SU1114981A1 - Stand for measuring frequency characteristics of substance properties - Google Patents

Abstract

STAND FOR MEASUREMENT OF FREQUENCY CHARACTERISTICS OF PROPERTIES OF SUBSTANCES, containing capacitive transducer, inductance coil, coil-magnetisation. amplifier, and a source connected to ground, a two-port circuit with controlled negative resistance, a controllable capacitor unit consisting of series-connected separating cond sensor and varicap, control unit, switch, key, memory unit, calculator, digital recorder and subtractor, to the first input of which is connected the output of the memory unit, which is limited by the fact that, in order to automate the measurement process, four functional converters, clock generator, binary counter, sampling-storage unit, decoder, time interval meter, register, digital-to-analog converter and digital meter with information input, start input, information output and output a counting stop, the inductance being connected to the feedback circuit of the impedance multiplier, one terminal of the bias coil is grounded, and the other terminal of the bias coil, the gate of the field-effect transistor and a control input of a two-pole with controlled negative resistance are connected to the outputs of the first, second and third functional converters, respectively, correspondingly, the input of the multiplier is impedan (LS, information input of the digital cumeter, the first clamp of the double pole luchnik with controlled negative resistance effect is connected to the first § input of the switch, to the second input of which a constant voltage is applied, the second terminal of the two-pole with controlled negative resistance, the varicap anode and one of the two plates of the capacitive sensor Ground, the non-grounded data plate is connected to the output of the switch 00 and through the key and the separation capacitor are connected to the varicap cathode and to the output of the sampling-storage unit; the first output of the control unit is connected to the control input of the switch, the trigger input of the digital meter and the first input of the time interval meter, the second output of the control unit is connected to the control input of the key and the control input of the sampling-storage unit, to the input of the control unit

Description

A clock output connected to the binary counter input, the output of which is connected in parallel with the inputs of the first, second and third function converters and to the memory block input, is connected, the information output of the digital meter is connected to the second input of the time interval meter via a decoder, the output of the stop signal the digital cumeter meter is connected to the control input of the register, the output of the time interval meter is connected to the information input of the register, the output of which is connected to the second input of the subtractor, the output of which is connected through serially connected digital-to-analog converter and the fourth functional converter is connected to the information input of the sampling-storage unit, and the information output and output of the counting signal of the digital meter and the output of the subtractor are connected to the digital inputs whose output is connected to the digital recorder.

The invention relates to a measurement technique and can be used to measure the frequency characteristics of the dielectric constant and the dielectric loss tangent for the purpose of nondestructive testing of various materials and products.

A stand for measuring the frequency characteristics of the dielectric properties of substances is known, which contains, a sweep generator, capacitive voltage dividers, vector phase meters, phase shifters, computing units and panoramic recording instruments. The sweep generator is connected to two dividers consisting of symmetrically mounted to each other and with respect to the input circuits of capacitive sensors and additional capacitors, and the outputs of the sensors, in the field of one of which the test substance is placed, are connected to the inputs of the computing units, the outputs which are connected to the panoramic recording devices. The sweep speed and speed of the recording devices are consistent. During the measurement, the amplitude and phase of the voltage on the plate, the sensor in which the test substance is placed, varies depending on the frequency characteristics of the dielectric CBovlCTB substance, and the amplitude and phase of the voltage on the plate of another sensor that does not interact with

the substance remains unchanged due to the constancy of the parameters of the divider into which it enters, c. fairly wide range of frequencies. According to frequency characteristics of c. and tg6 the frequency characteristics of the dielectric parameters of the substance tll can be determined by calculation.

However, since the device does not provide any hardware sampling tools for analog-digital and digital-analog conversion, it is possible to use only analog vector-phase meters, which do not provide sufficient accuracy and sensitivity of tgtf measurements for practice at frequencies above 10-15 MHz, since the full identity of the phase-frequency and amplitude-frequency characteristics of the input circuits of devices connected to the outputs of the working and reference voltage dividers is ensured,

