Abstract:
A measurement device is presented being configured to be connectable to an analyzer unit (comprising a network analyzer). The measurement device comprises a measuring unit and a calibration and control unit connected to and integral with the measuring unit. The calibration and control unit is configured to enable connection of the measuring unit to the analyzer unit. The calibration and control unit comprises a number of terminals of known RF reflection coefficients respectively and comprises a memory utility carrying recorded data indicative of said RF reflection coefficients and recorded data indicative of RF transfer coefficients of the calibration and control unit. This configuration enables calculation of the RF response of the measuring unit while remaining integral with the calibration and control unit.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is generally in the field of calibration techniques, and relates to a calibration method and system for use with an RF measurement device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Calibration is an important process in any measurement technique: the higher the precision of calibration, the better are the measurement results, i.e. the higher the measurement system sensitivity to small measurement variations. 
         [0003]    The RF calibration procedure is aimed at correcting measurement errors, thus ensuring that the response of a device under test (DUT) is recorded correctly. According to the conventional techniques of the kind specified, a vector network analyzer (VNA), including a signal transmitter/receiver, is frequently used; calibration loads are measured when connected to the appropriate signal ports of the VNA, such connection being carried out each time the calibration is to be performed. A calibration device includes a set of calibration loads or terminals with known loads. The calibration consists of switching (manually or automatically) between the calibration loads for sequentially measuring the response of the VNA for each of these loads, and by this determining how the signal propagation between the signal transmitting/receiving plane and the DUT connection plane is affected by a signal transmitting media in between these planes. Upon completing the calibration procedure, the calibration device is replaced by the DUT. 
         [0004]    Some examples of the calibration procedure of the kind specified are described in U.S. Pat. No. 5,434,511 and U.S. Pat. No. 6,914,436. 
       GENERAL DESCRIPTION 
       [0005]    There is a need in the art to facilitate an RF calibration procedure so as to, on the one hand, enable higher degree of precision of a calibration process, and, on the other hand, eliminate or at least significantly reduce a need for replacement between the calibration device and the DUT (measurement device). The latter feature is very important for various applications, for example those which do not allow frequent disconnection of a measurement device and/or do not allow access to the measurement device. 
         [0006]    The present invention solves the above problem by providing a novel RF calibration method and system, allowing immediate transfer of the signal transmitting/receiving plane to the plane of connection to the measurement device. This is implemented by using the calibration device having loads or terminals of the known (previously tested) loads, and integrating such calibration device with the measurement device. 
         [0007]    According to one broad aspect of the invention, there is provided a measurement device configured to be connectable to an analyzer unit, comprising a network analyzer, the measurement device comprising a measuring unit and a calibration and control unit connected to and integral with said measuring unit, said calibration and control unit being configured to enable connection of the measuring unit to the analyzer unit, said calibration and control unit comprising a number of terminals of known RF reflection coefficients respectively and comprising a memory utility carrying recorded data indicative of said RF reflection coefficients and recorded data indicative of RF transfer coefficients of the calibration and control unit, thereby enabling calculation of the RF response of the measuring unit while remaining integral with the calibration and control unit. 
         [0008]    As the calibration and control unit is integral with the measurement unit, the calibration process (of the RF response) of the measuring unit may be carried out at any desired time, e.g. continuously or periodically. 
         [0009]    The calibration unit comprises a controller utility, and a controllably operable switch enabling selective connection of each one of said terminals and said measurement unit to the network analyzer. The controller utility of the calibration and control unit is connectable to a controller of the analyzer unit. 
         [0010]    According to another broad aspect of the invention, there is provided a calibration and control unit configured to be interconnected between an analyzer unit and a measuring unit by connectors capable for RF signal transmission, and to be connected to the analyzer unit via a data transmitting connector, the calibration and control unit comprising: a number of terminals of known RF reflection coefficients respectively; and a memory utility comprising recorded data indicative of the RF reflection coefficients of the terminals and recorded data indicative of RF transfer coefficients of the calibration and control unit. 
         [0011]    According to yet another broad aspect of the invention, there is provided a method for use in calibrating a measurement device by connecting it to an analyzer unit comprising a network analyzer, the method comprising: 
         [0012]    providing a calibration and control unit comprising: a number of terminals of known RF reflection coefficients respectively, and a memory utility carrying recorded data indicative of said RF reflection coefficients and recorded data indicative of RF transfer coefficients of the calibration and control unit; 
         [0013]    integrating said calibration and control unit with the measuring unit, in a manner allowing connection of the measuring unit to the analyzer unit via said calibration and control unit; and 
         [0014]    calibrating the measuring unit by utilizing said known RF reflection coefficients and the RF transfer coefficients of the calibration and control unit, to determine an RF response of the measuring unit while remaining integral with the calibration and control unit. 
