JPH05291945A - Phase difference measurement circuit for synchronizing signal - Google Patents

Abstract

PURPOSE:To attain the measurement of a phase difference inexpensively with high accuracy by allowing an AC component amplifier circuit to eliminate a DC component from a measured signal and a reference signal and using a phase measurement circuit so as to measure the phase of the reference signal inputted to a phase shift circuit and its output signal. CONSTITUTION:AC component amplifier circuits 11, 11a, 11b receive an input signal at its negative input terminal via smoothing circuits 8b, 8c, 8d. Then the input signal is directly given to a positive input terminal to disregard a circuit phase error to eliminate a DC component from a measured signal Vx and a reference signal VREF. Moreover, the signal VREF, inputted to a phase shift circuit 14 and its output signal are converted into a pulse signal whose duty is 50% by 1st and 2nd shaping circuits 3, 3a, and the phase difference is measured by using a phase measurement circuit 16 calculating the mutual phase difference as a mean value for plural periods by the time count system with high accuracy. Furthermore, a phase angle is adjusted for the phase shift circuit 14 so that a sine component output of a synchronization rectifier circuit 2 is made zero.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous signal phase difference measuring circuit capable of stably measuring a phase difference between an alternating current measuring signal and a reference signal which repeats at a specific frequency with high accuracy and with little secular change. In particular, the reference signal is also supplied from the outside, and in addition, it relates to a phase difference measuring circuit capable of performing highly accurate measurement even when the DC component contained in the reference signal and the measurement signal is large. It can be used, for example, when the transmitted light and the received light of an optical signal intensity-modulated to KHz are voltage-converted by a pin photodiode and the mutual phase difference is measured.

[0002]

2. Description of the Related Art A synchronous rectification method is known as a method for measuring a phase difference between a signal repeated at a specific cycle and a reference signal with high accuracy, and an application example thereof is already disclosed in JP-B-61-115.
It is disclosed in Japanese Patent Publication No. 45, Japanese Utility Model Publication No. 3-13710, and the like. These methods are using switches,
The timing for turning the switch on and off is made from the synchronization signal. When the waveform of the sync signal is not a pulse, it is converted into a pulse waveform by an analog comparator.

FIG. 3 shows a reference signal V _REF to be measured.
And the measurement signal V _X. Letting the phase difference between these signals be θ, the DC components of these signals be B _R , B _X , and the amplitudes be A _R , A _X , period T, frequency f, and angular velocity ω, V _REF = B _R + A _R sin ωt ... _{· V X = B X + a} X sin (ωt + θ) ··· ω = 2πf = 2π / T ··· Figure 4 is a phase difference measuring circuit diagram of a conventional synchronous rectification system reference signal and the measurement signal that applies. R1 to R11 are resistors,
C1 to C7 are capacitors. C1 and R1 and C2 and R2 form a DC component removing circuit. 1, 1a are operational amplifiers (hereinafter referred to as OP amplifiers), which are R3, R4 and OP.
The gain of the measurement signal in the amplifier 1 is G times (G = 1 + R4 / R
It constitutes the amplifier of 3).

A circuit 2 surrounded by a chain double-dashed line shows a two-channel synchronous rectification circuit. This circuit is composed of high-speed analog switches 7, 7a and R5, R6, R6 and an OP amplifier 1a having two contacts in one circuit. 1 inverting amplifier.

Reference numeral 3 is a comparator which produces a square wave signal S ₀ having a duty ratio of 50% which is turned on and off at a zero cross point of the AC portion of the reference signal V _REF . 4 is an integrated circuit (74HC40) for a phase lock loop circuit (hereinafter referred to as a PLL circuit)
46), a low-pass filter composed of a 90 ° phase comparator 5, a voltage controlled oscillator 6, and external R7 and C3
A circuit is constructed to produce a square wave signal S ₉₀ with a phase delay of 90 degrees from the S ₀ signal.

Analog switches 7 and 7 of the synchronous rectification circuit 2
Since a is turned on / off using the S ₀ signal and the S ₉₀ signal, the synchronous components, that is, the sin component and the cos component, which are delayed by 0 ° and 90 ° with respect to the reference signal in the measurement signal, are DC voltage. To be extracted.

Reference numerals 8 and 8a denote smoothing circuits composed of R8, C4, R9 and C5, and R10, C6, R11 and C7, respectively, and their time constants are several tens of times more than that of the measurement signal. , The signals of the synchronous rectification circuit 2 are set to DC voltages V _XS and V _XC without pulsating current. Let DC voltage be V _XS and V _XC ,
When the phase difference between the reference signal and the measurement signal is θ, the following relationship holds.

