RU2237252C2 - Device for transforming reactive element parameters to frequency - Google Patents

Abstract

FIELD: radio engineering.

SUBSTANCE: device has frequency phase automatic phase-lock control system having phase-shifting circuit and resonance three-terminal network in feedback circuit. The three-terminal network has grounded chain having reactive element with parameters C_x and L_x looked for. The phase detector used in the system is of cosine type and voltage-controlled oscillator is rectangular pulse oscillator.

EFFECT: obtained growing relationship of frequency depending on reactive element parameter.

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Description

The invention relates to measuring technique and can be used to determine the parameters of elements of electrical circuits, in dielcometry, conductometry, when measuring the parameters of contactless inductive and capacitive sensors.

Known converters of parameters of reactive elements into frequency, containing an LC-oscillatory circuit with the desired reactive element (Lx or Cx) in the amplifier feedback loop [1, 2].

The disadvantage of these devices is inversely proportional to the square root of the dependence of the frequency on the magnitude of the desired parameter of the reactive element, i.e.

Also known are devices implemented on the basis of RC generators containing, during a timing chain, the desired reactive element Cx.

The disadvantage of this type of device is the inversely proportional nature of the dependence of the output frequency on Cx i.e.

Closest to the proposed is a capacitance-frequency converter, which uses a phase-locked loop and parametric modulation [3].

The disadvantage of this device is inversely proportional to the square root of the dependence of the output frequency on the magnitude of the desired reactive element.

The purpose of the invention is to obtain an increasing dependence of the output frequency on the parameter value of the desired reactive element, i.e. F≈ (F0 + kCx), where F0 is a certain initial frequency; k is the coefficient of proportionality.

This goal is achieved by the fact that in the converter of the parameters of the reactive elements into the frequency, consisting of a phase-locked loop, the feedback circuit of which includes a resonant three-terminal device containing a reactive element with the desired parameters, uses a cosine phase detector, a rectangular pulse generator controlled by voltage, and also a phase-shifting circuit, and the first input of the cosine phase detector is connected directly to the output of the rectangular pulse generator, controllably voltage, and the second through a phase-shifting circuit and a resonant three-terminal network containing a reactive element with the desired parameters in the grounded branch, the output of the cosine phase detector is connected to the input of the low-pass filter, the output of the latter is connected to the first input of the square-wave pulse generator, controlled by voltage, to the second input the generator receives a voltage that determines the initial frequency of operation of the generator, controlled by the voltage, the output of which is the output of the Converter.

Figure 1 presents the structural diagram of the Converter.

The converter of the parameters of the reactive elements into the frequency contains: a cosine phase detector 1, a rectangular pulse generator 2, voltage-controlled, phase-shifting circuit 3, a resonant three-terminal network, representing a complex voltage divider, the grounded branch of which contains a reactive element with the desired parameters and additional elements forming with the last resonant oscillation circuit 4, low-pass filter 5.

The inputs of the phase detector 1 are connected to the output of the voltage-controlled rectangular pulse generator 2 directly and through the phase-shifting circuit 3, in series with which the resonant three-terminal 4 is connected, the output of the cosine phase detector 1 is connected to the input of the low-pass filter 5, the output of which is connected to the first input of the generator 2, controlled by voltage, the second input of which receives a voltage that determines the initial frequency of its oscillations, the output of the generator 2 is the output of the Converter.

The device operates as follows.

The reactive element is connected to the grounded branch of the resonant three-terminal network, which is a complex voltage divider included in the feedback circuit of the phase-locked loop. The value of the parameter of the desired reactive element determines the phase shift between the signals arriving at the first input of the cosine phase detector directly from the generator, and at the second through the phase-shifting circuit that sets the operating mode, in series with which the resonant three-terminal device containing the desired reactive element is connected. The phase synchronization mode used in the converter allows the system to automatically reach the frequency that is a function of the desired parameter of the reactive element.