Closest to the invention is a device for measuring the dielectric characteristics of substances, comprising a capacitive measuring cell and a graduated measuring capacitor, connected through a switch to a measuring circuit consisting of an inductance coil and an auxiliary variable capacitor, supplying an amplitude detector and a control unit tuning a measuring circuit into a resonance, a unit for calculating the ratio of two voltages in series with a key, a memory unit and subtractor. Increasing the accuracy of measurements with simultaneous linearization of the output characteristic of the measured resistive conductivity is achieved by the fact that the first inputs of the subtractor and the unit for calculating the ratio of two voltages are connected to the output of the amplitude detector and through the key to the input of the memory unit whose output is connected to the second input of the reader, and the output j of the readout is connected to the second input j of the calculator of the ratio of two voltages, the control input of the switch and the key connected to the output of the unit controlling the measurement setting Yelnia loop into resonance. The circuit is tuned in resonance with an auxiliary capacitor connected to the measuring cell, but with the graduated (this closed) capacitor switched off, replacing the capacitance of the measuring cell. The measurement circuit is powered by a sinusoidal voltage generator. Determination of the dielectric characteristics of the substance filling the measuring | The cell is produced by comparing the voltages at the output of the amplitude detector with the cell connected and with the graduated capacitor connected. When a graduated measuring capacitor is connected to the circuit, the voltage UQ at the detector output is stored by the memory unit. This voltage is equal to the amplitude of the oscillations in the circuit in the absence of losses. With a measuring cell connected, this voltage is equal to U, respectively. The subtractor performs the UoU operation, and the block is the ratio of two voltages to the operation (Up,) / 1) V, which, with the gain of this unit, equal to the proper quality factor of the circuit, QQ allows one to fix a voltage numerically equal to the measured conductivity a substance equivalent to dielectric loss 2. However, in the known device, the reading of the dielectric constant values of the measured substance is possible only visually on a scale of a graduated capacitor, and this is a source of subjective errors and complicates the processing of information. In addition, the mechanical tuning of the auxiliary and graduated capacitors is inaccurate, and since the voltages are Ug and Uj. - the essence of the amplitude of oscillations in resonance, then this setting introduces a significant error in the result of measuring the loss of a substance, especially when measuring low-loss substances, since in this case two similar voltages are subtracted into the device (() - Changing the measurement frequency requires replacing the loop inductance loop and rebuilding the power generator, which makes it difficult to measure the frequency dependencies of the properties of substances and reduces the functionality of the device. expanding the frequency range R improving measurement accuracy. The goal is achieved by having a capacitance sensor, an inductance coil, a bias coil placed on the same magnetic core with an inductance coil, a multiplier and a turnout consisting of an amplifier with an open circuit, to measure the frequency characteristics of the properties of substances. a feedback circuit and a field-effect transistor, a drain connected to the output of the amplifier, and a source connected to the ground, a two-port circuit with a controlled negative resistance, a control capacitance box consisting of a series-connected separation capacitor and a varicap, a control unit, a switch, a key, a memory unit, a calculator, a digital recorder and a subtractor, to the first input of which the output of the memory unit is connected, four functional converters are introduced; generator, binary counter, sampling-storage unit, decoder, time interval meter, register, digital-to-analog converter and digital meter with information input, start input, information output and the output of the stop counting signal, where the inductor is connected to the feedback circuit of an impedance multiplier, one terminal of the bias coil is grounded, and the other terminal of the bias coil, the gate of the field-effect transistor and the control input of a two-pole with controlled negative resistance are connected to the outputs of the first, the second and third of its functional transducers, respectively, the input of the multiplier of the impedance, the information input of the digital meter, the first clip of the two-terminal with controlled negative resistance The event is connected to the first input of the switch, to the second input of which a constant voltage is applied, the second terminal of a two-terminal with controlled negative resistance, the varicap anode and one of the two plates of the capacitive sensor are grounded, the non-grounded cover of the sensor is connected to the output of the transducer, and through a switch and separator the capacitor is connected to the varicap cathode and to the output of the sampling-storage unit; the first output of the control unit is connected to the control input of the switch, the trigger input of the digital meter and the first input ohm of the time interval meter, the second output of the control unit is connected to the control input of the key and the control input of the sampling-storage unit, while the output of the clock generator of the clock generator connected to the input of the binary counter, the output of which is connected in parallel with the inputs .first, second, and third functional: transducers and with the input of the memory unit, the information output of the digital cumeter is connected to the second input of the time interval meter via a decoder, the output of the stop signal The digital meter counter is connected to the control input of the register, the output of the time interval meter is connected to the information input of the register, the output of which is connected to the second input of the subtractor, the output of which is connected to the information input of the sampling unit through the serially connected digital-to-analog converter and storage, and the information output and signal output of the stop meter of the digital cumeter, as well as the output of the subtractor are connected to the inputs of the calculator, the output of which is The digital video recorder. eleven . Fig, 1 shows the schematic wall yes; in fig. 2 -. timing charts explaining his work. The stand contains an inductance coil 1, an impedance multiplier 2 consisting of an amplifier 3 with an open feedback loop and a field-effect transistor 4, a bias coil 5, a two-terminal 6 with adjustable negative active c. Resistance, a control capacitance unit 7, a switch -, key 8, capacitive sensor 9, key 10, isolating to capacitor 11, varicap 12, first 13, second 14 and third 15 functional converters, clock generator 16, control unit 17, binary counter 18, sampling-storage unit 19, fourth functional transformation A transmitter 20, a digital-analog converter 21, a memory block 22, a digital meter 23, a subtractor 24, a decoder 25, a time interval meter 26, a register 27, a calculator 28 and a digital recorder 29. The digital meter 23 contains a pulse former 30, separate control key 31 closing and closing, pulse counter 32, rectifier 33j low-pass filter 34 and comparator 35. Inductor 1 closes the open feedback circuit of amplifier 3 of multiplier 2 impedance so that the input circuit of amplifier 3 together with capacitive sensor 9 obr zuet in parallel kolebatelnp a contour. Structurally, the inductance coil 1 is placed on a common magnetic core with a magnet coil 5. In parallel to this oscillatory circuit, a two-port 6 with adjustable negative active resistance is also connected. In parallel with the capacitive sensor 9, a varicap 12 is connected by means of a key 10. An information input of a digital meter 23 is also connected to the ungrounded common point of the oscillating circuit. The capacitive sensor 9 is connected either parallel to the inductor 1 or a constant voltage source (UQ) via a switch 8. Gate field-effect transistor 4 and the control input of the two-terminal 6 are connected to the outputs of the first 13 and third 15 functional converters, and the non-earth clamp of the bias coil is connected to the output The second functional converter 14, the inputs of the first, second and third-functional converters 13-15, as well as the input of the memory block 22 are connected to the output of the binary counter 18. The output of the memory block 22 is connected to the first, and the register 27 to the second input of the subtractor 24 , the output of which is through a serially connected digital-to-analog converter 21, a fourth functional converter 20 and a sampling-storage unit 19 to the varicap cathode 12. The clock generator 16 is connected to the input of the control unit 17 and the binary counter 18. The information input of the digital cumeter 23 is connected via the imaging unit 30 and the key 31 to the information input of the pulse counter 32, as well as through the rectifier 33, the low-pass filter 34 and the comparator 35 to the first control input of the key 31. To the second input of the comparator 35 is fed the constant voltage, while the output of the comparator 35, which is simultaneously the output of the stop signal of the digital cumeter 23, is connected to the control input of the register 27. The output of the counter 32 pulses, which is simultaneously the information output of the digital cumeter 23, Connected via the decoder 25 with the second input of the meter 26 time intervals, the output of which is connected to the information input of the register 27. The second control input of the key 31 with separate control closure and opening and the installation input of the counter 32 pulses combined to form the trigger input of the digital meter 23. The first output unit 17 is connected to the control input of the switch 8, the start input of the digital cumeter and the first input of the time interval meter 26. The second output of the control unit 17 is connected to the pilot inputs of the key 10 and the sample-storage unit 19. Information output cumer 23 and its output signal to stop the account, as well as the output of the subtractor 24 is connected to the inputs of the transmitter 28, the output of which is connected to the digital recorder 29.