Θ = tan ⁻¹ (V _XS / −V _XC ) ... Therefore, the output voltages V _XS and V _XC are converted into digital values by using the analog / digital converters 9 and 9a, and the milocomputer 10 is converted. The phase difference can be calculated and displayed (not shown).

[0009]

However, in the above construction, there is a phase difference Θ between the offset and the variation Δθ between the reference signal and the measurement signal as expressed by the following equation. When attempting to measure a minute change such as a degree below with high accuracy, there are the following problems.

Θ = Θ + Δθ ... Capacitor and resistance C for cutting off the DC component in the signal
Regarding the phase characteristic of the circuit constituted by 1, R1 and C2, R2, a phase lead of about 2.9 degrees occurs even when a time constant of 20 times the measured signal V _X is selected as a design value.
Therefore, the relationship with C1, R1 and C2, R2 must be adjusted so that their time constants are equal, and in addition, an element that is stable against changes in the surrounding environment must be selected, which is expensive.
Also, because this circuit is a high-pass filter, when the reference signal or measurement signal is distorted, its harmonic components are emphasized, and the error due to the odd harmonics that cannot be removed by synchronous rectification becomes large. There is.

Further, θ is calculated by an equation, but since the change of θ with respect to V _XS and V _XC is not linear, for example, Δ
For 0.1 degree change of θ, at θ = 0 degree, V _xc = 1
If 0.000 and V _xs = 0.000 are obtained, θ
= When [Delta] [theta] = 0.1 °, V _XC = 10.000, the _{V XS = 0.0017 θ = Θ =} 45 _{degrees, V XC = 7.072, V XS} = 7.072 Δθ = 0.1 ° , Ie, θ = Θ + Δθ = 45.1 degrees, V _XC = 7.059 and V _XS = 7.083.

Therefore, as is clear from the above equation, 0.
An attempt to measure a phase change with an accuracy of 01 degree is expensive because an analog / digital converter with an accuracy of 1 / 100,000 is required. Further, when the phase difference accuracy between the 90 ° delayed signal generated by the PLL circuit 4 and the 0 ° signal of the reference signal is slightly different from 90 °, for example, if the difference is 0.1 °, there will be almost no error at θ = 0 °. There is a problem that a large error appears near θ = Θ = 45 degrees.

In view of the above problems of the prior art, the present invention enables highly accurate and stable measurement with little secular change, and high accuracy measurement even when the DC component contained in the reference signal and the measurement signal is large. It is an object of the present invention to provide a synchronization signal phase difference measuring circuit capable of performing the above.

[0014]

The first invention of the present invention is as follows:
A first AC component amplification circuit composed of a differential amplifier, which has a measurement signal as an input signal, is connected to a negative input terminal via a smoothing circuit, and is directly connected to a positive input terminal,
A two-channel synchronous rectification circuit for extracting a sin component and a cos component synchronized with the reference signal from the output of the first AC component amplification circuit; and a first smoothing circuit connected to the sin component output of the synchronous rectification circuit. , A second smoothing circuit (output voltage V _XC ) connected to the cos component output, and an amplifier (gain N times, output voltage V _NXS ) connected to the first smoothing circuit,
A phase shift circuit capable of arbitrarily changing the phase of the reference signal, a second AC component amplifier circuit having the same configuration as the first AC component amplifier circuit connected to the output of the phase shift circuit, and a second AC component. A first shaping circuit for converting the output signal of the component amplification circuit into a pulse signal with a duty ratio of 50%, and a sync pulse for generating a sync pulse signal with a duty ratio of 50% having a 90 degree phase difference from the signal of the first shaping circuit. The generator circuit, the third AC component amplifier circuit having the same configuration as the first AC component amplifier circuit connected to the input of the phase shift circuit, and the output signal of the third AC component amplifier circuit are pulsed with a duty ratio of 50%. A second shaping circuit for converting to a signal, and a phase measuring circuit for calculating a phase difference φ between the first shaping circuit and the second shaping circuit as an average value for a plurality of cycles by a time counting method. Output of shaping circuit and sync pulse generation circuit It is connected as a timing signal for synchronous rectification circuit, the phase difference ^{θ, θ = φ + tan -1} (V
_NXS / N · V _XC ) is provided.

