JP5494548B2 - Voltage measuring device - Google Patents

Description

  The present invention relates to a voltage measuring device that measures the voltage of an object to be measured by capacitive coupling.

  Conventionally, for example, the voltage of the object to be measured is measured by capacitive coupling without directly contacting the object to be measured, as in the case of an electrocardiogram detecting device that detects an electrocardiogram by contacting an electrode with a human body through clothing or the like. There are known voltage measuring devices that can be used (see, for example, Patent Documents 1 and 2).

JP 2002-350473 A Special table 2009-534108 gazette

  However, as in Patent Documents 1 and 2 described above, in a voltage measurement device that measures voltage by capacitive coupling, the impedance between the object to be measured and the voltage measurement device (hereinafter referred to as source impedance) and the input impedance of the voltage measurement device Thus, since the voltage value of the device under test is divided, the voltage value input to the voltage measuring device decreases as the source impedance increases. For this reason, in order to evaluate the voltage value of the device under test, it is necessary to measure the source impedance.

  By the way, as a method of measuring the source impedance, an AC voltage having a predetermined frequency is applied to the object to be measured, and the voltage value of the object to be measured in a state where the AC voltage is applied is measured by the voltage measuring device. It has been known. That is, the magnitude of the source impedance is evaluated based on the ratio between the value of the AC voltage applied to the object to be measured and the AC voltage value measured by the voltage measuring device. However, in this source impedance measuring method, since a capacitance is also formed between the voltage application unit that applies an AC voltage to the object to be measured and the object to be measured, the source impedance cannot be measured accurately.

  The present invention has been made in view of these problems, and an object thereof is to provide a technique for accurately measuring the impedance between the object to be measured and the voltage measuring device.

  In the voltage measuring device according to claim 1, which is made to achieve the above object, the voltage sensor detects the voltage of the object to be measured without contact with the object to be measured by capacitive coupling, and the voltage applying unit includes An AC voltage having the set first AC frequency is applied to the object to be measured. Further, the input impedance changing means changes the input impedance of the voltage sensor at a second AC frequency set in advance so as to be different from the first AC frequency. Then, the measuring means measures the intensity of the frequency modulated by the first AC frequency and the second AC frequency among the voltages detected by the voltage sensor while the input impedance changing means is operating, and determines the source impedance. measure.

  In the voltage measuring apparatus configured as described above, the voltage value of the object to be measured is divided by the source impedance and the input impedance. Therefore, the larger the source impedance, the more the fluctuation of the detection voltage of the voltage sensor due to the fluctuation of the input impedance. growing. That is, as the source impedance increases, the intensity of the frequency modulated by the second AC frequency is modulated when the input impedance is varied at the second AC frequency, and the intensity of the AC component having the first AC frequency increases. Becomes smaller.

  Since there is a correlation between the magnitude of the source impedance, the intensity of the AC component having the first AC frequency, and the intensity of the AC component having a frequency modulated by the second AC frequency, the first AC frequency The source impedance can be measured by calculating the intensity of the alternating current component having a frequency and the intensity of the alternating current component having a frequency modulated by the second alternating frequency.

Thereby, the impedance between the object to be measured and the voltage sensor can be accurately measured regardless of the capacitance formed between the voltage applying means and the object to be measured.
Further, in the voltage measuring device according to claim 1, as described in claim 2, the measuring means adds a second AC frequency to the first AC frequency as a frequency modulated by the second AC frequency. Using the first modulation frequency and the second modulation frequency obtained by subtracting the second AC frequency from the first AC frequency, the intensity of the AC component having the first AC frequency, the intensity of the AC component having the first modulation frequency, and the second The source impedance may be measured by calculating a ratio with at least one of the intensities of AC components having a modulation frequency.

  According to the voltage measuring apparatus configured as described above, the data indicating the correlation between the intensity of the AC component having the first AC frequency and the source impedance, the intensity of the AC component having the first and second AC frequencies, and the source impedance. The source impedance can be measured by preparing only the data indicating the correlation between the ratio and the source impedance in advance without preparing two data including the data indicating the correlation with the source impedance in advance. .

  Moreover, in the voltage measuring device according to claim 1 or 2, when the second AC frequency is higher than the signal frequency desired for measurement and lower than the first AC frequency, as described in claim 3. Good.

