Patent Publication Number: US-11382190-B2

Title: Defrosting apparatus 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 contact with the food load, low power 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 the weight of the food load, and a timer may be used to control cessation of the operation. 
     Although good defrosting results are possible using such systems, the dynamic changes to the food load impedance may result in inefficient defrosting of the food load. In addition, inaccuracies inherent in determining the duration of the defrosting operation based on weight 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. 
    
    
     
       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. 4  is a schematic diagram of a single-ended variable inductance matching network, in accordance with an example embodiment; 
         FIG. 5  is a schematic diagram of a single-ended variable inductance network, in accordance with an example embodiment; 
         FIG. 6  is an example of a Smith chart depicting how a plurality of inductances in an embodiment of a variable impedance matching network may match the input cavity impedance to an RF signal source; 
         FIG. 7  is a cross-sectional, side view of a defrosting system, in accordance with an example embodiment; 
         FIG. 8  is a perspective view of a portion of a defrosting system, in accordance with an example embodiment; 
         FIG. 9  is a simplified block diagram of a balanced defrosting apparatus, in accordance with another example embodiment; 
         FIG. 10  is a schematic diagram of a double-ended variable impedance matching network, in accordance with another example embodiment; 
         FIG. 11  is a schematic diagram of a double-ended variable impedance network, in accordance with another example embodiment; 
         FIG. 12  is a perspective view of a double-ended variable impedance matching network module, in accordance with an example embodiment; 
         FIG. 13  is a perspective view of an RF module, in accordance with an example embodiment; 
         FIG. 14  is a flowchart of a method of operating a defrosting system with dynamic load matching, in accordance with an example embodiment; and 
         FIG. 15  is a chart plotting cavity match setting versus RF signal source match setting through a defrost operation for two different loads. 
     
    
    
     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 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. 
       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 ,  960 ,  FIGS. 3, 9 ), a control panel  120 , one or more radio frequency (RF) signal sources (e.g., RF signal source  320 ,  920 ,  FIGS. 3, 9 ), a power supply (e.g., power supply  326 ,  926 ,  FIGS. 3, 9 ), a first electrode  170  (e.g., electrode  340 ,  940 ,  FIGS. 3, 9 ), a second electrode  172  (e.g., electrode  950 ,  FIG. 9 ), impedance matching circuitry (e.g., circuits  334 ,  370 ,  934 ,  972 ,  FIGS. 3, 9 ), power detection circuitry (e.g., power detection circuitry  330 ,  930 ,  FIGS. 3, 9 ), and a system controller (e.g., system controller  312 ,  912 ,  FIGS. 3, 9 ). 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 ,  962 ,  FIGS. 3, 9 ) 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  914 ,  FIG. 9 ) 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 weight 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 ,  912 ,  FIGS. 3, 9 ) 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 ,  920 ,  FIGS. 3, 9 ) 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 ,  930 ,  FIGS. 3, 9 ) continuously or periodically measures the reflected power along a transmission path (e.g., transmission path  328 ,  928 ,  FIGS. 3, 9 ) between the RF signal source (e.g., RF signal source  320 ,  920 ,  FIGS. 3, 9 ) and the electrode(s)  170 ,  172 . Based on these measurements, the system controller (e.g., system controller  312 ,  912 ,  FIGS. 3, 9 ) 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 ,  920 ,  FIGS. 3, 9 ), a power supply (e.g., power supply  326 ,  926 ,  FIGS. 3, 9 ), a first electrode (e.g., electrode  340 ,  940 ,  FIGS. 3, 9 ), a second electrode  172  (e.g., containment structure  366 , electrode  950 ,  FIGS. 3, 9 ), impedance matching circuitry (e.g., circuits  334 ,  370 ,  934 ,  972 ,  FIGS. 3, 9 ), power detection circuitry (e.g., power detection circuitry  330 ,  930 ,  FIGS. 3, 9 ), and a system controller (e.g., system controller  312 ,  912 ,  FIGS. 3, 9 ). 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), infrared (IR) sensor(s), and/or weight 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), IR sensor(s), and/or weight 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 weight sensor(s) are positioned under the load  364 , and are configured to provide an estimate of the weight of the load  364  to the system controller  312 . 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 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 an 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. 9  and described later (e.g., including connectors  936 ,  938  and a conductor  928 - 3  such as a coaxial cable between the connectors  936 ,  938 ). 
     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 a more specific embodiment, the variable impedance matching network  370  includes a plurality of fixed-value lumped inductors (e.g., inductors  412 - 414 ,  712 - 714 ,  812 - 814 ,  FIGS. 4, 7, 8 ) 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. 4, 5 ), which may be located inside or outside of the cavity  360 . The inductance value provided by each of the variable inductance 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 input 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  326  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 range of about 10.0 megahertz (MHz) to about 100 MHz and/or from 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 decibels (dB) to about 15 dB. 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  924 ,  FIG. 9 ), 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 input impedance.” The cavity input impedance changes during a defrosting operation as the temperature of the load  364  increases. The cavity input 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 chamber input 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 chamber input 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 input 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. 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, this indicates that the system  300  is not adequately matched to the cavity input 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 or the S11 parameter 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  364 . 
     More specifically, the system controller  312  may provide control signals over control path  316  to the variable matching circuit  970 , 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 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 . 
     The variable matching network  370  may have any of a variety of configurations. 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  600 ,  FIG. 6 ). 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. 4 and 5 . According to an embodiment, 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). 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). 
       FIG. 4  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 4.66 MHz to about 4.68 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 “power amplifier 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  FIGS. 7 and 8 , the set  430  of lumped inductors  412 - 415  may be 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. 4  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. 5  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. 4 ), 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 ,  523 , 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. 5 , 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×I, 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 
                 Switch 
                 Switch 
                 Switch 
                 Total 
               
               
                   
                 511 state 
                 512 state 
                 513 state 
                 514 state 
                 network 
               
               
                   
                 (501 
                 (502 
                 (503 
                 (504 
                 inductance 
               
               
                 Network 
                 value = 
                 value = 
                 value = 
                 value = 
                 (w/o trace 
               
               
                 state 
                 I) 
                 2 × I) 
                 4 × I) 
                 8 × I) 
                 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 
                 open 
                 closed 
                 6 × I 
               
               
                 7 
                 open 
                 open 
                 open 
                 closed 
                 7 × I 
               
               
                 8 
                 closed 
                 closed 
                 closed 
                 open 
                 8 × I 
               
               
                 9 
                 open 
                 closed 
                 closed 
                 open 
                 9 × I 
               
               
                 10 
                 closed 
                 open 
                 closed 
                 open 
                 10 × I  
               
               
                 11 
                 open 
                 open 
                 closed 
                 open 
                 11 × I  
               
               
                 12 
                 closed 
                 closed 
                 open 
                 open 
                 12 × I  
               
               
                 13 
                 open 
                 closed 
                 open 
                 open 
                 13 × I  
               
               
                 14 
                 closed 
                 open 
                 open 
                 open 
                 14 × I  
               