The stand works as follows.

Measurement of the frequency characteristics of VEP properties (such as relative dielectric constant and tangent of dielectric loss angle tg cG) is performed by determining their values on a grid of fixed frequencies

O - {)) N-l

given

frequency range F In this case, the frequency response is represented by a set of values t (f), (f), ..., t (f) i (f N-1) j a frequency response tgrf - a set

tgcf (f), tg cf (f,), ..., tgrf (), .. values. , tg (f (f at the specified frequency grid. The number of frequencies in the grid and their specific values are selected when designing a particular stand, which only reduces to writing a specific array of frequencies (periods) values to memory block 22 and setting the first, second and third functional converters

13-15 to the corresponding conversion modes, according to the selected frequency grid. The values of (f) and tg f (f) at any frequency f are measured by determining the parameters of a parallel oscillatory circuit formed by the input circuit of amplifier 3 of multiplier 2 impedance OB, two-pole 6 with negative resistance, capacitive sensor 9 and connected in series with coupling capacitor 11 and varicap 12 before and after the measurement is applied to the capacitive

sensor 9. At the same time, the determination of the frequency tg rf (f) is made from the values of the Q-factors Q (f) and Q (f) of this oscillating circuit before and after the substance has been introduced

accordingly, the determination of (f,) is made by changing the capacitance of the varicap 12, necessary to compensate for the i-th frequency offset caused by introducing the studied

substances into the capacitive sensor 9. The transition from one measurement frequency to another is accomplished by changing the magnitude of the simulated inductance obtained at

input terminals of the amplifier 3 multiplier 2 impedances, the input impedance multiplier impedances, i.e. input impedance 2., uelite 3, whose feedback circuit is closed by impedance.) avno

--one( )

bh

I-ku

where .L is the inductance of coil 1

inductance j KH gain

amplifier 3 in voltage.