         [0015]    The recorded data indicative of said RF reflection coefficients and of RF transfer coefficients of the calibration and control unit is provided by carrying out a pre-calibration procedure (phase  1  calibration) before integrating the calibration and control unit within the measurement device. 
         [0016]    According to yet further aspect of the invention, there is provided a method for use in RF calibrating a measuring unit by connecting it to an analyzer unit comprising a network analyzer, the method comprising integrating a calibration and control unit with the measuring unit, in a manner allowing connection of the measuring unit to the analyzer unit via said calibration and control unit, where said calibration and control unit comprises a number of terminals of known RF reflection coefficients respectively and comprises a memory utility in which data indicative of said RF reflection coefficients and data indicative of RF transfer coefficients of the calibration and control unit is provided and stored before said calibration and control unit is integrated with the measuring unit, thereby enabling the RF calibration of the measuring unit when required without a need for disconnecting the measuring unit and the calibration and control unit. 
         [0017]    Applications for which the invention may be useful include: environmental monitoring, for examples in oceans, in remote locations; geophysical, for example sensing in bore-holes, sensing in quarries; industrial, for example production line monitoring, monitoring in processing plants; medical: implantable devices, use in sterile environment. Conditions in which the use of the invention may be advantageous include: sensor position is variable relative to console; inaccessibility to sensor location; sensor is disposable; sensor is operated in a hazardous environment; sensor is sterile; sensor is operated in a range of temperatures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  is a block diagram of a measurement system according to the invention; 
           [0020]      FIGS. 2A and 2B  show examples of the configuration of the measurement system of  FIG. 1 ; 
           [0021]      FIGS. 3 and 4  exemplify the configuration of a calibration and control unit suitable to be used in the measurement device of the present invention; 
           [0022]      FIGS. 5A and 5B  show a flow chart of an example of a calibration method of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0023]    Referring to  FIG. 1 , there is illustrated, by way of a block diagram, a measurement system, generally designated  10 , configured and operable according to the invention. The system  10  includes a measurement device  12  connectable to an analyzer  16 . The measurement device  12  of the present invention includes a measuring unit (probe)  12 A and a calibration and probe control (CPC) unit  12 B. The analyzer  16  includes a network analyzer  14 , and also a suitable communication unit (not shown) for handling digital and/or analog communication with the CPC unit  12 B. The network analyzer  14  may be of any known suitable type and therefore need not be described in details, except to note that it is configured and operable for transmitting and receiving RF signals. Network analyzer  14  may be configured and operable as a vector network analyzer (VNA), for recording both the relative amplitude and phase of RF signals. 
         [0024]    Network analyzer  14  is configured for transmitting and receiving RF signals via its signal ports, analyzing the received signals to determine the amplitude and, optionally, phase thereof which are indicative of the signal interaction with calibration loads, and delivering the calibration correction parameters. Network analyzer  14  is also configured for measuring a RF response of the measurement device  12  using the calibration correction parameters. 
         [0025]    Additionally, the analyzer  16  is configured with additional features, responsible for security issues to prevent reuse of the measurement device  12  or installation of other non-authorized measurement device in the system, as will be described further below. Analyzer  16  may also provide at least one of the following facilities to measurement device  12 : electrical power supply, means for handling digital and/or analog communication with measurement device  12 , vacuum/pressure communication  19 , a liquid dispensing line, optical signal communication, ultrasound signal communication, as well as provide control and power to an ablative/cutting apparatus/tool in measurement device  12 , user and/or machine input and/or output, and control of other types of probes to be used in measurement device  12 . 
         [0026]    Reference is made to  FIGS. 2A and 2B  illustrating specific but not limiting examples of the configuration of the measurement system  10 . The measurement device  12  includes a probe  12 A and a CPC  12 B integral with the probe  12 A, which are accommodated in a common housing  12 C. The probe  12 A includes a sensor  11  with its support circuit  13  and is connected to the CPC  12 B via a cable with an appropriate connector. 