The combined use of a voltage-controlled rectangular pulse generator and a cosine phase detector is due to:

1. The impossibility of obtaining an increasing dependence of the frequency on the parameter of the reactive element when using a sine phase detector operating with a harmonic signal, because in this case, the dependence has the form

2. The impossibility of using a cosine phase detector operating on a harmonic signal, since capturing at a phase shift of 90 degrees will automatically lead the system to the upper or lower boundary of the capture zone, where the frequency does not depend on the magnitude of the desired parameter.

Due to the complexity of the visual analytical representation of the converter operation caused by a large amount of numerical operations related to determining the system response to a pulsed action caused by the use of a rectangular pulse generator and a cosine phase detector, the experimental results are presented as evidence of operability.

Figure 2 presents the circuit diagram of the implemented Converter; figure 3 - obtained on the implemented version of the Converter, the dependence of the output frequency on the value of the capacitance taken as the desired reactive element; figure 4 - waveform of the signals at the inputs of the phase detector in capture mode, where 1 is the signal from the output of the generator, and 2 is the signal from the output of the three-terminal network.

The converter of the parameters of the reactive elements into the frequency is assembled on the basis of the integrated circuit КР174ХА12, which is a ready-made phase-locked loop. Experimental studies were performed on a specially made stand. The stand structure included:

1) a printed circuit board with a converter circuit;

2) switching clamps for connecting model elements;

3) interface circuits preventing the influence of cable connections of measuring instruments on the operating mode of the converter;

4) a gearbox with a vernier scale to enable multiple repeated measurements of capacitance values.

Highly stable capacitances of the KM type were used as elements defining the parameters of a low-pass filter and a voltage-controlled oscillator timing circuit.

The output frequency of the converter was measured with a BZ-63 frequency meter.

Given the high frequency of operation of the microcircuit on which the converter is implemented, it should be noted that additional phase shifts can occur due to the heterogeneity of the channels of the phase detector, which can either be used to eliminate additional phase-shifting circuits, or, if necessary, be compensated by them.

The given oscillogram shows that during operation the capture occurs at a phase shift close to 90 degrees.

The obtained dependence of the output frequency on the magnitude of the desired reactive element can be approximated, within the capture zone, which is about 10% of the initial frequency of the generator controlled by voltage, linear sections, which allows to obtain a converter with a linear frequency dependence of the capacitance.

In addition, this converter was tested over a year in an experimental sample of a sand moisture meter, where it showed the following results:

For the range of variation in the capacitance of the sensitive element of the hygrometer from 0 to 70 pF, the operating frequency of 4 MHz and the humidity range from 0-14%, the measurement error did not exceed 1% humidity.

It should be noted that the stand circuit is characterized by a weak dependence of the function of converting Cx to frequency on the value of the shunt resistor, which varies in the range from 1 MΩ to 360 Ω.

Sources of information

1. Kaufman M., Sidman A. A practical guide to the calculation of circuits in electronics: a Handbook. In 2 vols. Trans. From English / Ed. Pokrovsky F.N. - M.: Energoatomizdat, 1991, - 368 p.: Ill.

2. Agalakov A.A. Measuring circuits of capacitive and inductive sensors. - M .: Instruments, 2001, No. 1, 18-20 p.

3. A.S. No. 798634. Roife V.S., BI No. 3, 1981.

Claims (1)

The converter of the parameters of the reactive elements into a frequency, which includes a resonant three-terminal device, which contains a reactive element with the desired parameters, characterized in that a cosine phase detector is introduced, the first input of which is connected directly to the output of the voltage-controlled rectangular pulse generator, the second input of the cosine phase detector connected to the output of the rectangular pulse generator through a resonant three-terminal network, representing a voltage divider, a grounded branch of a cat It contains a reactive element with the desired parameters and additional elements forming a resonant oscillatory circuit with the latter, the output of the cosine phase detector is connected to the input of the low-pass filter, the output of the latter is connected to the first input of the square-wave generator controlled by voltage, voltage is applied to the second input of the specified generator, defining the initial frequency of its operation, and the output of the specified generator is the output of the Converter.

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