According to the voltage measuring device configured as described above, it is possible to eliminate the superposition of the second AC frequency and the first AC frequency in a desired ECG frequency band.
Further, in the voltage measuring device according to any one of claims 1 to 3, as described in claim 4, the changing means modulates the first AC frequency and the second AC frequency after modulation. It is also possible to detect the amplitude of the generated frequency and change the amplitude intensity of the first AC frequency.

  According to the voltage measuring device configured as described above, the first alternating current can be measured within the input voltage range of the voltage sensor in accordance with the amplitude of the frequency modulated by the first alternating frequency and the second alternating frequency. The frequency amplitude can be adjusted.

  Further, in the voltage measuring device according to any one of claims 1 to 4, the device under test is located in the vicinity of the commercial power source, and electromagnetic waves generated from the commercial power source are generated in the device under test. In the case of transmission, as described in claim 5, the voltage applying means may be a commercial power source that supplies an AC voltage with the first AC frequency as a commercial frequency.

  According to the voltage measuring device configured as described above, the source impedance can be measured without separately preparing an AC power source having the first AC frequency.

1 is a circuit diagram showing a configuration of an electrocardiogram detection device 1. FIG. It is a circuit diagram which shows the structure of a lock-in amplifier. It is a graph which shows the frequency distribution of a cardiac potential signal, an oscillation signal, and an alternating current signal, and the frequency distribution of the detection signal of sensors 71 and 72. It is a graph which shows the relationship between the output voltage of the sensors 71 and 72, and the resistance value of the resistors 71c and 72c. It is a graph which shows the relationship between the intensity | strength of the component of frequency fcm, and source impedance Zc.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a circuit diagram showing a configuration of an electrocardiogram detection device 1 of the present embodiment.
The electrocardiogram detection device 1 is mounted on a vehicle and, as shown in FIG. 1, an external voltage input unit 2, a buffer amplifier 3, a frequency conversion unit 5, a phase shifter group 6, a sensor group 7, a lock-in amplifier group 8, a low pass. A filter (hereinafter also referred to as LPF) group 9, an A / D converter group 10, and a control unit 11 are provided.

  Out of these, the external voltage input unit 2 includes an AC power supply 21 and an application electrode 22. The AC power supply 21 generates an AC signal having a frequency fc (for example, 1 kHz) sufficiently larger than the frequency of the cardiac potential. The application electrode 22 is installed in a driver's seat (not shown), and applies an AC signal generated by the AC power source 21 to the driver B via clothes or the like.

The control unit 11 includes an oscillation unit 11a. The oscillating unit 11a outputs a signal that oscillates at a frequency corresponding to a variable frequency fr of resistance values of resistors 71c and 72c (described later).
The buffer amplifier 3 receives an AC signal generated by the AC power source 21, amplifies the input signal with a predetermined amplification degree, and outputs the amplified signal.

  Further, as shown in FIG. 2A, the frequency converter 5 mixes the AC signal (frequency fc) from the AC power source 21 and the oscillation signal (frequency fr) from the control unit 11 so that the frequency becomes (fc). ± fr), and a band pass filter B1 that selectively passes only signals in the frequency band near (fc-fr) out of the output signals from the mixer M1. The oscillation signal whose frequency is (fc−fr) is output.

  As shown in FIG. 1, the phase shifter group 6 includes phase shifters 61 and 62. The phase shifter 61 generates a reference signal to be input to the lock-in amplifier group 8 by shifting the phase of the AC signal output from the buffer amplifier 3 by 90 °. Furthermore, the phase shifter 62 generates a reference signal to be input to the lock-in amplifier group 8 by shifting the phase of the AC signal output from the frequency converter 5 by 90 °.

  The sensor group 7 includes capacitive coupling sensors 71, 72, 73. The capacitive coupling sensors 71, 72, 73 detect the cardiac potential Vd generated when the driver B comes into contact with the measurement electrodes 71a, 72a, 73a installed in the driver's seat (not shown) via clothes or the like. .

  First, the capacitive coupling sensor 71 (72, 73) includes a measurement electrode 71a (72a, 73a), a voltage follower 71b (72b, 73b), and a resistor 71c (72c, 73c). Among these, the voltage follower 71b (72b, 73b) is a known impedance conversion circuit in which the measurement electrode 71a (72a, 73a) which is the input side has a high impedance and the output side has a low impedance. The non-inverting input terminal of the voltage follower 71b (72b, 73b) receives a signal from the measurement electrode 71a (72a, 73a).

