF signal may be a signal having a power level in a range of about 50 W to about 500 W, although different power levels alternatively may be used. 
     In block  1422 , power detection circuitry (e.g., power detection circuitry  330 ,  730 ,  730 ′,  730 ″,  1180 ,  FIGS. 3, 7, 11 ) then periodically measures the reflected power and, in some embodiments, the forward power along the transmission path (e.g., path  328 ,  728 ,  1148 ,  FIGS. 3, 7, 11 ) between the RF signal source and the electrode(s), and provides those measurements to the system controller. The system controller again may determine a ratio between the reflected and forward signal powers, and may determine the S11 parameter for the system based on the ratio and/or may determine the VSWR for the system based on the reflected and forward signal powers. The system controller may store the received power measurements, and/or the calculated ratios, VSWR, and/or S11 parameters for future evaluation or comparison, in an embodiment. According to an embodiment, the periodic measurements of the forward and reflected power may be taken at a fairly high frequency (e.g., on the order of milliseconds) or at a fairly low frequency (e.g., on the order of seconds). For example, a fairly low frequency for taking the periodic measurements may be a rate of one measurement every 10 seconds to 20 seconds. 
     In block  1424 , the system controller may determine, based on one or more reflected signal power measurements, one or more calculated reflected-to-forward signal power ratios, and/or one or more calculated S11 parameters, and/or one or more calculated VSWR values, whether or not the match provided by the variable impedance matching network is acceptable. For example, the system controller may use a single reflected signal power measurement, a single calculated reflected-to-forward signal power ratio, or a single calculated S11 parameter in making this determination, or may take an average (or other calculation) of a number of previously-received reflected signal power measurements, previously-calculated reflected-to-forward power ratios, or previously-calculated Si i parameters or VSWR values in making this determination. To determine whether or not the match is acceptable, the system controller may compare the received reflected signal power, the calculated ratio, VSWR value, and/or S11 parameter to one or more corresponding thresholds, for example. For example, in one embodiment, the system controller may compare the received reflected signal power to a threshold of, for example, 5 percent (or some other value) of the forward signal power. A reflected signal power below 5 percent of the forward signal power may indicate that the match remains acceptable, and a ratio above 5 percent may indicate that the match is no longer acceptable. In another embodiment, the system controller may compare the calculated reflected-to-forward signal power ratio to a threshold of 10 percent (or some other value). A ratio below 10 percent may indicate that the match remains acceptable, and a ratio above 10 percent may indicate that the match is no longer acceptable. When the measured reflected power, or the calculated ratio or S11 parameter, or the VSWR is greater than the corresponding threshold (i.e., the comparison is unfavorable), indicating an unacceptable match, then the system controller may initiate re-configuration of the variable impedance matching network by returning to block  1410 . 
     As discussed previously, the match provided by the variable impedance matching network may degrade over the course of a defrosting operation due to impedance changes of the load (e.g., load  364 ,  764 ,  1164 ,  FIGS. 3, 7, 11 ) as the load warms up. It has been observed that, over the course of a defrosting operation, an optimal cavity match may be maintained by adjusting the cavity match inductance or capacitance and by also adjusting the RF signal source inductance or capacitance. Referring again to  FIG. 15 , for example, an optimal match for the first type of load at the end of a defrosting operation is indicated by point  1514 , and an optimal match for the second type of load at the end of a defrosting operation is indicated by point  1524 . In both cases, tracking of the optimal match between initiation and completion of the defrosting operations involves gradually decreasing the inductance of the cavity match and increasing the inductance of the RF signal source match. 
     According to an embodiment, in block  1410  when re-configuring the variable impedance matching network, the system controller may take into consideration this tendency. More particularly, when adjusting the match by reconfiguring the variable impedance matching network in block  1414  or  1814 , the system controller initially may select states of the variable inductance networks for the cavity and RF signal source matches that correspond to lower inductances (for the cavity match, or network  411 ,  FIG. 4A ) and higher inductances (for the RF signal source match, or network  410 ,  FIG. 4B ). Similar processes may be performed in embodiments that utilize variable capacitance networks for the cavity and RF signal source. By selecting impedance values that tend to follow the expected optimal match trajectories (e.g., those illustrated in  FIG. 15 ), the time to perform the variable impedance matching network reconfiguration process (e.g., in block  1410 ) may be reduced, when compared with a reconfiguration process that does not take these tendencies into account. 
