JP3138315B2 - Asynchronous calibration circuit and frequency detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an asynchronous calibration circuit for asynchronously calibrating an AC signal and a frequency detection circuit capable of easily detecting the frequency of the AC signal whose approximate value is known.

[0002]

2. Description of the Related Art Generally, a measurement system has a calibration device for calibrating its own voltage level. For example, in a measurement system that handles AC signals, prior to measurement, a relationship between a level value represented by the measurement system and its true level value is obtained using a reference AC signal as a reference. In the case where the measurement system has a digital signal processor (DSP) 61 as shown in FIG. 6, the calibration is often performed by arithmetic processing based on the DSP. This is because resources can be more effectively used indirectly by digital detection using an existing DSP than by directly detecting a voltage level by analog detection by newly introducing a detection circuit or the like into a measurement system.

FIG. 6A shows a configuration of an analog section employed in a linear tester or the like, and converts an AC signal generated by an arbitrary waveform generator 62 having a D / A converter into an A / D signal.
In addition to the linear IC under test (DUT) 64 of the conversion system, the DSP 61 directly processes the detection signal from the detection signal. 6B, a digital signal is added to the DUT 64 of the D / A conversion system, the analog detection signal is converted into a digital signal by the A / D converter 63, and the DSP 61 performs DSP processing. Here, the DSP process samples the signal and performs FF
The voltage level is obtained by applying T (fast Fourier transform).

To calibrate by DSP processing, first,
It is necessary to obtain the reference voltage of the reference AC signal and the detection voltage of the detected AC signal by the FFT processing, and then correct and correct the difference between the detection voltage and the reference voltage. F
According to the principle of FT, in order to accurately determine the reference voltage of the reference AC signal, its frequency must be known accurately. However, when the reference signal is an AC signal, the frequency tends to deviate from the target frequency with the passage of time. Therefore, it is necessary to accurately know the true frequency or correct the target frequency. For this reason, a conventional digital calibration circuit using an FFT employs a synchronization method that does not require such a method.

Next, a specific example of the calibration circuit of the synchronous system will be described. FIG. 4 shows a calibration circuit for a signal generation circuit in the measurement system. The signal generation circuit 40 converts the data read from the latch circuit 41 into D / A in synchronization with the clock CLK0.
The analog signal is obtained by conversion by the converter 42 and the filter 46
To generate an AC signal having a desired frequency and voltage level required for measurement. The calibration circuit 49 that performs the voltage calibration of the signal generation circuit 40 has a reference AC signal generator 44 that generates a reference AC signal. The reference AC signal is synchronized with the clock CLK0 for the reason described above. The clock CLK1 via the synchronization adjustment circuit 43
In the circuit 44. AC voltage detection circuit 45
Is composed mainly of a DSP, has a function of obtaining the voltage of an AC signal by FFT processing, and obtains a reference voltage generated by the reference AC signal generator 44, and then switches the switch 47.
And the voltage of the detected AC signal generated by the signal generation circuit 40 is obtained. Then, the voltage of the detected AC signal is calibrated by correcting the difference with respect to the reference voltage. As described above, the relationship between the true value and the value represented by the signal generation circuit is obtained and calibration is performed.

In the measurement system, the frequency of the reference signal generated by the reference AC signal generator may be measured in experiments, inspections, or the like. In such a case, the voltage level can be conventionally observed. In many cases, a spectrum analyzer used as a computer is used as it is, or a frequency counter is used.

[0007]

However, the above-mentioned prior art has the following disadvantages.

(1) If a conventional synchronization method is used for the calibration circuit for the AC signal, a synchronization circuit for synchronizing with the clock of the measurement system is required inside the reference AC signal generator. The hardware load on the container increases. In addition, since the circuit must be routed for synchronization, the degree of freedom in design is limited.

(2) On the other hand, when simply measuring the frequency of a reference signal or the like generated by a reference AC signal generator, it is too much to use a general-purpose device such as a spectrum analyzer or a frequency counter as in the past. Is too large. For example, the AC signal is not completely unknown,
There is a case where the frequency of an AC signal whose approximate value of the frequency is known in advance is detected. In particular,
This is a case where the frequency set as a target of 1 kHz in the design stage has deviated from the target frequency with the passage of time, but it is desired to know the true frequency. In such a case, it is very convenient if the detection can be easily performed using, for example, components in the system, instead of using an oversized external device such as a general-purpose device.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an asynchronous calibration circuit which solves the above-mentioned disadvantages of the prior art and can perform highly accurate calibration while operating asynchronously. Another object of the present invention is to provide a simple and accurate frequency detection circuit.