  The resistors 71c and 72c are variable resistors, and are configured such that the resistance value varies between a preset upper limit value and a lower limit value in synchronization with the oscillation frequency of the control unit 11. One ends of the resistors 71c and 72c are connected to the non-inverting input terminals of the voltage followers 71b and 72b, respectively, and the other ends of the resistors 71c and 72c are connected to the output terminal of the voltage follower 73b. Thereby, the capacitive coupling sensors 71 and 72 detect the potential with reference to the output potential of the voltage follower 73b.

The resistor 73c is a fixed resistor. One end of the resistor 73c is connected to the non-inverting input terminal of the voltage follower 73b, and the other end is grounded.
Next, the lock-in amplifier group 8 includes lock-in amplifiers 81, 82, 83, and 84.

  The lock-in amplifier 81 calculates the amplitude and phase difference (Real signal, Imag signal) between the signal having the frequency fc in the detection signal from the capacitive coupling sensor 71 and the AC signal (frequency fc) from the AC power supply 21. The signal shown is output. The lock-in amplifier 82 has an amplitude and a phase difference between a signal having a frequency (fc−fr) of the detection signal from the capacitive coupling sensor 71 and an AC signal (frequency fc−fr) from the frequency converter 5 ( A signal indicating a Real signal and an Imag signal is output.

  The lock-in amplifier 83 calculates the amplitude and phase difference (Real signal, Imag signal) between the signal having the frequency fc in the detection signal from the capacitive coupling sensor 72 and the AC signal (frequency fc) from the AC power supply 21. The signal shown is output. The lock-in amplifier 84 has an amplitude and phase difference (Real) between a signal having a frequency (fc−fr) of the detection signal from the capacitive coupling sensor 72 and an AC signal (frequency fc−fr) from the AC power supply 21. Signal indicating Imag signal).

The lock-in amplifiers 81, 82, 83, 84 are composed of mixers M11, M12 and low-pass filters F11, F12 as shown in FIG. 2 (b).
The mixer M11 includes a detection signal from the capacitive coupling sensor 71 or the capacitive coupling sensor 72 (hereinafter simply referred to as a detection signal in the description of FIG. 2B) and an AC signal (from the AC power source 21 or the frequency converter 5). Hereinafter, it is simply referred to as an AC signal in the description of FIG. The low-pass filter F11 extracts a DC component from the output signal from the mixer M11. As a result, if the phase difference between the detection signal and the AC signal is α, a signal proportional to cos (α) is output. Hereinafter, this signal is referred to as a Real signal.

  The mixer M12 mixes the detection signal with an AC signal whose phase is shifted by 90 ° by the phase shifter 61 or the phase shifter 62. The low-pass filter F12 extracts a DC component from the output signal from the mixer M12. Thereby, if the phase difference between the detection signal and the AC signal is α, a signal proportional to sin (α) is output. Hereinafter, this signal is referred to as an Imag signal.

  That is, the phase difference between the detection signal and the AC signal can be calculated from the Real signal and the Imag signal output from each of the lock-in amplifiers 81, 82, 83, and 84.

  In the lock-in amplifier 81 (83), the detection signal from the capacitive coupling sensor 71 (72) and the alternating current signal (frequency fc) from the alternating current power source 21 are input to the mixer M11 and from the capacitive coupling sensor 71. When the detection signal and the output signal (frequency fc) from the phase shifter 61 are input to the mixer M12, the signal having the frequency fc among the detection signals from the capacitive coupling sensor 71 (72) and the AC power source 21 A Real signal and an Imag signal indicating a phase difference from the AC signal (frequency fc) are output.

  In the lock-in amplifier 82 (84), the detection signal from the capacitive coupling sensor 71 (72) and the AC signal (frequency fc-fr) from the frequency converter 5 are input to the mixer M11 and from the capacitive coupling sensor 71. Of the detection signal and the output signal from the phase shifter 62 are input to the mixer M12, the signal having the frequency (fc-fr) among the detection signals from the capacitive coupling sensor 71 (72), and the frequency conversion unit 5 The Real signal and the Imag signal indicating the phase difference from the AC signal (frequency fc−fr) from the signal are output.

  Next, the low pass filter (LPF) group 9 includes low pass filters 91 and 92 as shown in FIG. The low-pass filter 91 (92) receives the detection signal from the capacitive coupling sensor 71 (72), and the cut-off frequency (for example, 50 Hz) set to be higher than the frequency of the cardiac potential among the input signals. Only signals having the following frequencies are selectively passed. Thereby, high frequency noise can be removed from the detection signal of the capacitive coupling sensor 71 (72).