         [0027]    In the example of  FIG. 2A , there is only one RF signal connection (RF port connection) between the analyzer  16  and the measurement device  12 . This RF connection may be used, for example, but not limited to, when the sensor used in the probe  12 A is configured for operation in reflection mode. Also, as exemplified in  FIG. 2A , an optical signal cable  15  is optionally provided for transmitting optical signals to and/or from the sensor  11  in the probe  12 A. In the example of  FIG. 2B , there are two RF signal connections (RF port connections) between the analyzer  16  and the measurement device  12 . These could be used, for example, but not limited to, when the sensor  11  used in the probe  12 A is configured for operation both in the reflection and transmission modes, or, for example, when sensor  11  is configured for operation with two reflection mode signals. As also shown in the example of  FIG. 2B , a vacuum/pressure communication line is used for providing vacuum/pressure communication  19  to the probe  12 A. 
         [0028]    It should be appreciated that embodiments of the present invention may utilize more than two RF signal connections between analyzer  16  and measurement device  12 . There may generally be n RF signal connections (RF port connections) between the analyzer unit and measurement device, n being an integer greater than zero. 
         [0029]      FIG. 2A  shows more specifically the interface of the analyzer  16  to the measurement device  12 . As shown cables C 1  and C 2  are provided, where cable C 1  is configured as a cable for RF signal transmission, for example a coaxial cable, and cable C 2  is configured for data transfer and control between units  12  and  16  and may be an USB cable, a coaxial cable, a cable for transmitting RS232 protocol data, a cable for transmitting GFSK protocol data, or any other cable known in the art as usable for supporting digital and/or analog communication. Cable C 2  may also support electrical power supply to measurement device  12 . The CPC unit  12 B is connected to cable C 1  via an RF grade connector C 3  (for example, male SMA), and is connected to cable C 2  via a connector C 4  (for example SMA, or USB), thus enabling the RF connection and data connection between measurement device  12  and analyzer  16 . On the probe  12 A side, the CPC unit  12 B is connected to the probe via an RF grade connector C 5  (for example SMA). The RF grade connectors are appropriately selected to provide an interface that can define a calibration plain and give repeatable measurement results. The RF grade connectors are used for connecting all RF signal transmission paths (ports) between measurement device  12  and analyzer  16 , and between CPC  12 B and probe  12 A. Examples of RF grade connectors include: N-Type, BNC, SMA SMB, MCX, MMCX, U.FL. 
         [0030]    Preferably, the CPC unit (printed circuit board) is enclosed within a housing, having an RF cover, to provide mechanical strength and electromagnetic immunity to the CPC unit  12 B. Mechanical strength of the housing enables better calibration by eliminating geometrical distortion, which may occur, for example, due to mechanical stresses or environmental changes of the CPC unit. This distortion may result in changes in the propagation of RF signals within the CPC unit, leading to degradation in calibration performance. Electromagnetic immunity of the housing enables better calibration by reducing RF interference of the CPC unit  12 B with the probe  12 A, and by reducing RF interference of external RF sources with the CPC unit  12 B. Connectors of CPC unit  12 B may be integrated into the housing. Housing may be constructed to enable operation of measurement device  12  in various environmental conditions, and to enable sterilization of the measurement device, by use of radiation and/or gas. 
         [0031]    It should be noted, although not specifically shown, that the measurement device  12  may optionally include output (e.g. LED indicators) and/or input (e.g. control button) elements. These elements are associated with the probe  12 A and used for controlling the operation thereof. 
         [0032]      FIG. 3  illustrates a non limiting example of the configuration of the CPC unit  12 B, suitable to be used in the system configuration of the above-described example of  FIG. 2A , where there is only one RF signal connection (RF port connection) between an analyzer  16  and a measurement device  12 . In this example, both data communication and power supply to measurement device  12  are delivered via a coaxial cable C 2  connected to connector C 4 , with the data communication signals “riding” (superimposed) on a DC power supply voltage. As shown, the CPC unit  12 B includes a switch  20  connected to a number of terminals, a controller/processor  22 , a memory utility  24  to be used as code and data storage, a power conversion/stabilization unit  26 , and a bias utility (BIAS-TEE)  28 . Bias utility  28  is used to separate between the DC voltage supply from the analyzer  16  and the data communication signals riding on that DC. This voltage supply is filtered and appropriately regulated by power conversion/stabilization unit  26  (e.g. 5 Volt are converted to 3.3 volt supply by a linear voltage regulator (LVR)). This filtered and appropriately regulated voltage supply provides operating voltages for switch  20 , controller/processor  22 , memory  24 , and other components within the measurement device that require electrical power to operate. 