               
                 15 
                 open 
                 open 
                 open 
                 open 
                 15 × I  
               
               
                   
               
            
           
         
       
     
     Referring again to  FIG. 4 , 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 input impedance that is present during a defrosting operation. 
       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, 4 ) may match the input cavity 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, 4 ). 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. 4 , 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. 
     A particular physical configuration of a defrosting system will now be described in conjunction with  FIGS. 7 and 8 . More particularly,  FIG. 7  is a cross-sectional, side view of a defrosting system  700 , in accordance with an example embodiment, and  FIG. 8  is a perspective view of a portion of defrosting system  700 . The defrosting system  700  generally includes a defrosting cavity  774 , a user interface (not shown), a system controller  730 , an RF signal source  720 , power supply and bias circuitry (not shown), power detection circuitry  780 , a variable impedance matching network  760 , a first electrode  770 , and a second electrode  772 , in an embodiment. In addition, in some embodiments, defrosting system  700  may include weight sensor(s)  790 , temperature sensor(s), and/or IR sensor(s)  792 . 
     The defrosting system  700  is contained within a containment structure  750 , in an embodiment. According to an embodiment, the containment structure  750  may define three interior areas: the defrosting cavity  774 , a fixed inductor area  776 , and a circuit housing area  778 . The containment structure  750  includes bottom, top, and side walls. Portions of the interior surfaces of some of the walls of the containment structure  750  may define the defrosting cavity  774 . The defrosting cavity  774  includes a capacitive defrosting arrangement with first and second parallel plate electrodes  770 ,  772  that are separated by an air cavity within which a load  764  to be defrosted may be placed. For example, the first electrode  770  may be positioned above the air cavity, and a second electrode  772  may be provided by a conductive portion of the containment structure  750  (e.g., a portion of the bottom wall of the containment structure  750 ). Alternatively, the second electrode  772  may be formed from a conductive plate that is distinct from the containment structure  750 . According to an embodiment, non-electrically conductive support structure(s)  754  may be employed to suspend the first electrode  770  above the air cavity, to electrically isolate the first electrode  770  from the containment structure  750 , and to hold the first electrode  770  in a fixed physical orientation with respect to the air cavity. 
     According to an embodiment, the containment structure  750  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  750  that corresponds to the second electrode  772  may be formed from conductive material and grounded. To avoid direct contact between the load  764  and the second electrode  772 , a non-conductive barrier  756  may be positioned over the second electrode  772 . 
     When included in the system  700 , the weight sensor(s)  790  are positioned under the load  764 . The weight sensor(s)  790  are configured to provide an estimate of the weight of the load  764  to the system controller  730 . The temperature sensor(s) and/or IR sensor(s)  792  may be positioned in locations that enable the temperature of the load  764  to be sensed both before, during, and after a defrosting operation. According to an embodiment, the temperature sensor(s) and/or IR sensor(s)  792  are configured to provide load temperature estimates to the system controller  730 . 
     Some or all of the various components of the system controller  730 , the RF signal source  720 , the power supply and bias circuitry (not shown), the power detection circuitry  780 , and portions  710 ,  711  of the variable impedance matching network  760 , may be coupled to a common substrate  752  within the circuit housing area  778  of the containment structure  750 , 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 ,  FIG. 12 ), which are housed within the circuit housing area  778  of the containment structure  750 . According to an embodiment, the system controller  730  is coupled to the user interface, RF signal source  720 , variable impedance matching network  760 , and power detection circuitry  780  through various conductive interconnects on or within the common substrate  752 . In addition, the power detection circuitry  780  is coupled along the transmission path  748  between the output of the RF signal source  720  and the input  702  to the variable impedance matching network  760 , in an embodiment. For example, the substrate  752  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. 
     The first electrode  770  is electrically coupled to the RF signal source  720  through a variable impedance matching network  760  and a transmission path  748 , in an embodiment. As discussed previously, the variable impedance matching network  760  includes variable inductance networks  710 ,  711  (e.g., networks  410 ,  411 ,  FIG. 4 ) and a plurality of fixed-value lumped inductors  712 - 715  (e.g., inductors  412 - 415 ,  FIG. 4 ). In an embodiment, the variable inductance networks  710 ,  711  are coupled to the common substrate  752  and located within the circuit housing area  778 . In contrast, the fixed-value lumped inductors  712 - 715  are positioned within the fixed inductor area  776  of the containment structure  750  (e.g., between the common substrate  752  and the first electrode  770 ). Conductive structures (e.g., conductive vias or other structures) may provide for electrical communication between the circuitry within the circuit housing area  778  and the lumped inductors  712 - 715  within the fixed inductor area  776 . 
     For enhanced understanding of the system  700 , the nodes and components of the variable impedance matching network  760  depicted in  FIGS. 7 and 8  will now be correlated with nodes and components of the variable impedance matching network  400  depicted in  FIG. 4 . More specifically, the variable impedance matching network  760  includes an input node  702  (e.g., input node  402 ,  FIG. 4 ), an output node  704  (e.g., output node  404 ,  FIG. 4 ), first and second variable inductance networks  710 ,  711  (e.g., variable inductance networks  410 ,  411 ,  FIG. 4 ), and a plurality of fixed-value inductors  712 - 715  (e.g., inductors  412 - 415 ,  FIG. 4 ), according to an embodiment. The input node  702  is electrically coupled to an output of the RF signal source  720  through various conductive structures (e.g., conductive vias and traces), and the output node  704  is electrically coupled to the first electrode  770 . 
     Between the input and output nodes  702 ,  704  (e.g., input and output nodes  402 ,  404 ,  FIG. 4 ), the variable impedance matching network  700  includes four lumped inductors  712 - 715  (e.g., inductors  412 - 415 ,  FIG. 4 ), in an embodiment, which are positioned within the fixed inductor area  776 . An enhanced understanding of an embodiment of a physical configuration of the lumped inductors  712 - 715  within the fixed inductor area  776  may be achieved by referring to both  FIG. 7  and to  FIG. 8  simultaneously, where  FIG. 8  depicts a top perspective view of the fixed inductor area  776 . In  FIG. 8 , the irregularly shaped, shaded areas underlying inductors  712 - 715  represents suspension of the inductors  712 - 715  in space over the first electrode  770 . In other words, the shaded areas indicate where the inductors  712 - 715  are electrically insulated from the first electrode  770  by air. Rather than relying on an air dielectric, non-electrically conductive spacers may be included in these areas. 