  Next, the A / D converter group 10 includes A / D converters 101 and 102. The A / D converters 101 and 102 convert the analog signals from the low-pass filters 91 and 92 into digital signals and output them to the control unit 11.

Next, the control unit 11 includes a multiplexer (hereinafter referred to as MUX) 11b, an AD conversion unit 11c, and an arithmetic control unit 11d in addition to the oscillation unit 11a.
The AD conversion unit 11c receives the Imag signal and Real signal from the lock-in amplifier group 8 and the digital signals from the A / D converters 101 and 102 via the MUX 11b. The AD conversion unit 11c converts the Imag signal and the Real signal from the lock-in amplifier group 8 into digital signals and outputs them to the calculation control unit 11d, and calculates the digital signals from the A / D converters 101 and 102 as they are. Output to the control unit 11d.

  Then, the arithmetic control unit 11d measures the voltage of the detection signal from the capacitive coupling sensors 71 and 72 while changing the resistance values of the resistors 71c and 72c at the variable frequency fr, thereby measuring the object to be measured and the capacitive coupling sensor. The impedance between 71 and 72 (hereinafter referred to as source impedance) is calculated.

Here, the calculation principle of the source impedance will be described with reference to FIGS.
FIG. 3A is a graph showing the frequency distribution of the cardiac potential signal, the oscillation signal (frequency fr) from the control unit 11, and the AC signal (frequency fc) from the AC power supply 21. FIG. 3B is a graph showing the frequency distribution of detection signals from the capacitive coupling sensors 71 and 72.

  As shown in FIG. 3A, the cardiac potential signal has a low frequency (see frequency fs in the figure), and the alternating current signal (frequency fc) from the alternating current power supply 21 has a sufficiently high frequency relative to the cardiac potential signal ( (See frequency fc in the figure). Then, the oscillation signal (frequency fr) from the control unit 11 is set to a frequency between the cardiac potential signal and the AC signal from the power source 21 (see the frequency fr in the figure).

  When the detection signals from the capacitive coupling sensors 71 and 72 are measured while changing the resistance values of the resistors 71c and 72c at the variable frequency fr, as shown in FIG. 3B, the frequency fcp (= fc + fr) is obtained. A component of frequency fcm (= fc−fr) appears.

  FIG. 4 is a graph showing the relationship between the output voltages of the capacitive coupling sensors 71 and 72 and the resistance values of the resistors 71c and 72c for each source impedance. In FIG. 4, the maximum resistance value Rm of the resistors 71c and 72c is 350 GΩ, and the source impedance Zc is 0.1 GΩ (see the curve L1), 0.5 GΩ (see the curve L2), and 1 GΩ (see the curve L3). , 2GΩ (see curve L4), 3GΩ (see curve L5), 4GΩ (see curve L6), 5GΩ (see curve L7), 6GΩ (see curve L8), 7GΩ (see curve L9), 8GΩ (See curve L10), 9GΩ (see curve L11), 10GΩ (see curve L12), 20GΩ (see curve L13). In FIG. 4, the maximum value of the output voltage of the capacitive coupling sensors 71 and 72 and the resistance value of the resistors 71c and 72c are standardized so that the maximum value is 1.

  As shown in FIG. 4, as the resistance value decreases from the maximum value 1, the output voltage decreases. For this reason, when the resistance value is varied at the frequency fr, a component that varies at the frequency fcp (= fc + fr) and the frequency fcm (= fc−fr) appears in the output voltage. The degree of fluctuation of the output voltage due to the fluctuation of the resistance value increases as the source impedance increases. That is, when the source impedance becomes smaller than the sensor input impedance, it falls into the left non-linear region in the figure, the modulation degree increases, and the fcp and fcm amplitudes increase.

  FIG. 5A is a graph showing the relationship between the intensity of the component of the frequency fcm and the source impedance Zc. The horizontal axis in FIG. 5A represents the ratio Zc / Rm between the source impedance Zc and the maximum resistance value Rm.

  As shown in FIG. 5A, as the source impedance Zc increases, the intensity P (fcm) of the component that fluctuates at the frequency fcm in the output voltage increases (see the curve L21 in the figure) and fluctuates at the frequency fc. The strength P (fc) of the component is reduced (see curve L22 in the figure).

  Therefore, the source impedance is calculated by varying the resistance values of the resistors 71c and 72c at the variable frequency fr and measuring the strengths of the components of the frequency fc and the frequency fcm in the output voltage of the capacitive coupling sensors 71 and 72. be able to.