A second invention of the present invention is the same as the first invention, except that the bandpass filter whose center frequency is set to the reference signal frequency and the reference signal is connected directly to the phase shift circuit or via the bandpass filter. And the changeover switch for selecting whether or not to do so. First, select the changeover switch to pass through the bandpass filter and measure the phase shift characteristic (phase shift angle Ψ) of the phase shift circuit. Return the changeover switch to connect the signal directly to the phase shift circuit, and change the phase difference θ to
It is characterized in that a means for calculating as θ = Ψ + tan ⁻¹ (V _NXS / N · V _XC ) is provided.

[0016]

According to the first aspect of the present invention, the first, second and third AC component amplifying circuits are connected to the negative input terminal of the input signal via the smoothing circuit to obtain the positive input. By directly connecting the terminals, the circuit phase error for removing the DC component from the measurement signal and the reference signal can be ignored.
A phase shift circuit capable of arbitrarily changing the phase of the reference signal, a second AC component amplifier circuit connected to the output of the phase shift circuit, and an output signal of the second AC component amplifier circuit are provided with a duty ratio of 50%. The first shaping circuit for converting the pulse signal into the pulse signal, the third AC component amplification circuit connected to the input of the phase shift circuit, and the output signal of the third AC component amplification circuit have a duty ratio of 50%.
The phase difference (φ = phase shift angle of the phase shift circuit) between the first shaping circuit and the second shaping circuit that is used as the pulse signal of The phase shift circuit characteristics can be measured with high accuracy by the calculating phase measuring circuit. The phase shift angle φ of the phase shift circuit is adjusted so that the sin component output of the synchronous rectifier circuit becomes zero, and the sin component is adjusted by the amplifier connected to the first smoothing circuit connected to the sin component output of the synchronous rectifier circuit. Of the phase difference between the two sync pulse signals of the sync rectifier circuit by amplifying
Even if there is a slight error in 0 degree, it does not affect the measurement accuracy, and it is possible to detect a minute phase angle with high sensitivity without using a high-precision and high-resolution analog-digital converter.

According to the second aspect of the present invention, a bandpass filter having a center frequency set to the reference signal frequency and a changeover switch are provided, and the reference signal is temporarily passed through the bandpass filter to shift the phase of the phase shift circuit. Since the characteristic (phase shift angle Ψ) can be measured, even if the reference signal contains noise or waveform distortion, it will be a beautiful sine wave with these removed, so measurement without error is possible. ..

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A sync signal phase difference detecting and measuring circuit according to an embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a circuit diagram of this embodiment, which includes resistors, capacitors, analog switches, OP amplifiers, comparators, and 2.
Channel synchronous rectification circuits, phase comparators, voltage controlled oscillators and the like having the same reference numerals as in the conventional example of FIG. 4 have the same functions and constants.

Although this embodiment has a small range of phase change, it is suitable when the amplitude voltage and the phase of the measurement signal change every moment and it is desired to measure the phase change with respect to the reference signal with high accuracy of 0.01 degree. Provide the circuit.

Reference numeral 1b is an OP amplifier, which includes resistors R12 and R12.
The phase of the reference signal is φ by 13, R14 and the capacitor C8.
The phase shift circuit 14 for shifting the frequency is constituted. The phase can be changed within the range of −90 ° ± 45 ° by the variable resistor R14. 12 and 12a are changeover switches, which are turned off 12a and turned on 12a during normal measurement.
When measuring the characteristic of the phase shift circuit 14, 12 is turned on and 12a is turned off.

Resistors R15 and R16 and capacitors C9 and C
Reference numeral 10 constitutes a smoothing circuit 8b, and its time constant is about the same as that of the resistors R8 and R9 and the capacitors C4 and C5 of the smoothing circuit 8 at the output stage. Compared with the conventional example, the number of stages of the low pass filter of the smoothing circuit 8b composed of resistors and capacitors is 2
Since it is a step, there is almost no AC component.

Reference numerals 11 and 11a are differential amplifiers, and their gains are G times and H times, respectively. The positive input terminal of the differential amplifier 11, is inputted as the measurement signal V _X, whereas for the DC component in the measurement signal V _X by smoothing circuit 8b is input to the negative input terminal of the measurement signal V _X Only the alternating-current component therein is amplified G times by the differential amplifier 11 and output. With such a circuit configuration, an error in which the phase shifts as in the conventional circuit can be ignored. It will be better if the number of filter stages is three.