         [0033]    It should be appreciated that power/voltage supply to measurement device  12  may be provided using a separate electrical connection within cable C 2 . In this case, there is no need to use a bias utility  28 . 
         [0034]    It should also be appreciated that power/voltage supply to measurement device  12  can be provided by a power/voltage source within the measurement device  12 , such as a battery. In this case, there is no need for cable C 2  to accommodate for power supply to the measurement device  12 . 
         [0035]    The switch  20  may be soldered to the CPC circuit (printed circuit board). The switch has connection ports J 1  and J 3  associated with connections C 3  and C 5 , and is also connected to a plurality of calibration loads. In the present example, the switch is associated with three calibration load terminals including a SHORT termination J 2 , OPEN termination J 4 , and LOAD termination J 5 . It is appreciated that other types of calibration loads may be used, as well as the number of calibration loads may be greater than three. The number and/or type of calibration loads may be selected in order to enhance the calibration accuracy. It is appreciated that switch  20  may have other than four switching states. Additionally, there may be more than one switch in the CPC unit, operable to provide a plurality of calibration loads. 
         [0036]    Memory utility  24  may include volatile and/or non-volatile memory types. Examples of volatile memory types include: static random access memory (RAM), dynamic RAM. Examples of non-volatile memory types include: EEPROM, EPROM, flash memory, read only memory. 
         [0037]    In some embodiments, memory  24  stores data characterizing the plurality of calibrations loads. Data characterizing the calibration loads may be frequency dependant. Data characterizing the calibration loads may also include data indicative of the dependence of the calibration loads&#39; values on environmental parameters/conditions (for example: temperature, humidity, acceleration, mechanical agitation). Memory  24  also stores data characterizing the CPC unit  12 B, which may be frequency dependant. This data may include the full 2-port complex parameters, transfer coefficients, (S 11 , S 22 , S 12 , and S 21 ) of the CPC unit. Data characterizing the CPC may also include data indicative of the dependence of the CPC response on environmental parameters/conditions (for example: temperature, humidity, acceleration, mechanical agitation). 
         [0038]    Memory  24  may also store data characterizing probe  12 A. This data may include RF calibration data for RF signal propagation between connector C 5  and the sensor. This data may also include information regarding the specific response of the sensor support circuit and/or the sensor. This data may also include data indicative of the dependence of the probe&#39;s response on environmental parameters/conditions (for example: temperature, humidity, acceleration, mechanical agitation). 
         [0039]    CPC unit may also include sensors to record values of environmental parameters. Also, the CPC unit may include sensors for determining the location/position of the measurement unit  12 , for example a GPS receiver. 
         [0040]    The CPC unit may be constructed such that RF signal propagation paths within it are in the form of Transmission lines (TL). The use of TL is useful, for example, for eliminating impedance mismatch, for reducing radiation, for reducing cross-talk within the CPC unit. Positioning of components on the CPC circuit board, and design of the CPC circuit board may be such as to reduce disturbance of RF signal propagation within the CPC unit. These disturbances may be associated with interference, losses, cross-talk, and noise. CPC unit may be constructed so that its size is typically less than half a wavelength of the RF signals that propagate through it. For example, for RF signals of about 1 GHz, the CPC&#39;s largest dimension size should be less than 10 cm. This is useful for eliminating cavity resonances within the CPC unit. CPC unit may be constructed so that its size is sufficiently small to minimize errors related to phase changes. 
         [0041]    As exemplified more specifically in  FIG. 4 , the ports J 1  and J 3  of the switch (which may for example be AS204-80 commercially available from Skyworks Solutions, or HMC345LP3 commercially available from Hittite Microwave Corporation) are connected to the connections C 3  and C 5  (in  FIG. 3 ) via 4700 pF capacitors. As also shown in the figure, the SHORT termination J 2  is shorted to the ground through a 4700 pF capacitor, and the LOAD termination J 5  is shorted to the ground through a resistor (50 Ohm SMT resistor) and through a 4700 pF capacitor. It should be understood that the ports of the switch are connected to 4700 pF capacitors in order to block the DC voltage that arises in use of certain types of switches. The open termination does not need any capacitor, and in the load termination it is best to place the capacitor after a resistor. It is appreciated that if the switch type used does not result in a DC voltage present at its ports (for example TS3V330 and/or TS5A3359 commercially available from Texas Instruments), there is no need to use capacitors. 