     In an embodiment, the first lumped inductor  712  has a first terminal that is electrically coupled to the input node  702  (and thus to the output of RF signal source  720 ), and a second terminal that is electrically coupled to a first intermediate node  721  (e.g., node  421 ,  FIG. 4 ). The second lumped inductor  713  has a first terminal that is electrically coupled to the first intermediate node  721 , and a second terminal that is electrically coupled to a second intermediate node  722  (e.g., node  422 ,  FIG. 4 ). The third lumped inductor  714  has a first terminal that is electrically coupled to the first intermediate node  721 , and a second terminal that is electrically coupled to the output node  704  (and thus to the first electrode  770 ). The fourth lumped inductor  715  has a first terminal that is electrically coupled to the output node  704  (and thus to the first electrode  770 ), and a second terminal that is electrically coupled to a ground reference node (e.g., to the grounded containment structure  750  through one or more conductive interconnects). 
     The first variable inductance network  710  (e.g., network  410 ,  FIG. 4 ) is electrically coupled between the input node  702  and a ground reference terminal (e.g., the grounded containment structure  750 ). Finally, the second variable inductance network  711  (e.g., network  411 ,  FIG. 4 ) is electrically coupled between the second intermediate node  722  and the ground reference terminal. 
     The description associated with  FIGS. 3-8  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. 9  is a simplified block diagram of a balanced defrosting system  900  (e.g., defrosting system  100 ,  210 ,  220 ,  FIGS. 1, 2 ), in accordance with an example embodiment. Defrosting system  900  includes RF subsystem  910 , defrosting cavity  960 , user interface  980 , system controller  912 , RF signal source  920 , power supply and bias circuitry  926 , variable impedance matching network  970 , two electrodes  940 ,  950 , and power detection circuitry  930 , in an embodiment. In addition, in other embodiments, defrosting system  900  may include temperature sensor(s), infrared (IR) sensor(s), and/or weight sensor(s)  990 , although some or all of these sensor components may be excluded. It should be understood that  FIG. 9  is a simplified representation of a defrosting system  900  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  900  may be part of a larger electrical system. 
     User interface  980  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  910  includes a system controller  912 , an RF signal source  920 , a first impedance matching circuit  934  (herein “first matching circuit”), power supply and bias circuitry  926 , and power detection circuitry  930 , in an embodiment. System controller  912  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  912  is operatively and communicatively coupled to user interface  980 , RF signal source  920 , power supply and bias circuitry  926 , power detection circuitry  930  (or  930 ′ or  930 ″), variable matching subsystem  970 , sensor(s)  990  (if included), and pump  992  (if included). System controller  912  is configured to receive signals indicating user inputs received via user interface  980 , to receive signals indicating RF signal reflected power (and possibly RF signal forward power) from power detection circuitry  930  (or  930 ′ or  930 ″), and to receive sensor signals from sensor(s)  990 . Responsive to the received signals and measurements, and as will be described in more detail later, system controller  912  provides control signals to the power supply and bias circuitry  926  and/or to the RF signal generator  922  of the RF signal source  920 . In addition, system controller  912  provides control signals to the variable matching subsystem  970  (over path  916 ), which cause the subsystem  970  to change the state or configuration of a variable impedance matching circuit  972  of the subsystem  970  (herein “variable matching circuit”). 
     Defrosting cavity  960  includes a capacitive defrosting arrangement with first and second parallel plate electrodes  940 ,  950  that are separated by an air cavity within which a load  964  to be defrosted may be placed. Within a containment structure  966 , first and second electrodes  940 ,  950  (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  960  (e.g., interior cavity  260 ,  FIG. 2 ). According to an embodiment, a distance  952  between the electrodes  940 ,  950  renders the cavity  960  a sub-resonant cavity, in an embodiment. 
     The first and second electrodes  940 ,  950  are separated across the cavity  960  by a distance  952 . In various embodiments, the distance  952  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  952  is less than one wavelength of the RF signal produced by the RF subsystem  910 . In other words, as mentioned above, the cavity  960  is a sub-resonant cavity. In some embodiments, the distance  952  is less than about half of one wavelength of the RF signal. In other embodiments, the distance  952  is less than about one quarter of one wavelength of the RF signal. In still other embodiments, the distance  952  is less than about one eighth of one wavelength of the RF signal. In still other embodiments, the distance  952  is less than about one 50th of one wavelength of the RF signal. In still other embodiments, the distance  952  is less than about one 100th of one wavelength of the RF signal. 
     In general, a system  900  designed for lower operational frequencies (e.g., frequencies between 10 MHz and 100 MHz) may be designed to have a distance  952  that is a smaller fraction of one wavelength. For example, when system  900  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  952  is selected to be about 0.5 meters, the distance  952  is about one 60th of one wavelength of the RF signal. Conversely, when system  900  is designed for an operational frequency of about 300 MHz (corresponding to a wavelength of about 1 meter), and distance  952  is selected to be about 0.5 meters, the distance  952  is about one half of one wavelength of the RF signal. 
     With the operational frequency and the distance  952  between electrodes  940 ,  950  being selected to define a sub-resonant interior cavity  960 , the first and second electrodes  940 ,  950  are capacitively coupled. More specifically, the first electrode  940  may be analogized to a first plate of a capacitor, the second electrode  950  may be analogized to a second plate of a capacitor, and the load  964 , barrier  962 , and air within the cavity  960  may be analogized to a capacitor dielectric. Accordingly, the first electrode  940  alternatively may be referred to herein as an “anode,” and the second electrode  950  may alternatively be referred to herein as a “cathode.” 
     Essentially, the voltage across the first and second electrodes  940 ,  950  heats the load  964  within the cavity  960 . According to various embodiments, the RF subsystem  910  is configured to generate the RF signal to produce voltages across the electrodes  940 ,  950  in a range of about 90 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  940 ,  950 , as well. 
     An output of the RF subsystem  910 , and more particularly an output of RF signal source  920 , is electrically coupled to the variable matching subsystem  970  through a conductive transmission path, which includes a plurality of conductors  928 - 1 ,  928 - 2 ,  928 - 3 ,  928 - 4 , and  928 - 5  connected in series, and referred to collectively as transmission path  928 . According to an embodiment, the conductive transmission path  928  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  928  may include unbalanced first and second conductors  928 - 1 ,  928 - 2  within the RF subsystem  910 , one or more connectors  936 ,  938  (each having male and female connector portions), and an unbalanced third conductor  928 - 3  electrically coupled between the connectors  936 ,  938 . According to an embodiment, the third conductor  928 - 3  comprises a coaxial cable, although the electrical length may be shorter or longer, as well. In an alternate embodiment, the variable matching subsystem  970  may be housed with the RF subsystem  910 , and in such an embodiment, the conductive transmission path  928  may exclude the connectors  936 ,  938  and the third conductor  928 - 3 . Either way, the “balanced” portion of the conductive transmission path  928  includes a balanced fourth conductor  928 - 4  within the variable matching subsystem  970 , and a balanced fifth conductor  928 - 5  electrically coupled between the variable matching subsystem  970  and electrodes  940 ,  950 , in an embodiment. 
     As indicated in  FIG. 9 , the variable matching subsystem  970  houses an apparatus configured to receive, at an input of the apparatus, the unbalanced RF signal from the RF signal source  920  over the unbalanced portion of the transmission path (i.e., the portion that includes unbalanced conductors  928 - 1 ,  928 - 2 , and  928 - 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  974 , in an embodiment. The balanced RF signals are conveyed over balanced conductors  928 - 4  to the variable matching circuit  972  and, ultimately, over balanced conductors  928 - 5  to the electrodes  940 ,  950 . 