Similarly, the differential amplifier 11a and the resistor R1
Only the AC component in the reference signal V _REF is amplified by the smoothing circuit 8c composed of 7, R18 and the capacitors C11, C12. Reference numeral 3 is the same comparator as in the conventional example, and the phase shift circuit 14 produces a pulse signal S _S0 having a duty ratio of 50% which is turned on and off at the zero cross point of the AC component delayed by φ degrees from the reference signal V _REF .

Similar to the conventional example, the PLL circuit 4 is composed of the 90-degree phase comparator 5, the voltage controlled oscillator 6, the resistor 7 and the capacitor C3. The signal S _S0 is input to the PLL circuit 4 and produces a pulse signal S _S90 which is 90 degrees phase delayed with the signal S _S0 . On the output side of the 2-channel synchronous rectification circuit 2, S
_{The S0} signal and the S _S90 signal are used to measure the sin component of the measurement signal and C
The OS component is converted into a DC signal voltage by the principle of synchronous rectification. Further, the resistors R8 and R9 and the capacitors C4 and C5, and the resistors R10 and R11 and the capacitors C6 and C7 constitute smoothing circuits 8 and 8a, and the synchronous component of the measurement signal is a DC voltage sin component V _XS without pulsation. And the cos component V _XC .

The OP amplifier 1c and the resistors R21 and R22 constitute a 100-fold amplifier, and the sin component has a 100-fold voltage V _NXS . 13 is a voltmeter.

Reference numeral 15 is a bandpass filter whose center frequency is the reference signal frequency. Also, the differential amplifier 11b and the resistor R
Only the AC component in the reference signal V _REF is amplified by the smoothing circuit 8d composed of 19, R20 and the capacitors C13, C14. Reference numeral 3a is a comparator, which has a duty ratio of 5 which is turned on and off at a zero cross point of the AC component of the reference signal V _REF.
A 0% pulse signal S _R is produced.

Reference numeral 16 surrounded by a two-dot chain line is a digital phase measuring circuit for accurately measuring the phase component φ degree of the phase shift circuit 14. The timing chart is shown in FIG. Reference numeral 17 is a crystal oscillator circuit having a frequency sufficiently higher than that of the reference signal. 18
Is a preset counter circuit composed of integrated circuits,
100 is preset, the signal S _R is counted, and 1
At the 00th time, the counting completion signal G _A turns on. 19, 19a
Is an inverter circuit, 20, 20a, 20b, 20c is 2
It is an input AND circuit. Reference numerals 21 and 21a are counter circuits composed of an integrated circuit or the like. 22 is a NAND circuit. When the signal S _R is on, the reset signal is not valid.

The procedure for measuring the phase with high accuracy will be described below. First, with the switch 12a turned on and the switch 12 turned off, the sin component voltage V _NXS of the measurement signal V _X.
The variable resistor R14 of the phase shift circuit 14 is adjusted so that becomes zero. Therefore, it is convenient to attach the voltmeter 13. In this state, the AC component synchronized with the reference signal of the measurement signal and the signal S _S90 are in phase with each other, and the signal S _S0 is deviated by just −90 °. As described above, since the sin component voltage V _NXS of the measurement signal is amplified 100 times as compared with the conventional zero, the zero point detection accuracy is high.

Next, referring to FIG. 2, the phase shift circuit 14
The phase shift characteristic of is measured. First, the microcomputer 1
The reset signal RS from 0 resets the counter circuits 18, 21, 21a when the pulse signal S _R is off. The reference signal V _REF becomes the pulse signal S _S0 that passes through the phase shift circuit 14 by the comparator 3, and is converted into the pulse signal S _R that does not pass through the phase shift circuit 14 by the comparator 3a. The phase difference between them is the angle φ. Since the AND circuit 20 generates the pulse signal S _{X which} is turned on for the angle φ, the counter circuit 21 counts the clock C _K1 of the crystal oscillator 17 while the pulse signal S _X is on. The counter circuit 21a counts the clock C _K2 while the signal S _R is on, that is, during the half of the period of the reference signal. As described above, when the preset counter 18 counts the pulse signal S _R 100 times, the AND circuit 20b stops the input of the clock of the crystal oscillator 17. Therefore, the count data C _X of the counter circuits 21 and 21a at this time point,
If C _T , the phase delay angle φ of the phase shift circuit 14 can be calculated by φ = 180 · (C _X / C _T ) ...