         [0042]    Turning back to  FIG. 3 , during the calibration process, automated switching between the terminations J 2 , J 4  and J 5  is carried out using the controlled switch  20  on the CPC unit  12 B. The S parameters, transfer coefficients, of the CPC (S 11 , S 12 , S 21 , S 22 ) and the reflection coefficient of each calibration termination path (S 11 open, S 11 short and S 11 load) are stored in the memory utility  24 . The operation of the memory utility  24  and the switching of the switch  20  are controlled by the microcontroller  22 . Optionally, this controller  22  also controls the output (e.g. LED indicators) and/or input (e.g. control button) elements within the measurement device  12 . The microcontroller  22  also communicates with a microcontroller (CPU) of the analyzer  16  via appropriate matching circuits for transferring data between analyzer  16  and measurement device  12 , data to be further used for the calibration process. Additionally, the microcontroller  22  is preferably configured and operable to authenticate the measurement device  12  and to disable the reuse of the measurement device, as will be described further below. 
         [0043]    According to the invention, the calibration procedure is a two-phase procedure including preliminary calibration (phase  1 ) which is carried out before integrating the CPC unit  12 B with the measurement device  12 . This allows for further “actual” calibration (phase  2 ) during the device  12  operation without a need for dissembling the CPC unit  12 B from the measurement device  12 A. Thus, in addition to the known in the art calibration procedure, which transfers the measurement plane from the VNA output to the input connector of a calibration unit, the measurement plane is immediately/instantaneously transferred from the analyzer output plane to the plane of connection to a measurement unit (probe  12 B) within the measurement device. 
         [0044]    Reference is made to  FIGS. 5A and 5B  exemplifying a flow chart of a calibration method of the present invention. This example relates to the system configuration of  FIG. 2A . It should however be understood that the same procedure may be employed when n RF signal connections (ports) are present and/or when the number of calibration loads is greater than three. 
         [0045]    Initially, as illustrated in  FIG. 5A , the calibration phase  1  is carried out off line, namely before the calibration unit is integrated with the measurement device. In this calibration phase  1 , the complex reflection coefficients (S 11 ) of the calibration loads S 11 open, S 11 short, and S 11 load are determined (step  100 ). Then, the full 2-port complex parameters, transfer coefficients, (S 11 , S 22 , S 12 , and S 21 ) of the CPC unit  12 B are determined (step  104 ). Step  104  is carried out when switch  20  is directed to port J 3 , that is the RF signal path within the switch is from port J 1  to port J 3 . The reflection coefficient S 11 open corresponds to S 11  of J 1  when connected to J 4 ; S 11 short corresponds to S 11  of J 1  when connected to J 2 ; and S 11 load corresponds to S 11  of J 1  when connected to J 5 . 
         [0046]    These S-parameters described above may be obtained in the standard manner known in the art. In the second stage of this calibration phase  1 , data indicative of the above described S-parameters and the calibration loads (S 11 open, S 11 short, and S 11 load) is stored in the memory utility  24  (steps  102  and  106 ). Optionally, the above described S-parameters data may be fitted, and the fitting parameters are stored in the memory utility  24 . The fitting enables the use of lesser memory storage space within memory utility  24 . 
         [0047]    Then, the measurement device is assembled by integrating a measuring unit (probe)  12 A with the CPC unit  12 B containing in its memory utility  24  data indicative of the above described S-parameters and the calibration loads (S 11 open, S 11 short, and S 11 load), measured during the phase  1  calibration. From this stage on, the measuring unit  12 A and the calibration and control unit  12 B are formed as an integral measurement device  12 . 
         [0048]    In operation of measurement device  12 , when connected to analyzer  16 , phase  2  calibration is carried out on line, utilizing the stored parameters, to calculate the corrections to the measured RF signals in order to transfer the signal measurement plane from the analyzer output plane to the plane of connection to the probe  12 B within the measurement device. This is exemplified in  FIG. 6B . The measured signal is corrected in two stages. 