     In an alternate embodiment, as indicated in a dashed box in the center of  FIG. 9 , and as will be discussed in more detail below, an alternate RF signal generator  920 ′ may produce balanced RF signals on balanced conductors  928 - 1 ′, which may be directly coupled to the variable matching circuit  972  (or coupled through various intermediate conductors and connectors). In such an embodiment, the balun  974  may be excluded from the system  900 . Either way, as will be described in more detail below, a double-ended variable matching circuit  972  (e.g., variable matching circuit  1000 ,  FIG. 10 ) is configured to receive the balanced RF signals (e.g., over connections  928 - 4  or  928 - 1 ′), to perform an impedance transformation corresponding to a then-current configuration of the double-ended variable matching circuit  972 , and to provide the balanced RF signals to the first and second electrodes  940 ,  950  over connections  928 - 5 . 
     According to an embodiment, RF signal source  920  includes an RF signal generator  922  and a power amplifier  924  (e.g., including one or more power amplifier stages). In response to control signals provided by system controller  912  over connection  914 , RF signal generator  922  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  922  may be controlled to produce oscillating signals of different power levels and/or different frequencies, in various embodiments. For example, the RF signal generator  922  may produce a signal that oscillates in a range of about 10.0 MHz to about 100 MHz and/or from 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). Alternatively, the frequency of oscillation may be lower or higher than the above-given ranges or values. 
     The power amplifier  924  is configured to receive the oscillating signal from the RF signal generator  922 , and to amplify the signal to produce a significantly higher-power signal at an output of the power amplifier  924 . 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  924  may be controlled using gate bias voltages and/or drain bias voltages provided by the power supply and bias circuitry  926  to one or more stages of amplifier  924 . More specifically, power supply and bias circuitry  926  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  912 . 
     The power amplifier may include one or more amplification stages. In an embodiment, each stage of amplifier  924  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. 9 , the power amplifier arrangement  924  is depicted to include one amplifier stage coupled in a particular manner to other circuit components. In other embodiments, the power amplifier arrangement  924  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. 9 , an alternate RF signal generator  920 ′ may include a push-pull or balanced amplifier  924 ′, which is configured to receive, at an input, an unbalanced RF signal from the RF signal generator  922 , to amplify the unbalanced RF signal, and to produce two balanced RF signals at two outputs of the amplifier  924 ′, where the two balanced RF signals are thereafter conveyed over conductors  928 - 1 ′ to the electrodes  940 ,  950 . In such an embodiment, the balun  974  may be excluded from the system  900 , and the conductors  928 - 1 ′ may be directly connected to the variable matching circuit  972  (or connected through multiple coaxial cables and connectors or other multi-conductor structures). 
     Defrosting cavity  960  and any load  964  (e.g., food, liquids, and so on) positioned in the defrosting cavity  960  present a cumulative load for the electromagnetic energy (or RF power) that is radiated into the interior chamber  962  by the electrodes  940 ,  950 . More specifically, and as described previously, the defrosting cavity  960  and the load  964  present an impedance to the system, referred to herein as a “cavity input impedance.” The cavity input impedance changes during a defrosting operation as the temperature of the load  964  increases. The cavity input impedance has a direct effect on the magnitude of reflected signal power along the conductive transmission path  928  between the RF signal source  920  and the electrodes  940 ,  950 . In most cases, it is desirable to maximize the magnitude of transferred signal power into the cavity  960 , and/or to minimize the reflected-to-forward signal power ratio along the conductive transmission path  928 . 
     In order to at least partially match the output impedance of the RF signal generator  920  to the chamber input impedance, a first matching circuit  934  is electrically coupled along the transmission path  928 , in an embodiment. The first matching circuit  934  is configured to perform an impedance transformation from an impedance of the RF signal source  920  (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  934  may have any of a variety of configurations. According to an embodiment, the first matching circuit  934  includes fixed components (i.e., components with non-variable component values), although the first matching circuit  934  may include one or more variable components, in other embodiments. For example, the first matching circuit  934  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  934  is configured to raise the impedance to an intermediate level between the output impedance of the RF signal generator  920  and the cavity input impedance. 
     According to an embodiment, and as mentioned above, power detection circuitry  930  is coupled along the transmission path  928  between the output of the RF signal source  920  and the electrodes  940 ,  950 . In a specific embodiment, the power detection circuitry  930  forms a portion of the RF subsystem  910 , and is coupled to the conductor  928 - 2  between the RF signal source  920  and connector  936 . In alternate embodiments, the power detection circuitry  930  may be coupled to any other portion of the transmission path  928 , such as to conductor  928 - 1 , to conductor  928 - 3 , to conductor  928 - 4  between the RF signal source  920  (or balun  974 ) and the variable matching circuit  972  (i.e., as indicated with power detection circuitry  930 ′), or to conductor  928 - 5  between the variable matching circuit  972  and the electrode(s)  940 ,  950  (i.e., as indicated with power detection circuitry  930 ″). For purposes of brevity, the power detection circuitry is referred to herein with reference number  930 , although the circuitry may be positioned in other locations, as indicated by reference numbers  930 ′ and  930 ″. 
     Wherever it is coupled, power detection circuitry  930  is configured to monitor, measure, or otherwise detect the power of the reflected signals traveling along the transmission path  928  between the RF signal source  920  and one or both of the electrode(s)  940 ,  950  (i.e., reflected RF signals traveling in a direction from electrode(s)  940 ,  950  toward RF signal source  920 ). In some embodiments, power detection circuitry  930  also is configured to detect the power of the forward signals traveling along the transmission path  928  between the RF signal source  920  and the electrode(s)  940 ,  950  (i.e., forward RF signals traveling in a direction from RF signal source  920  toward electrode(s)  940 ,  950 ). 
     Over connection  932 , power detection circuitry  930  supplies signals to system controller  912  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  912  may calculate a reflected-to-forward signal power ratio, or the S11 parameter. 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, this indicates that the system  900  is not adequately matched to the cavity input impedance, and that energy absorption by the load  964  within the cavity  960  may be sub-optimal. In such a situation, system controller  912  orchestrates a process of altering the state of the variable matching circuit  972  to drive the reflected signal power or the S11 parameter 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  964 . 
     More specifically, the system controller  912  may provide control signals over control path  916  to the variable matching circuit  972 , which cause the variable matching circuit  972  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  972 . Adjustment of the configuration of the variable matching circuit  972  desirably decreases the magnitude of reflected signal power, which corresponds to decreasing the magnitude of the S11 parameter and increasing the power absorbed by the load  964 . 