[0011]

An asynchronous calibration circuit according to the present invention is a calibration circuit for calibrating the voltage level of an AC signal to be calibrated formed by D / A converting digital data output in synchronization with a clock. A reference AC signal generator for generating a reference AC signal for calibrating the voltage level of the uncalibrated AC signal asynchronously with the clock, sampling the AC signal to be calibrated and the reference AC signal, and digitally sampling the sampled value. An AC voltage detection circuit for detecting each voltage value of the AC signal by performing signal processing, wherein the AC voltage detection circuit detects a voltage level of the AC signal to be calibrated and a reference signal generated from a reference AC signal generator. The voltage level of the AC signal is detected as in the following a to e,
When the detected reference value is within the allowable range, the difference between the reference value and the detection value of the AC signal to be calibrated is corrected,
Alternatively, the difference between the target value and the detected value of the AC signal to be calibrated is corrected.

A. An asynchronous reference AC signal generated from a reference AC signal generator is input to the AC voltage detection circuit, and a sampling measurement section including a corresponding frequency on a sampling frequency corresponding to the frequency of the AC signal is set. Select some sampling frequencies that define the section, b. By sampling and processing the AC signal at the selected sampling frequency, a voltage level indicating the proximity to the corresponding frequency is obtained for each sampling frequency; c. Comparing the obtained voltage levels, a sampling point having a small proximity is excluded from the measurement section, and a narrow section defined by the remaining sampling points is set as a new measurement section, and d. The sampling frequency is converged by repeatedly selecting the sampling frequency within the new measurement section as described above, obtaining the voltage level at that frequency, and further narrowing the next measurement section, e. The voltage level of the reference AC signal at the converged sampling frequency is set as a reference value.

In this case, in order to perform more accurate calibration, it is further detected whether or not the true frequency of the reference AC voltage generated from the reference AC signal generator is within an allowable range, and It may be determined whether the signal generator and the AC voltage detection circuit are correct or not.

On the other hand, a frequency detecting circuit according to the present invention is a frequency detecting circuit for detecting a frequency of an AC signal whose approximate value is known, wherein the sampling measurement includes a corresponding frequency on a sampling frequency corresponding to the frequency of the AC signal. A section is set, sampling frequencies for forming the measurement section are selected at several points at equal intervals, and the AC signal is sampled and processed at the selected sampling frequency, thereby obtaining a voltage level indicating the proximity to the corresponding frequency. The obtained voltage level is obtained for each sampling frequency, the obtained voltage levels are compared, sampling points having low proximity are excluded from the measurement section, and a narrowed section formed by the remaining sampling points is set as a new measurement section. Select the sampling frequency again as described above, and find the voltage level at that frequency. By repeating the operation of further narrowing down the next measurement interval, it converges the sampling frequency is from converged sampling frequency which was set to determine the true frequency of the corresponding frequency to the AC signal.

In the present invention, the degree of proximity refers to a degree indicating how close a sampling frequency is to a corresponding frequency by a voltage level obtained from a certain sampling frequency. In addition, the method of obtaining the voltage level at each sampling frequency includes, for example,
(Fast Fourier Transform). The sampling points to be selected need not be at equal intervals.

[0016]

As described above, the calibration circuit of the present invention employs an asynchronous system. However, there is concern about making the calibration circuit asynchronous with the system because the frequency of the reference AC signal in the calibration circuit deviates from the target frequency. The difference is that it is treated as the frequency at the time of design or setting, processed as it is, and calibrated based on the incorrect reference voltage (in this regard, the conventional one adopts the synchronization method. And the reference voltage is determined by one measurement). However, the uneasiness factor is that after accurately knowing the true frequency of the reference AC signal using the above-described frequency detection circuit of the present invention and performing self-calibration,
This can be resolved by performing the original calibration. In this case, since the calibration target in the present invention is the calibration of the voltage level, the phase of the reference AC signal is of course not a problem, and the frequency can be adjusted to the target frequency if the value is within the allowable range. No correction is required. It is only necessary to know the frequency of the reference AC signal accurately. If the frequency is accurately known, the voltage level of the reference AC signal can be accurately known, and here, the reference value can be accurately known for the first time. When calibration is performed with the reference value that is accurately determined in this way, highly accurate calibration can be performed asynchronously. Hereinafter, the principle of the present invention will be described.

In order to accurately know the voltage level of an AC signal using FFT, it is necessary to satisfy, for example, equation (1). Therefore, it is necessary to accurately know the frequency F _{0 of the} AC signal. .