In this way, the counter circuits 21, 21a
In addition, since the data for 100 cycles is integrated, the average value data for 100 times can be obtained, so that the phase measurement resistant to noise can be performed.

On the other hand, regarding the output of the synchronous rectification circuit 2,
Similar to the conventional example, by using the analog-digital converters 9 and 9a, a sin component c synchronized with the reference signal of the measured signal, c
The DC voltage V _NXS and V _XC , which are os components, are converted into digital values, and the phase variation Δθ is calculated by the microcomputer 10.
To calculate. Since the DC voltage V _NXS, which is the sin component, is amplified 100 times by the amplifier using the OP amplifier 1c, Δθ is calculated by the following equation.

Δθ = tan ⁻¹ (V _NXS / −100 · V _XC ) ... Therefore, the phase difference θ between the reference signal and the measurement signal can be accurately calculated as θ = Δθ + φ. DC voltage V of the above sin component
_{Since NXS} is always adjusted to a value close to zero by the adjustment of the phase shift circuit 14 as described above, it is sufficient that the resolution of the analog-digital converter 9 is 10 bits.
os component DC voltage V _XC analog-digital converter 9
As for the resolution of a, since it is always used near the full scale, 10 bits is enough. Therefore, since an inexpensive analog-digital converter can be used, the measuring circuit as a whole becomes inexpensive.

By the way, the reference signal V _REF is usually a clean sine wave with little distortion, but it may contain noise or harmonic distortion depending on the measurement target.
In such a case, first, the voltmeter 13 is adjusted to the minimum value, and the changeover switch 12a is turned off and 12 is turned on. Then, the reference signal V _REF is input to the phase shift circuit 14 via the bandpass filter 15. Since the band-pass filter 15 removes the harmonic components and noise contained in the reference signal, the stable pulse signals S _S0 and S _R are input to the digital phase measuring circuit 16, so that the phase shift circuit 1
The phase shift angle Ψ of 4 is stable and accurate. This value is stored in the microcomputer.

At the time of measurement, the variable resistor R1 of the phase shift circuit 14
4 allowed to its leave without tampering, the switch 12 is turned off to turn on the 12a, and inputs the measurement signal V _X which phase difference varies in a range of a few degrees. Then, the phase difference is calculated by the following formula.

Θ = Δθ + Ψ In this way, since the phase shift characteristic of the phase shift circuit 14 with respect to the reference signal frequency can be accurately calculated by using the band-pass filter 15, the voltage level of the reference signal is low and noise components are generated. Accurate phase difference measurement is possible even when riding.

[0037]

As described above, according to the first aspect of the present invention, the configuration of the first, second and third AC component amplifying circuits is such that the input signal is passed through the smoothing circuit to the negative input terminal. Then connect,
Since it is directly connected to the positive input terminal, the circuit phase error for removing the DC component from the measurement signal and the reference signal can be ignored, and the cost can be reduced. Further, after the reference signal and the output signal input to the phase shift circuit are converted into pulse signals having a duty ratio of 50% by the first and second shaping circuits, the mutual phase difference (φ = phase shift angle of the phase shift circuit). ) Is used as the average value of a plurality of cycles by the time counting method, the phase difference measurement can be performed with high accuracy. Further, the phase shift angle φ of the phase shift circuit is adjusted so that the sin component output of the synchronous rectifier circuit becomes zero, and the direct current close to 0 volt of the first smoothing circuit to which the sin component output of the synchronous rectifier circuit is connected. By amplifying the voltage with an amplifier and performing analog-to-digital conversion, even if there is a slight error in the phase difference of 90 degrees of the synchronous pulse signal of the synchronous rectifier circuit, it does not affect the measurement accuracy, and the accuracy and resolution are high. It is possible to measure minute phase angles with high sensitivity without using the analog-digital converter of.

Further, according to the second aspect of the present invention, a bandpass filter having a center frequency set to the reference signal frequency and a changeover switch are provided, and the reference signal is passed through the bandpass filter to shift the phase shift characteristic of the phase shift circuit. Since the (phase shift angle Ψ) can be measured, even if the reference signal includes noise and waveform distortion, a clean sine wave is obtained by removing them, and therefore measurement without error is possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a synchronization signal phase difference measuring circuit according to an embodiment of the present invention.

FIG. 2 is a timing diagram of the digital phase difference measuring circuit according to the same embodiment.

FIG. 3 is an explanatory diagram of a reference signal and a measurement signal.