     As discussed above, the variable matching circuit  972  is used to match the input impedance of the defrosting cavity  960  plus load  964  to maximize, to the extent possible, the RF power transfer into the load  964 . The initial impedance of the defrosting cavity  960  and the load  964  may not be known with accuracy at the beginning of a defrosting operation. Further, the impedance of the load  964  changes during a defrosting operation as the load  964  warms up. According to an embodiment, the system controller  912  may provide control signals to the variable matching circuit  972 , which cause modifications to the state of the variable matching circuit  972 . This enables the system controller  912  to establish an initial state of the variable matching circuit  972  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  964 . In addition, this enables the system controller  912  to modify the state of the variable matching circuit  972  so that an adequate match may be maintained throughout the defrosting operation, despite changes in the impedance of the load  964 . 
     The variable matching circuit  972  may have any of a variety of configurations. For example, the circuit  972  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  972  is implemented in a balanced portion of the transmission path  928 , the variable matching circuit  972  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  928 , 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 ,  FIG. 4 ). According to more specific embodiments, the variable matching circuit  972  includes a variable inductance network (e.g., double-ended network  1000 ,  FIG. 10 ). The inductance, capacitance, and/or resistance values provided by the variable matching circuit  972 , which in turn affect the impedance transformation provided by the circuit  972 , are established through control signals from the system controller  912 , as will be described in more detail later. In any event, by changing the state of the variable matching circuit  972  over the course of a treatment operation to dynamically match the ever-changing impedance of the cavity  960  plus the load  964  within the cavity  960 , the system efficiency may be maintained at a high level throughout the defrosting operation. 
     The variable matching circuit  972  may have any of a wide variety of circuit configurations, and non-limiting examples of such configurations are shown in  FIGS. 10 and 11 . For example,  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 ,  900 ,  FIGS. 1, 2, 9 ), in accordance with an example embodiment. 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 in a ladder arrangement between the input  1001  and output  1002 . For example, when connected into system  900 , the first input  1001 - 1  may be connected to a first conductor of balanced conductor  928 - 4 , and the second input  1001 - 2  may be connected to a second conductor of balanced conductor  928 - 4 . Similarly, the first output  1002 - 1  may be connected to a first conductor of balanced conductor  928 - 5 , and the second output  1002 - 2  may be connected to a second conductor of balanced conductor  928 - 5 . 
     In the specific embodiment illustrated in  FIG. 10 , circuit  1000  includes a first variable inductor  1011  and a first fixed inductor  1015  connected in series between input  1001 - 1  and output  1002 - 1 , a second variable inductor  1016  and a second fixed inductor  1020  connected in series between input  1001 - 2  and output  1002 - 2 , a third variable inductor  1021  connected between inputs  1001 - 1  and  1001 - 2 , and a third fixed inductor  1024  connected between nodes  1025  and  1026 . 
     According to an embodiment, the third variable inductor  1021  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  920 ,  FIG. 9 ) as modified by the first matching circuit (e.g., circuit  934 ,  FIG. 9 ), or more particularly to match the impedance of the final stage power amplifier (e.g., amplifier  924 ,  FIG. 9 ) as modified by the first matching circuit (e.g., circuit  934 ,  FIG. 9 ). According to an embodiment, and as will be described in more detail in conjunction with  FIG. 11 , the third variable inductor  1021  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  1000  is provided by the first and second variable inductors  1011 ,  1016 , and fixed inductors  1015 ,  1020 , and  1024 . Because the states of the first and second variable inductors  1011 ,  1016  may be changed to provide multiple inductance values, the first and second variable inductors  1011 ,  1016  are configurable to optimally match the impedance of the cavity plus load (e.g., cavity  960  plus load  964 ,  FIG. 9 ). For example, inductors  1011 ,  1016  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  1015 ,  1020 ,  1024  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  1011 ,  1015 ,  1016 ,  1020 ,  1021 ,  1024  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  1011  and  1016  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  1002 - 1  and  1002 - 2  are balanced. 
     As discussed above, variable matching circuit  1000  is a double-ended circuit that is configured to be connected along a balanced portion of the transmission path  928  (e.g., between connectors  928 - 4  and  928 - 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  928 . 
     By varying the inductance values of inductors  1011 ,  1016 ,  1021  in circuit  1000 , the system controller  912  may increase or decrease the impedance transformation provided by circuit  1000 . Desirably, the inductance value changes improve the overall impedance match between the RF signal source  920  and the cavity input 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  912  may strive to configure the circuit  1000  in a state in which a maximum electromagnetic field intensity is achieved in the cavity  960 , and/or a maximum quantity of power is absorbed by the load  964 , and/or a minimum quantity of power is reflected by the load  964 . 
       FIG. 11  is a schematic diagram of a double-ended variable impedance matching network  1100 , in accordance with another example embodiment. Network  1100  includes a double-ended input  1101 - 1 ,  1101 - 2  (referred to as input  1101 ), a double-ended output  1102 - 1 ,  1102 - 2  (referred to as output  1102 ), and a network of passive components connected in a ladder arrangement between the input  1101  and output  1102 . The ladder arrangement includes a first plurality, N, of discrete inductors  1111 - 1114  coupled in series with each other between input  1101 - 1  and output  1102 - 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  1116 - 1119  coupled in series with each other between input  1101 - 2  and output  1102 - 2 . Additional discrete inductors  1115  and  1120  may be coupled between intermediate nodes  1125 ,  1126  and the output nodes  1102 - 1 ,  1102 - 2 . Further still, the ladder arrangement includes a third plurality of discrete inductors  1121 - 1123  coupled in series with each other between inputs  1101 - 1  and  1101 - 2 , and an additional discrete inductor  1124  coupled between nodes  1125  and  1126 . For example, the fixed inductors  1115 ,  1120 ,  1124  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  1111 - 1114  may be considered a first variable inductor (e.g., inductor  1011 ,  FIG. 10 ), the series arrangement of inductors  1116 - 1119  may be considered a second variable inductor (e.g., inductor  1016 ,  FIG. 10 ), and series arrangement of inductors  1121 - 1123  may be considered a third variable inductor (e.g., inductor  1021 ,  FIG. 10 ). To control the variability of the “variable inductors”, network  1100  includes a plurality of bypass switches  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143 , where each switch  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143  is coupled in parallel across the terminals of one of inductors  1111 - 1114 ,  1116 - 1119 ,  1121 , and  1123 . Switches  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143  may be implemented as transistors, mechanical relays or mechanical switches, for example. The electrically conductive state of each switch  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143  (i.e., open or closed) is controlled using control signals  1151 - 1154 ,  1156 - 1159 ,  1161 ,  1163  from the system controller (e.g., control signals from system controller  912  provided over connection  916 ,  FIG. 9 ). 