F ₀ / f _c = M / N (1) where f _c : sampling frequency of the AC signal M: cycle number of the AC signal included in the sample number N N: sample number In the case of a reference AC signal generated by a reference AC signal generator which may fluctuate thereafter and have a frequency value different from the initial value, when the voltage is to be accurately known using FFT, First, it is necessary to accurately know the true frequency F ₀ of the AC signal, and then it is necessary to know the voltage level at the frequency F ₀ in a two-step procedure.

By the way, when an AC signal of a frequency F ₀ is sampled at an arbitrary frequency f and the voltage level is observed by applying FFT, as shown in FIG. 5, the sampling frequency f is the sampling frequency corresponding to the frequency F _{0 of the} AC signal. When the frequency coincides with the corresponding frequency f ₀ , the equation (1) is satisfied, so that the voltage level shows a peak. However, as the frequency deviates from the corresponding frequency f ₀ , the expression (1) is not satisfied, so that the voltage level gradually decreases. Conversely, if the sampling frequency is shifted, the level changes, and if a frequency showing the maximum level is found, it becomes the corresponding frequency of the AC signal. The present invention utilizes this phenomenon. Therefore, when detecting the frequency of the AC signal, by using the FFT to find the magnitude or peak of the voltage level and sequentially reducing the frequency, the corresponding frequency f of the AC signal is finally determined.
_{0, and} thus the detected frequency F ₀ can be accurately grasped.

In this case, if it is known in advance that the corresponding frequency is within a specific range, the range is set as an initial measurement section, and the measurement section is set so that a sampling point indicating a higher voltage level remains in that section. As the frequency is gradually reduced, the frequency in the final measurement section becomes as close as possible to the corresponding frequency, and can be regarded as the corresponding frequency. Since the present invention checks the frequency step by step,
The true frequency can be detected much faster than when the search is performed over all frequencies, and the frequency can be detected with high resolution. Here, the fact that the true frequency can be detected means that the calibration system is operating normally, and if the true frequency cannot be detected or the detected frequency exceeds the allowable value. Has an error in the calibration system. Therefore, detecting the true frequency is nothing but performing self-diagnosis of the calibration system, that is, self-calibration. If an accurate reference value is obtained based on the detected true frequency, highly accurate calibration can be performed.

Note that simple frequency detection can be performed with high accuracy based on the above principle.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 shows an embodiment of the frequency detection circuit which enables the calibration circuit to be asynchronous. In the figure, the AC signal generator 31 generates an AC signal having a frequency F _0. Although this F ₀ fluctuates from a target value in advance, it does not fluctuate greatly, and therefore it is assumed that the approximate value is known. FF
In order to recognize F ₀ using T, the voltage level at the time of sampling the AC signal is important.

The A / D converter 32 samples the AC signal from the AC signal generator 31 and converts it into a digital signal in accordance with the clock CLK output from the oscillator 33.
The oscillator 33 outputs three types of clocks CLK having different frequencies over time during one measurement. Therefore, the oscillator 33 is constituted by a programmable oscillator whose frequency is determined based on a command from the digital signal processing unit 34. The three clock frequencies have two frequency measurement sections.
In order to be able to split, the upper and lower points of the section,
And the midpoint frequency is selected.

The digital signal processing unit 34 may be a versatile CPU which is strong in control, or a DSP which is strong in operation. Here, an FFT that mainly performs DSP processing is performed, and a voltage level is obtained from the sampling signal. A / D
Each digital signal output from the converter 32 is processed, and the frequency of the clock output from the oscillator 33 increases as the frequency approaches the corresponding frequency f ₀ corresponding to the frequency F _{0 of the} AC signal. The maximum voltage level is obtained, the obtained voltage levels at each clock frequency are compared with each other, a section in which the corresponding frequency f ₀ of the AC signal is present is selected, and the frequency obtained by further dividing this section is obtained. The oscillation frequency commands are output to the oscillator 33 so as to output different clocks. It should be noted that whether the voltage level of the AC signal is viewed as a peak value, an effective value, or other criteria is completely arbitrary.

The oscillator 33 generates a sampling clock CLK having a different frequency in a time series manner based on an oscillation frequency command from the CPU 34 and outputs the clock CLK to the A / D converter 32. As described above, three types of clocks CLK are output in one step. This clock frequency must be at least twice the frequency f _{0 according} to the sampling theorem.