         [0049]    Measurement device  12  is first connected to analyzer unit  16  (step  200 ), and thus to network analyzer utility  14  thereof, via cables C 1 , C 2 , and additional cables if there are more signal ports. Each cable may be connected separately. Optionally, all cables may be incorporated into a dedicated connector, which is connected to a matching connector on analyzer  16 . The use of a dedicated connector is advantageous, as it provides easier connection, faster connection and removal, and a high quality RF signal connection path. Connector may incorporate other cables and/or connections between analyzer unit  16  and measurement device  12 . For example: vacuum/pressure communication line, liquid dispensing line, optical signal cable, ultrasound sensor cable, ablative/cutting apparatus/tool cable, or additional cables. Following connection, the stored reflection coefficient data (S 11 ) of each of the calibration loads within the CPC unit and the full 2-port parameters (S 11 , S 22 , S 12 , S 21 ) of the CPC are retrieved from memory  24  of the CPC unit to the analyzer  16  (step  202 ). 
         [0050]    Then, after the measurement device  12  is positioned at a measurement location, the switch  20  is operated to selectively direct the RF signal path from network analyzer utility  14  to each of the calibration loads (J 2 , J 4  and J 5 ) within the CPC unit, and network analyzer utility  14  concurrently measures the reflection coefficients of each of the calibration loads within the CPC unit, and measured data recorded in the analyzer  16  (step  204 ). Here, 
         [0051]    Γ 1  is the measured reflection coefficient corresponding to the switch directed to open; 
         [0052]    Γ 2  is the measured reflection coefficient corresponding to the switch directed to short; 
         [0053]    Γ 3  is the measured reflection coefficient corresponding to the switch directed to load. 
         [0054]    Measuring Γ 1 , Γ 2 , and Γ 3  only when measurement device  12  is in place ensures that the calibration accuracy will not be affected by spatial variations in the relative position of measurement device  12  and analyzer unit  16 , and by changes in the positioning of the cables connecting them. 
         [0055]    The analysis of the reflection and transmission coefficients provides that: 
         [0000]    
       
         
           
             
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         [0000]    wherein Γ m  is the measured reflection coefficient; Γ a  is the actual reflection coefficient;
 
Ed=S 11  is the directivity error; Ert=S 21 *S 12  is the reflection tracking error; and
 
Es=S 22  is the source match error.
 
         [0056]    When correcting a linear system, relationship between the measured reflection coefficient and the actual reflection coefficient is: 
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         [0057]    After measuring the reflections from each one of the calibration terminations (Γ 1 , Γ 2 , Γ 3 ) and obtaining the actual reflections from the memory (S 1 =S 11 open, S 2 =S 11 short, S 3 =S 11 load) as determined during the preliminary (phase  1 ) calibration, the three errors can be calculated using the following formulas: 
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         [0000]    wherein Corr 1  is the result of the first stage of the correction to the RF signal reflection, and it accounts to transferring the measurement plane from the network analyzer output plane to the entrance plane of the CPC unit (C 3 ). 
         [0058]    Following this correction, the signals measured when the switch was directed to the calibration loads within the CPC unit (Γ 1 , Γ 2 , Γ 3 ) are not used anymore, until an additional calibration sequences is initiated. 
         [0059]    From this point on the switch  20  is directed to J 3 , that is, the RF signal path within the switch is from J 1  to J 3 . Measurements by sensor may be performed at will, or automatically. 
         [0060]    For each measurement performed by the sensor, an additional compensation for the CPC unit internal 2-port response is performed by use of the S parameters (S 11 , S 22  and S 12 S 21 ) of the CPC unit that were retrieved from the memory utility: 
         [0000]    
       
         
           
             
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                       · 
                       
                         
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                               · 
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         [0000]    wherein Corr 2  is the result of the second stage of the correction to the RF signal reflection, and it accounts to transferring the measurement plane from the entrance plane of the CPC (C 3 ) to the input plane of the probe (C 5 ). The Corr 2  corrected RF signal reflection is the final, fully calibrated, RF response signal for reflection. 
         [0061]    It should be appreciated that when there is more than one RF port connection, multiple calibration and probe control (CPC) units (such as calibration and probe control unit  12 B) are incorporated within measurement device  12 ; stages  1  and  2  of the above-described calibration process are performed for each RF signal port connection, resulting in obtaining the fully calibrated, RF response signal for each of the RF ports. As shown in  FIG. 2B , a single control line (C 2 ) may be used to control the operation of all the multiple calibration and probe control (CPC) units. 