     In an embodiment, sets of corresponding inductors in the two paths between input  1101  and output  1102  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  1102 - 1  and  1102 - 2  are balanced. For example, inductors  1111  and  1116  may constitute a first “set of corresponding inductors” or “paired inductors” with substantially equal values, and during operation, the states of switches  1131  and  1136  are controlled to be the same (e.g., either both open or both closed), at any given time. Similarly, inductors  1112  and  1117  may constitute a second set of corresponding inductors with equal inductance values that are operated in a paired manner, inductors  1113  and  1118  may constitute a third set of corresponding inductors with equal inductance values that are operated in a paired manner, and inductors  1114  and  1119  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  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143  are open, as illustrated in  FIG. 11 , substantially all current flowing between input and output nodes  1101 - 1 ,  1102 - 1  flows through the series of inductors  1111 - 1115 , and substantially all current flowing between input and output nodes  1101 - 2 ,  1102 - 2  flows through the series of inductors  1116 - 1120  (as modified by any current flowing through inductors  1121 - 1123  or  1124 ). This configuration represents the maximum inductance state of the network  1100  (i.e., the state of network  1100  in which a maximum inductance value is present between input and output nodes  1101 ,  1102 ). Conversely, when all switches  1131 - 1134 ,  1136 - 1139 ,  1141 , and  1143  are closed, substantially all current flowing between input and output nodes  1101 ,  1102  bypasses the inductors  1111 - 1114  and  1116 - 1119  and flows instead through the switches  1131 - 1134  or  1136 - 1139 , inductors  1115  or  1120 , and the conductive interconnections between the input and output nodes  1101 ,  1102  and switches  1131 - 1134 ,  1136 - 1139 . This configuration represents the minimum inductance state of the network  1100  (i.e., the state of network  1100  in which a minimum inductance value is present between input and output nodes  1101 ,  1102 ). 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  1131 - 1134  or  1136 - 1139 , inductors  1115  or  1120 , and the conductive interconnections between nodes  1101 ,  1102  and the switches  1131 - 1134  or  1136 - 1139 . For example, in the minimum inductance state, a trace inductance for the series combination of switches  1131 - 1134  or  1136 - 1139  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  1100 . 
     Starting from the maximum inductance state in which all switches  1131 - 1134 ,  1136 - 1139  are open, the system controller may provide control signals  1151 - 1154 ,  1156 - 1159  that result in the closure of any combination of switches  1131 - 1134 ,  1136 - 1139  in order to reduce the inductance of the network  1100  by bypassing corresponding combinations of inductors  1111 - 1114 ,  1116 - 1119 . 
     Similar to the embodiment of  FIG. 10 , in circuit  1100 , the first and second pluralities of discrete inductors  1111 - 1114 ,  1116 - 1119  and fixed inductor  1124  correspond to a “cavity matching portion” of the circuit. Similar to the embodiment described above in conjunction with  FIG. 5 , in one embodiment, each inductor  1111 - 1114 ,  1116 - 1119  has substantially the same inductance value, referred to herein as a normalized value of I. For example, each inductor  1111 - 1114 ,  1116 - 1119  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  1101 - 1  and  1102 - 2 , and the maximum inductance value between input node  1101 - 2  and  1102 - 2  (i.e., when all switches  1131 - 1134 ,  1136 - 1139  are in an open state) would be about NxJ, plus any trace inductance that may be present in the network  1100  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)×I (plus trace inductance). 
     As also explained in conjunction with  FIG. 5 , above, in an alternate embodiment, the inductors  1111 - 1114 ,  1116 - 1119  may have different values from each other. For example, moving from the input node  1101 - 1  toward the output node  1102 - 1 , the first inductor  1111  may have a normalized inductance value of I, and each subsequent inductor  1112 - 1114  in the series may have a larger or smaller inductance value. Similarly, moving from the input node  1101 - 2  toward the output node  1102 - 2 , the first inductor  1116  may have a normalized inductance value of I, and each subsequent inductor  1117 - 1119  in the series may have a larger or smaller inductance value. For example, each subsequent inductor  1112 - 1114  or  1117 - 1119  may have an inductance value that is a multiple (e.g., about twice or half) the inductance value of the nearest downstream inductor  1111 - 1114  or  1116 - 1118 . The example of Table 1, above, applies also to the first series inductance path between input and output nodes  1101 - 1  and  1102 - 1 , and the second series inductance path between input and output nodes  1101 - 2  and  1102 - 1 . More specifically, inductor/switch combinations  1111 / 1131  and  1116 / 1156  each are analogous to inductor/switch combination  501 / 511 , inductor/switch combinations  1112 / 1132  and  1117 / 1157  each are analogous to inductor/switch combination  502 / 512 , inductor/switch combinations  1113 / 1133  and  1118 / 1158  each are analogous to inductor/switch combination  503 / 513 , and inductor/switch combinations  1114 / 1134  and  1119 / 1159  each are analogous to inductor/switch combination  504 / 514 . 
     Assuming that the trace inductance through series inductors  1111 - 1114  in the minimum inductance state is about 10 nH, and the range of inductance values achievable by the series inductors  1111 - 1114  is about 10 nH (trace inductance) to about 400 nH, the values of inductors  1111 - 1114  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  1116 - 1119  may be similarly or identically configured. Of course, more or fewer than four inductors  1111 - 1114  or  1116 - 1119  may be included in either series combination between input and output nodes  1101 - 1 / 1102 - 1  or  1101 - 2 / 1102 - 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  1111 - 1114 ,  1116 - 1119  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 input impedance that is present during a defrosting operation. 
     As with the embodiment of  FIG. 10 , the third plurality of discrete inductors  1121 - 1123  corresponds to an “RF signal source matching portion” of the circuit. The third variable inductor, comprising the series arrangement of inductors  1121 - 1123 , where bypass switches  1141  and  1143  enable inductors  1121  and  1123  selectively to be connected into the series arrangement or bypassed based on control signals  1161  and  1163 . In an embodiment, each of inductors  1121 - 1123  may have equal values (e.g., values in a range of about 1 nH to about 100 nH. In an alternate embodiment, the inductors  1121 - 1123  may have different values from each other. For example, moving from the first input node  1101 - 1  toward the second input node  1101 - 2 , the first inductor  1121  may have a normalized inductance value of J, and each subsequent inductor  1122 ,  1123  in the series may have a larger or smaller inductance value. For example, inductor  1122  may have a value of 2*J, and inductor  1123  may have a value of 4*J, in some embodiments. 
     It should be understood that the variable impedance matching circuits  1000 ,  1100  illustrated in  FIGS. 10 and 11  are but two 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 networks, or may include passive networks that include 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. 
     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,  FIG. 12  is a perspective view of an example of a module  1200  that includes a double-ended variable impedance matching network (e.g., network  1000 ,  1100 ,  FIGS. 10, 11 ), in accordance with an example embodiment. The module  1200  includes a printed circuit board (PCB)  1204  with a front side  1206  and an opposite back side  1208 . The PCB  1204  is formed from one or more dielectric layers, and two or more printed conductive layers. Conductive vias (not visible in  FIG. 12 ) may provide for electrical connections between the multiple conductive layers. At the front side  1206 , 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  of the PCB  1204 . Similarly, at the back side  1208 , 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  of the PCB  1204 . 