     In an alternate embodiment, the system controller may instead iteratively test each adjacent configuration to attempt to determine an acceptable configuration. For example, referring again to Table 1, above, if the current configuration corresponds to state  12  for the cavity matching network and to state  3  for the RF signal source matching network, the system controller may test states  11  and/or  13  for the cavity matching network, and may test states  2  and/or  4  for the RF signal source matching network. If those tests do not yield a favorable result (i.e., an acceptable match), the system controller may test states  10  and/or  14  for the cavity matching network, and may test states  1  and/or  5  for the RF signal source matching network, and so on. 
     In actuality, there are a variety of different searching methods that the system controller may employ to re-configure the system to have an acceptable impedance match, including testing all possible variable impedance matching network configurations. Any reasonable method of searching for an acceptable configuration is considered to fall within the scope of the inventive subject matter. In any event, once an acceptable match is determined in block  1413  or  1813 , the defrosting operation is resumed in block  1420 , and the process continues to iterate. 
     Referring back to block  1424 , when the system controller determines, based on one or more reflected power measurements, one or more calculated reflected-to-forward signal power ratios, one or more calculated VSWR values, and/or one or more calculated S11 parameters, that the match provided by the variable impedance matching network is still acceptable (e.g., the reflected power measurements, calculated ratio, or S11 parameter, or VSWR is less than a corresponding threshold, or the comparison is favorable), the system may evaluate whether or not an exit condition has occurred, in block  1426 . In actuality, determination of whether an exit condition has occurred may be an interrupt driven process that may occur at any point during the defrosting process. However, for the purposes of including it in the flowchart of  FIG. 14A , the process is shown to occur after block  1424 . 
     In any event, several conditions may warrant cessation of the defrosting operation. For example, the system may determine that an exit condition has occurred when a safety interlock is breached. Alternatively, the system may determine that an exit condition has occurred upon expiration of a timer that was set by the user (e.g., through user interface  380 ,  780 ,  FIGS. 3, 7 ) or upon expiration of a timer that was established by the system controller based on the system controller&#39;s estimate of how long the defrosting operation should be performed (e.g., based on a heating time determined by the system controller at block  1418  or  1818  based on the previously identified optimized RF signal power level and based on the previously identified amount of energy estimated to be required for defrosting the load). In still another alternate embodiment, the system may otherwise detect completion of the defrosting operation. 
     If an exit condition has not occurred, then the defrosting operation may continue by iteratively performing blocks  1422  and  1424  (and the matching network reconfiguration process  1410 , as necessary). When an exit condition has occurred, then in block  1428 , the system controller causes the supply of the RF signal by the RF signal source to be discontinued. For example, the system controller may disable the RF signal generator (e.g., RF signal generator  322 ,  722 ,  FIGS. 3, 7 ) and/or may cause the power supply and bias circuitry (e.g., circuitry  326 ,  726 ,  FIGS. 3, 7 ) to discontinue provision of the supply current. In addition, the system controller may send signals to the user interface (e.g., user interface  380 ,  780 ,  FIGS. 3, 7 ) that cause the user interface to produce a user-perceptible indicia of the exit condition (e.g., by displaying “door open” or “done” on a display device, or providing an audible tone). The method may then end. 
     It should be understood that the order of operations associated with the blocks depicted in  FIGS. 14A-D  corresponds to an example embodiment, and should not be construed to limit the sequence of operations only to the illustrated order. Instead, some operations may be performed in different orders, and/or some operations may be performed in parallel. 
     The connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the subject matter. In addition, certain terminology may also be used herein for the purpose of reference only, and thus are not intended to be limiting, and the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     As used herein, a “node” means any internal or external reference point, connection point, junction, signal line, conductive element, or the like, at which a given signal, logic level, voltage, data pattern, current, or quantity is present. Furthermore, two or more nodes may be realized by one physical element (and two or more signals can be multiplexed, modulated, or otherwise distinguished even though received or output at a common node). 
     The foregoing description refers to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element is directly joined to (or directly communicates with) another element, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element is directly or indirectly joined to (or directly or indirectly communicates with) another element, and not necessarily mechanically. Thus, although the schematic shown in the figures depict one exemplary arrangement of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the depicted subject matter. 