The change in the value of K and is practically carried out by changing the active load of the amplifier 3, which used the channel resistance of the field-effect transistor 4, the value of the channel resistance is determined by the output voltage of the first functional converter 13, which is applied to the gate and the source of the field transmitter 4. Change

iiu

-) times non-inductance at (1 +

Ch - Ki

(enough to cover a wide frequency range, therefore, to extend this range, inductor 1 is placed on a common magnetic core with bias coil 5, which is designed to change the magnetic induction of the core and the inductance L, the principle of controlling the inductance of the main coil with an additional bias coil is widely used in a sweep generator. The current flowing through the bias coil 5 is determined by the second function converter 14. In the process Adjusting the simulated inductance changes its quality. To compensate for these changes, which degrade the accuracy of the measurements, a 9-pole dual-pole correction unit has been introduced with an adjustable negative active resistance, the magnitude of which is controlled by the voltage supplied from the output of the third functional converter. 15. Negative resistance of the 6-pole 6 is chosen large enough to provide incomplete compensation for the total losses of the parallel oscillatory circuit throughout the working range. azone

where G is the active component of the conductivity of the input circuit of the digital cumeter 23J

The active component of the input circuit of the amplifier 3, i.e. loss conductivity in a parallel equivalent circuit of simulated inductance; Gijj- negative conductivity

two-port 6; conductivity in parallel equivalent circuit of an empty capacitance sensor 9; 0, the conductivity component introduced by the varicap and downstream circuit of the sampling-storage unit 19 is active. The principle of operation of the digital cumeter 5–23 is based on the constant logarithmic decrement of the amplitudes and oscillation periods of the oscillating circuit. Before measuring the quality factor, capacitive sensor 9 is charged by means of switch 8 to a potential equal to UQ. Prior to start-up, the key 31 with separate control of the closure and opening is open, and the pulse counter 32 is reset. At the time of the start of the switch 8, the capacitive i sensor 9 is connected to the simulated inductance and the switch 31 is closed. The damped voltage fluctuations on the circuit with frequency fd are fed through a pulse shaper 30 to the pulse counter 32 and simultaneously to the rectifier 33 and the low-pass filter 34 to apply the voltage envelope, which is fed to the first input of the comparator 35, the second input of which is constant. the voltage equal to iJo / t, the moment of decreasing the envelope to the level Uo / 5, is triggered by a comparator, which cancels the key 31. The number of pulses counted by the counter 32 is numerically equal to the quality factor of the oscillating circuit.

The whole process of measuring the frequency characteristics of a substance is divided into N two-stroke channels, each of which corresponds to one of the clock grids.