         [0062]    Optionally, an additional stage of correction to the RF signal reflection may be incorporated, for transferring the measurement plane from the input plane of the probe (C 5 ) to the sensor measurement plane or sensor support circuit measurement plane. This correction is similar in form to the correction for Corr 2 , and it uses the parameters characterizing the 2-port response of the probe cable and optionally the probe support circuit. These 2-port response parameters are pre-measured and are stored in the CPC memory  24 . In operation they are retrieved from memory  24  by the analyzer unit  16 . 
         [0063]    As described above, an additional calibration sequences (re-calibration) may be initiated “at any time”. 
         [0064]    The re-calibration may be invoked by the user, by the analyzer (either on a periodic basis, or based on some inputs), and/or by measurement device  12  (either on a periodic basis, or based on some inputs). This re-calibration is advantageous when spatial, temporal, environmental, or a combination of these conditions, have been changed. These changes can lead to degradation in the calibration of the RF signals, and thus re-calibration is important of keeping the accuracy level of the measured RF signals. 
         [0065]    In particular, calibration can be performed before each measurement, that is, “calibration on the fly”. 
         [0066]    The procedure of re-calibration “picks-up” from stage  3  of phase  2  of the calibration process. That is, when switch is selectively directed towards each of the calibration loads within the CPC, the reflection coefficients (S 11 ) are measured again and recorded in analyzer  16 . Next, correction Corr 1  is derived, as already detailed above. Following this correction, the switch  20  is directed to J 3 , that is, the RF signal path within the switch is from J 1  to J 3 . Measurements by sensor may be performed at will, or automatically. For each measurement performed by the sensor, the additional correction Corr 2  is carried out, as detailed above. Specifically, when working in mode of “calibration on the fly”, following each measurement by sensor, the procedure of re-calibration is re-initiated. 
         [0067]    As already described, re-calibration may be initiated due to sensing of environmental changes. Changes in environmental parameters may affect calibration, as the RF properties of components related to the path of the RF signal propagation may depend on environmental parameters. These changes can lead to degradation in the calibration of the RF signals, and thus re-calibration is important of keeping the accuracy level of the measured RF signals when values of environmental parameters are changed. 
         [0068]    Sensing of environmental parameters may be performed by sensors in analyzer  16  and/or in measurement device  12 . As already detailed, the data characterizing the dependence of components of measurement device  12  on the values of environmental parameters is stored in memory  24 . All the stages of the calibration procedure, described with reference to  FIGS. 5A-5B , can use the data characterizing the dependence of components of measurement device  12  on the values of environmental parameters, to enhance the accuracy of the calibration procedure. 
         [0069]    Sensing of environmental parameters can be performed during the measurement procedure. Changes in environmental parameters can be used to trigger (initiate) re-calibration. The sensing of environmental parameters can be performed autonomously by measurement device  12 . Triggering of re-calibration is initiated by measurement device  12 , when change(s) in environmental parameter(s) is/are higher than corresponding threshold(s) stored in memory  24 . The sensing of environmental parameters can be performed by analyzer  16 , by recording the values of environmental parameters, from appropriate sensors in measurement device  12  and/or from appropriate sensors in analyzer  16 . Triggering of re-calibration is initiated by analyzer  16 , when changes in environmental parameters are higher than thresholds stored in the database of analyzer  16 . 
         [0070]    Optionally, values of environmental parameters are continuously, or selectively, saved/stored/recorded in memory  24  and/or database of analyzer  16 . 
         [0071]    Memory utility  24  may also store Identification (ID) data of the specific measurement device, optionally in an encrypted form. As indicated above, the present invention also provides for preventing undesirable reuse of the measurement device or installation of other non-authorized measurement device in the system. To this end, each probe  12 A and/or each CPC unit  12 B is/are associated with a unique ID data. During production of a measurement device, its unique ID data is stored in memory  24 . In operation, microcontroller of the measurement device or of analyzer  16  operates to access the respective ID data in the memory utility, and carries out an identification process (for example: reads ID of the probe and/or CPC unit, performs authentication of ID data, compares ID data to database in analyzer  16 , compares measured data to specific response of the sensor), and only thereafter allows initiation of the use of measurement device  12 . 
         [0072]    Memory  24  may also store measurement device usage data, optionally in an encrypted form. For example: time of use, number of uses, number of calibration sequences performed, number of executed connections of measurement device  12  to analyzer  16 , number of measurements performed. This measurement device usage data may be used to monitor/“enforce”/limit/control the way in which the device  12  is used.