     In an example embodiment, a thermal increase system may include a radio frequency (RF) signal source, at least one variable impedance network that includes at least one variable passive component having at least one current variable component value, and a controller. The at least one variable impedance network may be coupled between the RF signal source and the electrode. The controller may be configured to determine an initial estimated mass of a load that is proximate to the electrode based at least on the at least one current variable component value of the at least one variable impedance network, to determine one or more desired signal parameters for the RF signal, including a desired RF power level, based on at least the estimated mass of the load, to control the RF signal source to supply an initial-mass-estimate-based RF signal with the one or more desired signal parameters, to determine a rate of change of a parameter of the RF signal source while the initial-mass-estimate-based RF signal is supplied, to determine a refined estimated mass of the load based on at least the rate of change of the parameter, to determine one or more refined signal parameters for the RF signal based on at least the refined estimated mass of the load, and to control the RF signal source to supply a refined-mass-estimate-based RF signal with the one or more refined signal parameters. The parameter may an S11 parameter, a voltage standing wave ratio, or a reflected power of the RF signal. 
     In one embodiment, the controller may be configured to determine an initial estimated amount of energy sufficient to warm the load to a desired temperature based on the initial estimated mass of the load, and to determine a refined estimated amount of energy sufficient to warm the load to the desired temperature based on at least the refined estimated mass of the load. The controller may be configured to determine the one or more desired signal parameters for the RF signals based on the initial estimated amount of energy sufficient to warm the load to the desired temperature, and to determine the refined signal parameters for the RF signal based on the refined estimated amount of energy sufficient to warm the load to the desired temperature. 
     In one embodiment, the thermal increase system may include a memory configured to store a look-up table (LUT) that includes a set of stored load mass values, a set of stored RF power levels, and a set of stored parameter rates of change, and that is organized into multiple entries, each entry including a stored load mass value of the set of stored load mass values, a stored RF power level of the set of stored RF power levels, and a stored parameter rate of change of the set of stored parameter rates of change. The controller may be configured to determine the refined estimated mass of the load by comparing the rate of change of the parameter to the set of stored parameter rates of change of the LUT and comparing the desired RF power level to the set of stored RF power levels of the LUT to identify a correlated entry of the multiple entries. The correlated entry may include a first stored parameter rate of change that correlates with the rate of change of the parameter and a first stored RF power level that correlates with the desired RF power level, and by identifying a first stored load mass value of the set of stored load mass values that corresponds to the correlated entry. The first stored load mass value may be determined by the controller to be the refined estimated mass of the load. The at least one variable impedance network may include a double-ended variable impedance network that includes first and second inputs, first and second outputs, a first variable passive component that is connected between the first input and the first output, a second variable passive component that is connected between the second input and the second output, and a third variable passive component that is connected between the first input and the second input. 
     In one embodiment, the one or more refined signal parameters include at least one signal parameter selected from a group that includes a frequency of the RF signal and an amplitude of the RF signal. 
     In an example embodiment, a thermal increase system may be coupled to a cavity for containing a load. The thermal increase system may include a radio frequency (RF) signal source configured to supply an RF signal, a transmission path electrically coupled between the RF signal source and first and second electrodes that are positioned across the cavity, an impedance matching network electrically coupled along the transmission path, and a controller. The impedance matching network may include one or more variable passive components. Each of the one or more variable passive components may have a current variable component value at a first evaluation time, and a current variable component value set includes the current variable component value of each of the one or more variable passive components. The controller may be configured to determine an initial estimated mass of the load based on at least the current variable component value set, to determine one or more desired signal parameters for the RF signal, including a desired RF power level, based on at least the initial estimated mass of the load, to modify the RF signal source to supply an initial-mass-estimate-based RF signal with the one or more desired signal parameters, to reconfigure the impedance matching network at a second evaluation time, to determine an elapsed time between the first evaluation time and the second evaluation time, to determine a refined estimated mass of the load based on at least the elapsed time, to determine one or more refined signal parameters for the RF signal based on at least the refined estimated mass of the load, and to modify the RF signal source to supply a refined-mass-estimate-based RF signal with the one or more refined signal parameters. 
     In one embodiment, the controller may be configured to determine an initial estimated amount of energy sufficient to warm the load to a desired temperature based on at least the initial estimated mass of the load, and to determine a refined amount of energy sufficient to warm the load to the desired temperature based on at least the refined estimated mass of the load. The controller may be further configured to determine the one or more desired signal parameters for the RF signal based on the initial estimated amount of energy sufficient to warm the load to the desired temperature, and to determine the refined signal parameters for the RF signal based on the refined estimated amount of energy sufficient to warm the load to the desired temperature. 