     According to an embodiment, the PCB  1204  houses an RF input connector  1238  (e.g., coupled to back side  1208  and thus not visible in the view of  FIG. 12 , but corresponding to connector  938 ,  FIG. 9 ) and a balun  1274  (e.g., coupled to back side  1208  and thus not visible in the view of  FIG. 12 , but corresponding to balun  974 ,  FIG. 9 ). The input connector  1238  is configured to be electrically connected to an RF subsystem (e.g., subsystem  310 ,  910 ,  FIGS. 3, 9 ) with a connection (e.g., connection  928 - 3 ,  FIG. 9 ) such as a coaxial cable or other type of conductor. In such an embodiment, an unbalanced RF signal received by the balun  1274  from the RF input connector  1238  is converted to a balanced signal, which is provided over a pair of balanced conductors (e.g., connections  928 - 4 ,  FIG. 9 ) to a double-ended input that includes first and second inputs  1201 - 1 ,  1201 - 2 . The connection between the input connector  1238  and the balun  1274 , and the connections between the balun  1274  and the inputs  1201 - 1 ,  1201 - 2  each may be implemented using conductive traces and vias formed on and in the PCB  1204 . In an alternate embodiment, as discussed above, an alternate embodiment may include a balanced amplifier (e.g., balanced amplifier  924 ′,  FIG. 9 ), which produces a balanced signal on connections (e.g., conductors  928 - 1 ′,  FIG. 9 ) that can be directly coupled to the inputs  1201 - 1 ,  1201 - 2 . In such an embodiment, the balun  1274  may be excluded from the module  1200 . 
     In addition, the PCB  1204  houses circuitry associated with a double-ended variable impedance matching network (e.g., network  972 ,  1000 ,  1100 ,  FIGS. 9-11 ). Accordingly, the circuitry housed by the PCB  1204  includes the double-ended input  1201 - 1 ,  1201 - 2  (e.g., inputs  1101 - 1 ,  1101 - 2 ,  FIG. 11 ), a double-ended output  1202 - 1 ,  1202 - 2  (e.g., outputs  1102 - 1 ,  1102 - 2 ,  FIG. 11 ), a first plurality of inductors  1211 ,  1212 ,  1213 ,  1214 ,  1215  (e.g., inductors  1111 - 1115 ,  FIG. 11 ) 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  1116 - 1120 ,  FIG. 11 ) 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  1121 - 1123 ,  FIG. 11 , 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  1124 ,  FIG. 11 ) coupled between nodes  1225  and  1226  (e.g., nodes  1125 ,  1126 ). 
     A plurality of switches or relays (e.g., not visible in the view of  FIG. 12 , but corresponding to switches  1131 - 1134 ,  1136 - 1139 ,  1141 ,  1143 ,  FIG. 11 , for example) may be coupled 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  1121 ,  1123 ,  FIG. 11 ) 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  1151 - 1154 ,  1156 - 1159 ,  1161 ,  1163 ,  FIG. 11 ), and thus to switch the inductors into or out of the circuit, as described previously. As shown in  FIG. 12 , fixed-value inductors  1215 ,  1220  (e.g., inductors  1115 ,  1120 ,  FIG. 11 ) may be formed from relatively large coils, although they may be implemented using other structures as well. As shown in the embodiment of  FIG. 12 , 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  940 ,  950 ,  FIG. 9 ) of the system. 
     In various embodiments, the circuitry associated with the RF subsystem (e.g., RF subsystem  310 ,  910 ,  FIGS. 3, 9 ) 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 ,  910 ,  FIGS. 3, 9 ), 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  910 ,  FIGS. 3, 9 ). Accordingly, the circuitry housed by the PCB  1302  includes system controller circuitry  1312  (e.g., corresponding to system controller  312 ,  912 ,  FIGS. 3, 9 ), RF signal source circuitry  1320  (e.g., corresponding to RF signal source  320 ,  920 ,  FIGS. 3, 9 , including an RF signal generator  322 ,  922  and power amplifier  324 ,  325 ,  924 ), power detection circuitry  1330  (e.g., corresponding to power detection circuitry  330 ,  930 ,  FIGS. 3, 9 ), and impedance matching circuitry  1334  (e.g., corresponding to first matching circuitry  334 ,  934 ,  FIGS. 3, 9 ). 
     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, 9 . 
     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 ,  980 ,  FIGS. 3, 9 ) and other functionality. Connector  1316  may be configured to connect with a variable matching circuit (e.g., circuit  372 ,  972 ,  FIGS. 3, 9 ) 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 ,  936 ,  FIGS. 3, 9 ) 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 ,  928 - 3 ,  FIGS. 3, 9 ) to a variable matching subsystem (e.g., subsystem  370 ,  970 ,  FIGS. 3, 9 ). In an alternate embodiment, components of the variable matching subsystem (e.g., variable matching network  370 , balun  974 , and/or variable matching circuit  972 ,  FIGS. 3, 9 ) also may be integrated onto the PCB  1302 , in which case connector  1336  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 ,  FIG. 12 ) may be electrically connected together, and connected with other components, to form a defrosting apparatus or system (e.g., apparatus  100 ,  200 ,  300 ,  900 ,  FIGS. 1-3, 9 ). For example, an RF signal connection may be made through a connection (e.g., conductor  928 - 3 ,  FIG. 9 ), such as a coaxial cable, between the RF connector  1338  ( FIG. 13 ) and the RF connector  1238  ( FIG. 12 ), and control connections may be made through connections (e.g., conductors  916 ,  FIG. 9 ), such as a multi-conductor cable, between the connector  1316  ( FIG. 13 ) and the connector  1230  ( FIG. 12 ). 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  940 ,  950 ,  FIG. 9 ) may be connected to the outputs  1202 - 1 ,  1202 - 2 . 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 ,  960 ,  FIGS. 1, 3, 9 ), 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. 14 and 15 . More specifically,  FIG. 14  is a flowchart of a method of operating a defrosting system (e.g., system  100 ,  210 ,  220 ,  300 ,  700 ,  900 ,  FIGS. 1-3, 7, 9 ) with dynamic load matching, in accordance with an example embodiment. 
     The method may begin, in block  1402 , when the system controller (e.g., system controller  312 ,  912 ,  FIGS. 3, 9 ) 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 ,  964 ,  FIGS. 3, 9 ) into the system&#39;s defrosting cavity (e.g., cavity  360 ,  960 ,  FIGS. 3, 9 ), 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 ,  980 ,  FIGS. 3, 9 ). 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 weight. 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 ,  990 ,  FIGS. 3, 7, 9 ) of the system. Information regarding the load weight may be received from the user through interaction with the user interface, or from a weight sensor (e.g., sensor  390 ,  790 ,  990 ,  FIGS. 3, 7, 9 ) of the system. As indicated above, receipt of inputs indicating the load type, initial load temperature, and/or load weight is optional, and the system alternatively may not receive some or all of these inputs. 
     In block  1404 , the system controller provides control signals to the variable matching network (e.g., network  370 ,  400 ,  972 ,  1000 ,  1100 ,  FIGS. 3, 4, 9-11 ) to establish an initial configuration or state for the variable matching network. As described in detail in conjunction with  FIGS. 4, 5, 10, and 11 , the control signals affect the inductances of variable matching networks (e.g., networks  410 ,  411 ,  1011 ,  1016 ,  1021 ,  FIGS. 4, 10 ) within the variable matching network. For example, the control signals may affect the states of bypass switches (e.g., switches  511 - 514 ,  1131 - 1134 ,  1136 - 1139 ,  1141 ,  1143 ,  FIGS. 5, 11 ), which are responsive to the control signals from the system controller (e.g., control signals  521 - 524 ,  1151 - 1154 ,  1156 - 1159 ,  1161 ,  1163 ,  FIGS. 5, 11 ). 
     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 ,  920 ,  FIGS. 3, 9 ) or the final stage power amplifier (e.g., power amplifier  325 ,  924 ,  FIGS. 3, 9 ), and a second portion of the variable matching network may be configured to provide a match for the cavity (e.g., cavity  360 ,  960 ,  FIGS. 3, 9 ) plus the load (e.g., load  364 ,  964 ,  FIGS. 3, 9 ). For example, referring to  FIG. 4 , 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. 