FIG. 4 is a circuit diagram of a synchronization signal phase difference measuring circuit of a conventional example.

[Explanation of symbols]

1, 1a, 1b, 1c Operational amplifier 2 2 channel synchronous rectification circuit 3, 3a Comparator (shaping circuit) 4 Phase lock loop circuit 7, 7a Analog switch 8, 8a, 8b, 8c, 8d Smoothing circuit 11, 11a , 11b Differential amplifier (AC component amplification circuit) 12, 12a Changeover switch 14 Phase shift circuit 15 Bandpass filter 16 Digital phase measurement circuit 17 Crystal oscillator 18 Preset counter circuit 21, 21a Counter circuit

Claims (2)

[Claims] 1. A first alternating-current component amplifying circuit constituted by a differential amplifier which uses a measurement signal as an input signal, is connected to a negative input terminal through a smoothing circuit, and is directly connected to a positive input terminal. And a two-channel synchronous rectification circuit for extracting a sin component and a cos component synchronized with the reference signal from the output of the first AC component amplification circuit, and a sin of the synchronous rectification circuit.
A first smoothing circuit connected to the component output, a second smoothing circuit (output voltage V _XC ) connected to the cos component output, and an amplifier (gain N times, output voltage) connected to the first smoothing circuit. V _NXS ), a phase shift circuit capable of arbitrarily changing the phase of the reference signal, and a second AC component amplifier circuit having the same configuration as the first AC component amplifier circuit connected to the output of the phase shift circuit. , A first shaping circuit for converting the output signal of the second AC component amplification circuit into a pulse signal having a duty ratio of 50%, and a duty ratio of 50% having a 90 degree phase difference different from that of the signal of the first shaping circuit.
Of the third AC component amplifier circuit, which has the same configuration as the first AC component amplifier circuit connected to the input of the phase shift circuit, and the third AC component amplifier circuit. Phase that calculates the phase difference φ between the second shaping circuit that makes the output signal a pulse signal with a duty ratio of 50% and the first shaping circuit and the second shaping circuit as an average value for a plurality of cycles by the time counting method The output of the first shaping circuit and the synchronization pulse generation circuit is connected as a timing signal of the synchronous rectification circuit, and the phase difference θ is θ = φ +
A phase difference measuring circuit for synchronizing signals, characterized in that it is provided with means for calculating tan ⁻¹ (V _NXS / N · V _XC ). 2. A first AC component amplifying circuit, which comprises a differential amplifier which uses a measurement signal as an input signal, is connected to a negative input terminal through a smoothing circuit, and is directly connected to a positive input terminal. Circuit, a 2-channel synchronous rectification circuit for extracting a sin component and a cos component synchronized with the reference signal from the output of the first AC component amplification circuit, and si of the synchronous rectification circuit.
a first smoothing circuit connected to the n component output, and a second smoothing circuit (output voltage V _XC ) connected to the cos component output;
An amplifier (gain N times, output voltage V _NXS ) connected to the first smoothing circuit, a phase shift circuit that can arbitrarily change the phase of the reference signal, and a band in which the center frequency is set to the reference signal frequency. The filter, the changeover switch for selecting whether the reference signal is directly connected to the phase shift circuit or the band-pass filter, and the first AC component amplifier circuit connected to the output of the phase shift circuit A second AC component amplifying circuit having a configuration; a first shaping circuit for converting an output signal of the second AC component amplifying circuit into a pulse signal having a duty ratio of 50%;
Duty ratio 5 with 90 degree phase difference from the signal of the shaping circuit
A synchronization pulse generation circuit for generating a 0% synchronization pulse signal, a third AC component amplification circuit having the same configuration as the first AC component amplification circuit connected to the input of the phase shift circuit, and a third AC component amplification The phase difference between the second shaping circuit that makes the output signal of the circuit a pulse signal with a duty ratio of 50% and the phase difference between the first shaping circuit and the second shaping circuit is calculated as an average value for a plurality of cycles by the time counting method. The output of the first shaping circuit and the synchronization pulse generation circuit is connected as a timing signal of the synchronous rectification circuit, and the changeover switch is switched by the phase measurement circuit to pass the reference signal through the bandpass filter. The phase shift characteristic (phase shift angle Ψ) of the phase shift circuit is measured, and the phase difference θ is calculated as θ = Ψ + tan ⁻¹ (V
_A circuit for measuring the phase difference of a synchronizing signal, which is provided with means for calculating by _NXS / N · V _XC ).