     In one embodiment, the thermal increase system may include a memory configured to store a look-up table (LUT) that includes a set of stored load masses, a set of stored RF power levels, and a set of stored elapsed times, and that is organized into multiple entries each entry including a stored load mass of the set of stored load masses, a stored RF power level of the set of stored RF power levels, and a stored elapsed time of the set of stored elapsed times. In one embodiment, the controller may be configured to determine the refined estimated mass of the load by comparing the elapsed time to the set of stored elapsed times of the LUT and comparing the desired RF power level to the set of stored RF power levels of the LUT to identify a correlated entry of the multiple entries, wherein the correlated entry includes a first stored elapsed time that correlates with the elapsed time and a first stored RF power level that correlates with the desired RF power level and by identifying a first stored load mass of the set of stored load masses that corresponds to the correlated entry of the multiple entries in the LUT. The first stored load mass may be determined by the controller to be the refined estimated mass of the load. 
     In one embodiment, the one or more refined signal parameters may include at least one signal parameter selected from a group that includes a frequency of the RF signal and an amplitude of the RF signal. 
     In an example embodiment, a method of operating a thermal increase system that includes a cavity within which a load is contained may include supplying, by a radio frequency (RF) signal source, one or more RF signals to a transmission path that is electrically coupled between the RF signal source and one or more electrodes that are positioned proximate to the cavity, detecting, by power detection circuitry, reflected signal power along the transmission path, modifying, by a controller, one or more component values of one or more variable passive components of an impedance matching network that is electrically coupled along the transmission path to reduce the reflected signal power, determining, by the controller, an initial estimated mass of the load at least based on one or more current component values of the one or more variable passive components, determining, by the controller, one or more desired signal parameters for the RF signal at least based on the initial estimated mass of the load, the one or more desired signal parameters including a desired RF power level, controlling, by the controller, the RF signal source to supply an initial-mass-estimate-based RF signal with the one or more desired signal parameters, determining, by the controller, a rate of change of a parameter of the RF signal source while the initial-mass-estimate-based RF signal is supplied, wherein the parameter is selected from a group consisting of: an S11 parameter, a voltage standing wave ratio, and a reflected power of the RF signal, determining, by the controller, a refined estimated mass of the load based on at least the rate of change of the parameter, determining, by the controller, one or more refined signal parameters for the RF signal based on at least the refined estimated mass of the load, and controlling, by the controller, the RF signal source to supply a refined-mass-estimate-based RF signal with the one or more refined signal parameters. 
     In one embodiment, the method may include determining, by the controller, an initial estimated amount of energy sufficient to warm the load to a desired temperature based on the initial estimated mass of the load, and determining, by the controller, a refined estimated amount of energy sufficient to warm the load to the desired temperature based on the refined estimated mass of the load. 
     In one embodiment, the desired signal parameters may be determined based on the initial estimated amount of energy sufficient to warm the load to the desired temperature, and wherein the refined signal parameters are determined based on the refined estimated amount of energy sufficient to warm the load to the desired temperature. 
     In one embodiment, determining the initial estimated mass of the load may include comparing, by the controller, the one or more current component values with multiple stored component value sets stored in a memory of the thermal increase system, identifying, by the controller, a correlated stored component value set from the multiple stored component value sets that correlates with the one or more current component values, determining, by the controller, an identified stored mass of a plurality of stored masses that corresponds to the correlated stored component value set, and determining, by the controller, the initial estimated mass of the load to be the identified stored mass. 
     In one embodiment, wherein determining the refined mass estimate of the load includes comparing, by the controller, the rate of change of the parameter with multiple stored parameter rates of change stored in a memory of the thermal increase system, comparing, by the controller, the desired RF power level with multiple stored RF power levels stored in the memory of the system, identifying, by the controller, a correlated entry stored in the memory of the system, the correlated entry including a stored parameter rate of change that correlates with the rate of change of the parameter, and including a stored RF power level that correlates with the desired RF power level, and a stored load mass, and determining, by the controller, the refined estimated mass of the load to be the stored load mass of the correlated entry. 
     In one embodiment, the one or more refined signal parameters may include at least one signal parameter selected from a group that includes a frequency of the RF signal and an amplitude of the RF signal. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.