     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, weight, and so on), and trace  1520  corresponds to a second load (e.g., having a second type, weight, 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. 4 , 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. 10 , this translates to a relatively high inductance for variable inductance networks  1011  and  1016 , and a relatively low inductance for variable inductance network  1021 . 
     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. 4 ) 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/weight/temperature information known to the system controller a priori. If no a priori load type/weight/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  or  1011 / 1016 ) 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  1021 ) of about 10 percent of the network&#39;s maximum value. Assuming each of the variable inductance networks has a structure similar to the 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  1011 / 1016 ) has state 12 (i.e., about 80 percent of the maximum possible inductance of network  411  or  1011 / 1016 ), and the RF signal source match network (e.g., network  410  or  1021 ) 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  1011 / 1016 ) 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  1021 ) 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  1011 / 1016 ) has state 6 (i.e., about 40 percent of the maximum possible inductance of network  411  or  1011 / 1016 ), and the RF signal source match network (e.g., network  410  or  1021 ) has state 2 (i.e., about 10 percent of the maximum possible inductance of network  410  or  1021 ). 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. 
     Referring again to  FIG. 14 , once the initial variable matching network configuration is established, the system controller may perform a process  1410  of adjusting, if necessary, 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. According to an embodiment, this process includes causing the RF signal source (e.g., RF signal source  320 ,  920 ,  FIGS. 3, 9 ) 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  940 ,  950 ,  FIGS. 3, 9 ), in block  1412 . The system controller may control the RF signal power level through control signals to the power supply and bias circuitry (e.g., circuitry  326 ,  926 ,  FIGS. 3, 9 ), 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 ,  924 ,  FIGS. 3, 9 ) that are consistent with the 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. A relatively low power level signal during the match adjustment process  1410  is desirable to reduce the risk of damaging the cavity 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 networks (e.g., due to arcing across the switch contacts). 
     In block  1414 , power detection circuitry (e.g., power detection circuitry  330 ,  930 ,  930 ′,  930 ″,  FIGS. 3, 9 ) then measures the reflected and (in some embodiments) forward power along the transmission path (e.g., path  328 ,  928 ,  FIGS. 3, 9 ) 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 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 for future evaluation or comparison, in an embodiment. 
     In block  1416 , 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, whether or not the match provided by the variable impedance matching network is acceptable (e.g., the reflected power is below a threshold, or the ratio is 10 percent or less, 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. 
     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  1418 , by reconfiguring the variable impedance matching network. For example, this 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 within the network (e.g., by causing the variable inductance networks  410 ,  411 ,  1011 ,  1016 ,  1021  to have different inductance states, or by switching inductors  501 - 504 ,  1111 - 1114 ,  1116 - 1119 ,  1121 ,  1123 ,  FIGS. 4, 5, 10, 11  into or out of the circuit). After reconfiguring the variable inductance network, blocks  1414 ,  1416 , and  1418  may be iteratively performed until an acceptable or best match is determined in block  1416 . 
     Once an acceptable or best match is determined, the defrosting operation may commence. Commencement of the defrosting operation includes increasing the power of the RF signal supplied by the RF signal source (e.g., RF signal source  320 ,  920 ,  FIGS. 3, 9 ) to a relatively high power RF signal, in block  1420 . Once again, the system controller may control the RF signal power level through control signals to the power supply and bias circuitry (e.g., circuitry  326 ,  926 ,  FIGS. 3, 9 ), 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 ,  924 ,  FIGS. 3, 9 ) that are consistent with the desired signal power level. For example, the relatively high power 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 ,  930 ,  930 ′,  930 ″,  FIGS. 3, 9 ) then periodically measures the reflected power and, in some embodiments, the forward power along the transmission path (e.g., path  328 ,  928 ,  FIGS. 3, 9 ) 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. The system controller may store the received power measurements, and/or the calculated ratios, 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, 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 S11 parameters 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, 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 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 again performing process  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 ,  FIG. 3 ) 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 decreasing the cavity match inductance (e.g., by decreasing the inductance of variable inductance network  411 ,  FIG. 4 ) and by increasing the RF signal source inductance (e.g., by increasing the inductance of variable inductance network  410 ,  FIG. 4 ). 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 the iterative process  1410  of 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  1418 , 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. 4 ) and higher inductances (for the RF signal source match, or network  410 ,  FIG. 4 ). By selecting impedances 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  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  1416 , the defrosting operation is resumed in block  1414 , 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, 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 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. 14 , 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 ,  980 ,  FIGS. 3, 9 ) 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. 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 ,  922 ,  FIGS. 3, 9 ) and/or may cause the power supply and bias circuitry (e.g., circuitry  326 ,  926 ,  FIGS. 3, 9 ) to discontinue provision of the supply current. In addition, the system controller may send signals to the user interface (e.g., user interface  380 ,  980 ,  FIGS. 3, 9 ) 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  FIG. 14  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. 
     An embodiment of a thermal increase system is coupled to a cavity for containing a load. The thermal increase system includes an RF signal source configured to supply an RF signal, a transmission path, an impedance matching network, power detection circuitry, and a controller. The transmission path is electrically coupled between the RF signal source and first and second electrodes that are positioned across the cavity. The impedance matching is electrically coupled along the transmission path, and the impedance matching network comprises a network of variable passive components. The power detection circuitry is configured to detect reflected signal power along the transmission path. The controller is configured to modify, based on the reflected signal power, one or more values of one or more of the variable passive components of the impedance matching network to reduce the reflected signal power. 
     An embodiment of a method of operating a thermal increase system that includes a cavity includes supplying, by an RF signal source, one or more RF signals to a transmission path that is electrically coupled between the RF signal source and first and second electrodes that are positioned across the cavity. The method further includes detecting, by power detection circuitry, reflected signal power along the transmission path, and modifying, by a controller, one or more values of one or more of variable passive components of an impedance matching network that is electrically coupled along the transmission path to reduce the reflected signal power. 
     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.