About -iN1 1114981 frequencies. The total loss resistance R is equal to g- / (GQ QL-GR GC + UV) The intrinsic frequency characteristic of the quality factor Q (f) of the oscillating circuit, i.e. the set of values Q (fo), Q (f} ,,,,, Q (fi-), ..., Q (f N), is determined for a specific capacitive sensor 9 by measuring on the stand itself when it starts up work. If necessary, Q (f) can be measured each time before measuring a specific substance. Clocking of the entire stand is carried out. With a tactile generator 16 whose output pulses are shown in the time diagram Vg (Fig. 2). range F is equal to two periods of the pulse sequence of the clock generator 16, which corresponds to The i-ro state of fixing the binary counter 1 is fixed (diagram 2). The binary counter 18 generates its output a sequence of codes corresponding to the decimal frequency grid numbers from O to N-1. Code combinations received from the output of the binary counter 18, the inputs of the first, second, and third functional converters 13-15 generate voltages at their outputs that control the setting of the value of the simulated inductance corresponding to the frequency f, and the value of the quality factor of the oscillating circuit Q (f ;,) not less than Qg. Functional transformation is required because the laws controlling the resonant frequency of a loop that coincides with f and its quality factor Q (f; j) are non-linear with respect to frequency. The first, second, and third functional converters 13-15 are code analogue converters and are built by a serial connection of a read-only memory (ROM) and a digital-to-analog converter (DAC). The address of the ROM is defined with binary counter 18, and the address itself is the control voltage code. The transition to another frequency grid is associated in this case only with the reprogramming of the ROM. The code of the binary counter 18 is also fed to the address input of the memory block 22, at the output of which a period code is formed corresponding to the frequencies of the given number i. When switching to another frequency grid, the contents of the cells of memory block 22 should also be changed. After filling the capacitance sensor with the substance, the stand is started. At the beginning of the first cycle of each cycle, the control unit generates, at its first pass, a pulse V (diagram V of Fig. 2), which nulls the pulse counter 32. The digital key 23 switches the key 31, and also switches the switch 8; in the initial state, the connecting capacitive sensor 9 is always connected to a constant voltage source (UQ /. The switch 8 and the key 31 are triggered with the greatest delay relative to the counter of 32 pulses. After the end of the pulse arriving at the launch of the digital cumeter 23, a decaying oscillatory process begins in the oscillatory circuit and the calculation of the value of the quality factor at a frequency f different from the required frequency f. the capacitance sensor filled with the substance is large.The Q values obtained in the first cycle of each cycle are therefore not used, and the first cycles are used only to simultaneously determine the frequency in order to return the oscillating circuit to the specified frequency. of these oscillations, which is performed using decoder 25, a time interval meter 26 and a register 27. The determination of the period is based on the measurement of the time interval, The ory is an integer number of damped oscillations of the contour, and this number is chosen equal to an integer power m of two. The time interval is counted from a predetermined front of the start pulse V.j obtained at the first output of the control unit 17 until the end of the 2nd pulse counted by the counter 32 pulses of the digital meter 23. The period measurement is illustrated by diagrams Vd (Fig. 2). The envelope on VV Vj, on the capacitive yarn sensor 9, is shown on the Vg diagram. After the time T since the start of the digital cumeter 23, the counter of 32 pulses counts 2 pulses, and the value of m is chosen so that T, for any possible values of quality factor, is less than the time 131 needed to calculate this quality factor, i.e. should always be equal to the ratio of 2 Q ,,. where Qmitii is the smallest admissible value of the quality factor of an oscillatory control. After counting the pulse, the decoder 25 at its output produces a pulse V, jj-, arriving at the second input of the meter 26 time intervals. The interval code formed by the time interval meter 26 is entered into register 27. Since the period is equal to the ratio of the measured interval to the number of pulses, and it is 2, the period code is obtained by simply shifting the contents of register 27 by m bits to the right. This operation is performed by applying to the control input of the register 27 a pulse for stopping the account VTJ- obtained from the output of the parameter 35 (diagram, figure 2). The change of state of register 27 is shown in the diagram Vj (Fig. 2), and the period code is stored in the register unchanged during the time T ". From the period code corresponding to the frequency fj, the calculator 24 subtracts the period code corresponding to the frequency that comes from the output of the memory block 22. Based on the difference in the periods, a change in the capacitance of the sensor 9 caused by the introduction of the substance into it is compensated. This is done in the second cycle of each i-ro cycle. The process of measuring the quality factor and the period of damped oscillations occurs similarly to the first cycle - the interval T corresponds to T, -, and the interval T T, (Fig. 2) with the only difference that in the second cycle with the key 10, the varicap 12 is connected in parallel with the capacitive sensor 9 The separation capacitor 11 is used to isolate the control circuit of the varicap 12 and the oscillating circuit in direct current. In the second cycle, a cash taken from the second output of control block 17 closes key 10 and sets block 19 of sampling-storage to memorizing the control varicop 12 tension obtained by converting the period difference code by means of the FAP 21 and fourth functional converter 20. Control the voltage on the varicap 12 is maintained by a sample-storage block 19 during the second cycle of each cycle. Since the difference is D f resonance, I frequency 2 jfTLTcTuCf where C is the capacitance of an empty capacitive sensor; iC is the increment of the capacitance of the sensor 9, caused by the introduction of a substance into it, then there is a connection between the difference between the oscillation periods. corresponding to the frequency difference U f and the magnitude of the DC 1 T; 4Ti-uTi) where T p- the period corresponding to the i-th grid frequency is known in advance. Since the dependence of the capacitance of the varicap 1 2 on the control voltage is also known (determined experimentally), for any frequency f, the fourth functional converter 20 matches the corresponding control voltage, which reduces the capacitance of the varicap by the value of c, which returns the resonant frequency of the circuit filled with capacitance sensor 9 is equal to. In the second cycle, the i-ro cycle digital meter 23 thus measures the value Q (), equal to the quality factor of the oscillating circuit at a frequency, this value, and also by the previously known value Q (fvj), the tangent of the dielectric loss angle is determined by the form tad (f-: -lUfT) Q (f) and the dielectric constant 6 is according to the formula 8U,) .. l .. I (, The calculation of the quantities Q () and Q ()) continues for time intervals T, and T, respectively and the values of these quantities in the counter 32 pulses remain unchanged during the time intervals T. and T., respectively (chart ma V 2). / At the time interval Tu of each 1st cycle, the code at the output of the subtractor 24, corresponding to the offset of the i-th resonant circuit 15 frequency, is also fixed. Synchronization of the data required for calculating tgcTCf) and (f) is in the calculator 28 is performed on the falling edge of the pulse supplied to the input of the calculator from the output, the signal to stop the counting of the digital cumeter 23 (Fig. 2). The output code LT of the subtractor 24, as well as the coders and coders stored in the memory 28, and C are used to calculate the dC with the subsequent computation of C (f), and the code Q (f) entered from the information output of the cumeter 23 together with the computer stored in memory 28 by Q (f) - to calculate tg (f (f)) The obtained values of the dielectric properties of the substance, corresponding to the frequency grid fg, f, ..., f (J.,, go to the recorder 29. Put the effect on The proposed device is achieved by the fact that the electronic tuning of the oscillating circuit on the frequency grid fg, f, ..., f (pos It automates the entire measurement process, as well as sampling the measured values, which improves the accuracy of calculating the properties of substances.The introduction of a digital cumeter into the stand makes it possible to simultaneously measure the Q-factor and the resonant frequency of the oscillating circuit containing a capacitive sensor, which makes it possible to compensate for the offset of the resonant frequency introducing the substance, and implementing in the stand a well-known and accurate capacitance variation method, which is widely used to measure dielectric properties PTS substances.