  FIG. 5B is a graph showing the relationship between the source impedance Zc and the ratio P (fc) / P (fcm) between the intensity P (fc) of the frequency fc component and the intensity P (fcm) of the frequency fcm component. It is. The horizontal axis in FIG. 5B is the ratio Zc / Rm between the source impedance Zc and the maximum resistance value Rm.

  As shown in FIG. 5B, the ratio P (fc) / P (fcm) shows a characteristic that decreases as the source impedance Zc increases. That is, the source impedance Zc can be uniquely determined by calculating the value of the ratio P (fc) / P (fcm).

  For this reason, the arithmetic control unit 11d stores in advance a correlation table indicating the correlation between the ratio P (fc) / P (fcm) and the source impedance Zc, and sets the value of the ratio P (fc) / P (fcm). After the calculation, the source impedance Zc is determined by referring to this correlation table.

  The arithmetic control unit 11d also detects the detection signal from the capacitive coupling sensors 71 and 72 every time the resistance values of the resistors 71c and 72c whose resistance values fluctuate at the variable frequency fr coincide with the preset measurement resistance values. The cardiac potential Vd is measured based on the above. That is, a voltage output is obtained when the input impedance shows a certain value.

  The arithmetic control unit 11d has an AGC (auto gain control) function, and the frequency of the detection signals from the capacitive coupling sensors 71 and 72 is Fc based on the Imag signal and the Real signal from the lock-in amplifier group 8. And the amplitude of the Imag signal and the Real signal of the AC signal (frequency fc) from the AC power supply 21 and the AC signal (frequency fc-fr) from the frequency converter 5 are calculated for the signal and the frequency of (fc-fr). Then, based on the amplitudes of the Imag signal and the Real signal calculated above, the absolute value of the amplitude is calculated for the signal having the frequency Fc and the signal having the frequency (fc−fr), and the capacitive coupling sensors 71 and 72 are detected. The amplitude of the AC signal of the AC power supply 21 is adjusted so that measurement can be performed within the voltage range and within the input voltage range of the A / D converters 101 and 102. .

  In the cardiac potential detection device 1 configured as described above, the capacitive coupling sensors 71 and 72 detect the cardiac potential Vd of the driver B without contact with the driver B by capacitive coupling, and the external voltage input unit 2 Then, an AC voltage having a preset frequency fc is applied to the driver B. Further, the resistors 71c and 72c are set in advance so as to be smaller than the frequency fc and larger than the electrocardiographic frequency between the preset upper limit value and lower limit value in synchronization with the oscillation frequency of the control unit 11. The resistance value (input impedance of the capacitive coupling sensors 71 and 72) is changed so as to continuously change and reciprocate at the frequency fr.

  Of the voltages detected by the capacitive coupling sensors 71 and 72 while the resistance values of the resistors 71c and 72c are changing, the strength of the alternating current component having the frequency fc and the frequency fcm (= fc−fr). The intensity of the alternating current component having

  In the electrocardiogram detection device 1 configured as described above, the driver B's impedance is determined by the impedance (source impedance) between the driver B and the external voltage input unit 2 and the resistance values (input impedance) of the resistors 71c and 72c. Since the heart voltage is divided, the detection voltage of the capacitive coupling sensors 71 and 72 due to the change of the input impedance increases as the source impedance increases. That is, as the source impedance increases, when the input impedance is varied at the frequency fr, the intensity of the AC component having the frequency fcm increases, while the intensity of the AC component having the frequency fc decreases.

  Since there is a correlation between the magnitude of the source impedance and the intensity of the AC component having the frequency fc and the intensity of the AC component having the frequency fcm, the AC having the intensity of the AC component having the frequency fc and the frequency fcm. By calculating the intensity of the component, the source impedance can be measured.

  Thereby, irrespective of the capacity | capacitance formed between the external voltage input part 2 and the driver | operator B, the impedance (source impedance) between the driver | operator B and the capacitive coupling sensors 71 and 72 is measured correctly. Can do.

  Further, the control unit 11 calculates a ratio P (fc) / P (fcm) between the intensity P (fc) of the component of the frequency fc and the intensity P (fcm) of the component of the frequency fcm. As a result, two data are prepared in advance: data indicating the correlation between the intensity of the AC component having the frequency fc and the source impedance, and data indicating the correlation between the intensity of the AC component having the frequency fcm and the source impedance. The source impedance can be measured by preparing in advance only data indicating the correlation between the ratio P (fc) / P (fcm) and the source impedance.

The variable frequency fr is higher than the electrocardiographic frequency and lower than the frequency fc. Thereby, the superposition of the frequency fr and the frequency fc in the desired electrocardiographic frequency band can be eliminated.
In the embodiment described above, the electrocardiogram detection device 1 is the voltage measuring device according to the present invention, the capacitive coupling sensors 71, 72, 73 are the voltage sensors according to the present invention, the external voltage input unit 2 is the voltage applying means according to the present invention, The section 11 and the resistors 71c and 72c are input impedance changing means in the present invention, the control section 11 is measuring means in the present invention, and the AGC 11b is changing means in the present invention.

  The driver B is the object to be measured in the present invention, the frequency fc is the first AC frequency in the present invention, the frequency fr is the second AC frequency in the present invention, the frequency fcp is the first modulation frequency in the present invention, and the frequency fcm is the present frequency. It is a 2nd modulation frequency in invention.

As mentioned above, although one Example of this invention was described, this invention is not limited to the said Example, As long as it belongs to the technical scope of this invention, a various form can be taken.
For example, in the above-described embodiment, the source impedance is measured using the intensity P (fc) of the component of the frequency fc and the intensity P (fcm) of the component of the frequency fcm. However, the intensity P ( The source impedance may be measured using fc) and the intensity of the component of the frequency fcp.

  In the above embodiment, the external voltage input unit 2 applies an AC voltage having the frequency fc to the driver B. However, when the device under test is located near the commercial power source and electromagnetic waves transmitted from the commercial power source are transmitted through the device under test, the source impedance is measured using the frequency fr as the commercial frequency. Also good. Accordingly, the source impedance can be measured without separately preparing an AC power source for applying an AC voltage to the object to be measured.

  The lock-in amplifier mechanism may add this function by digital mixing. As a result, the hardware of the lock-in amplifiers 81 to 84 becomes unnecessary and can be built in the control unit 11.

  DESCRIPTION OF SYMBOLS 1 ... Cardiac potential detection apparatus, 2 ... External voltage input part, 3 ... Buffer amplifier, 5 ... Frequency conversion part, 6 ... Phase shifter group, 7 ... Sensor group, 8 ... Lock-in amplifier group, 9 ... LPF group, 10 ... A / D converter group, 11 ... control unit, 11a ... oscillation unit, 11b ... MUX, 11c ... AD conversion unit, 11d ... calculation control unit, 21 ... AC power supply, 22 ... applied electrode, 61,62 ... phase shifter, 71, 72, 73 ... capacitive coupling sensor, 71a, 72a, 73a ... measurement electrode, 71b, 72b, 73b ... voltage follower, 71c, 72c, 73c ... resistor, 81, 82, 83, 84 ... lock-in amplifier, 91, 92 ... low pass filter, 101, 102 ... A / D converter

Claims (5)

A voltage measuring device for measuring the voltage of an object to be measured,
A voltage sensor that detects the voltage of the measurement object without contact with the measurement object by capacitive coupling;
Voltage applying means for applying an AC voltage having a preset first AC frequency to the object to be measured;
Input impedance changing means for changing the input impedance of the voltage sensor at a second AC frequency set in advance different from the first AC frequency;
Of the voltages detected by the voltage sensor while the input impedance changing means is operating, the intensity of the frequency modulated by the first AC frequency and the second AC frequency is measured, and the source impedance is measured. A voltage measuring device comprising: a measuring means for performing The measuring means includes
As a frequency modulated by the second AC frequency, a first modulation frequency obtained by adding the second AC frequency to the first AC frequency, and a second modulation frequency obtained by subtracting the second AC frequency from the first AC frequency. To calculate the ratio between the intensity of the AC component having the first AC frequency and the intensity of the AC component having the first modulation frequency and the intensity of the AC component having the second modulation frequency. The voltage measurement apparatus according to claim 1, wherein the source impedance is measured. The voltage measurement apparatus according to claim 1, wherein the second AC frequency is higher than a signal frequency desired for measurement and lower than the first AC frequency. The apparatus further comprises a changing means for detecting an amplitude of a frequency modulated by the first AC frequency and the second AC frequency after modulation and changing an amplitude intensity of the first AC frequency. The voltage measuring device according to claim 3. The voltage measuring device according to any one of claims 1 to 4, wherein the voltage application unit is a commercial power supply that supplies an AC voltage with the first AC frequency as a commercial frequency.