Claims (1)

STAND FOR MEASURING FREQUENCY CHARACTERISTICS OF SUBSTANCES OF SUBSTANCES, comprising a capacitive sensor, an inductor, a magnetizing coil located on one magnetic core with an inductor, an impedance multiplier consisting of an amplifier with an open feedback circuit and a field-effect transistor, an output source, and an output transistor connected to ground, a two-terminal with controlled negative resistance, a unit of controlled capacitance, consisting of a series-connected isolation capacitor as well as a varicap, control unit, switch, key, memory unit, calculator, digital recorder and subtractor, to the first input of which the output of the memory unit is connected, which means that, in order to automate the measurement process, four functional converter, clock, binary counter, sampling and storage unit, decoder, interval meter, register, digital-to-analog converter and digital meter with information input, start input, information output and signal output ki counting, and the inductor is included in the feedback circuit of the impedance multiplier, one bias coil terminal is grounded, and the other bias terminal, field-effect transistor gate and the control input of a two-terminal with controlled negative resistance are connected to the outputs of the first, second, and third functional converters § accordingly, the input of the impedance multiplier, the information input of a digital meter, the first terminal of a two-terminal with a controlled negative. resistance are connected to the first input of the switch, to the second input of which a constant voltage is applied, the second terminal of the two-terminal with controlled negative resistance, the varicap anode and one of the two capacitance sensor plates are grounded, the non-grounded sensor cover is connected to the switch output and is connected to the cathode through a key and an isolation capacitor varicap and to the output of the sample-storage unit, the first output of the control unit is connected to the control input of the switch, the start input of the digital meter and the first Meter input time intervals, the second output of the control unit is connected to the control input of the key and the control input of the sample-and-hold unit ,, wherein the entry to the control unit 1 1 14981 the output of the clock generator is connected, which is also connected to the input of the binary counter, the output of which is connected in parallel with the inputs of the first, second and third functional converters and with the input of the memory block, the information output of the digital meter is connected to the second input of the time interval meter through a decoder, the output the stop signal of the digital meter account is connected to the control input of the register, the output of the time interval meter is connected to the information input of the register, the output of which is connected to the second the input of the subtractor, the output of which through the digital-to-analog converter and the fourth functional converter are connected in series to the information input of the sample-storage unit, and the information output and the output of the stop signal of the digital cumeter account, as well as the output of the subtractor, are connected to the inputs of the computer, the output of which is connected to the digital recorder.