Next, the operation of the above-described frequency detection circuit will be described with reference to FIGS. A frequency region including the detected frequency f ₀ and allowable in the calibration is set in advance, and this is set as an initial sampling measurement section S1. Here, the allowable frequency region is a section in which FFT processing can be performed, and is a section corresponding to a section in which the variation of the detected frequency occurs.

First Step In order to divide the initial measurement section S1 into two, the following three measurement points are set in the sampling measurement section S1. The upper limit frequency f ₁ of the sampling measurement section S1,
The lower limit frequency f ₃ and the frequency f ₂ in the middle of the measurement range. In the illustrated example, the corresponding frequency f corresponding to the detected frequency
₀ it is assumed that lies between f ₂ and f _3. A frequency command is given from the digital signal processing unit 34 to the oscillator 33, and a clock CLK having three sampling frequencies is given from the oscillator 33 to the A / D converter 32 in a time series manner. The order of the applied clocks CLK is arbitrary. The A / D converter 32 samples the AC signal of the frequency F ₀ input thereto at the frequency of each clock CLK, and feeds back the sampling signal to the digital signal processing unit 34. In the digital signal processing unit 34 obtains these frequencies f _1, f _2, the voltage level v ₁ corresponding to each frequency by applying the FFT to the sampled value at _{f 3, v 2, v 3} .
Then, the magnitude of these voltage levels is determined.

Second Step The magnitude relationship between the voltage levels in the first step is represented by v ₁ <v ₂ <
When v was _3, the voltage level at the sampling frequency multiplied by the FFT as described above has a maximum when the sampling frequency f is reduced enough away from the frequency f ₀ of the alternating current signals were made consistent increase enough to close the opposite Therefore, the detected frequency f ₀ exists between f ₃ and f ₂ corresponding to v ₂ and v ₃ . Accordingly, the digital signal processing unit 34 determines the frequencies f ₂ and f ₃ corresponding to the remaining voltage levels v ₂ and v ₃ except for the smallest voltage level v _1.
And a section which is half of the initial section S1 is defined as a new measurement section S2. And this measurement section S2
2 equal portions upper limit frequency f2, the lower limit frequency f _3, to obtain a measuring point of the three points to the intermediate frequency f ₄ newly. Then, similarly to the first step, the voltage levels at these three points are obtained, and the magnitude is determined. In this case, since the voltage levels at f ₂ and f ₃ have already been obtained, the previous values can be used. However, when trying to obtain higher detection accuracy, it is preferable to obtain it again.

Third Step In the second step, the magnitude relationship between the voltage levels is expressed as v ₂ <v ₃ <v
When I was _4, f ₃ and f4 two equal portions, to obtain now the upper limit frequency f _4, the lower limit frequency f _3, the measurement points 3 points to the intermediate frequency f _5. Then, it is the same as the first step.

Hereinafter, the steps are repeated in the same manner, so that sampling points having a large voltage level remain.
By repeating the number of times of measurement up to the desired resolution, it is possible to finally know the corresponding f _{0 and} eventually the true frequency F ₀ of the AC signal. It is generally considered that the frequency value finally obtained will be the average of the upper and lower frequency limits.If the number of measurements is repeated up to the measurement resolution limit, which of the upper and lower frequency limits will be used? It is completely arbitrary to take the average of the upper and lower limit frequencies or to take any other method. This is because there is no difference between them at high resolution. In other words, the same result can be obtained regardless of which one is adopted due to the limit due to the detection error. Incidentally, if the number of steps is n, the resolution can be represented by 1/2 ⁿ . If the point frequency coincides with the corresponding frequency f ₀ during the repetition, a predicted peak voltage is obtained, and the sampling frequency at that time becomes a value corresponding to the frequency f ₀ .
The measurement is stopped there.

As described above, according to this embodiment, the AC signal to be calibrated is sampled at three sampling frequencies within the set frequency range, and the sampling signal is subjected to FFT processing to measure the voltage level. From the measurement result, the corresponding frequency f ₀ or the detected frequency F ₀ was found in the process of sequentially narrowing the measurement range, such as measuring the frequency range to be measured by halving the set frequency range. The detection probability is increased, and the detection time can be significantly reduced. In addition, since the resolution increases by 1/2 ⁿ in the process of narrowing the measurement range, highly accurate detection becomes possible. In the above embodiment, the measurement section
By measuring at three points, the upper limit, the lower limit, and the middle point, the measurement section becomes 2
One is divided into, in any that determine each time whether the f ₀ enters, has been described for determining the frequency gradually narrowing the interval, in principle to measure the measurement period at 4 or more points It is also possible.

As described above, according to this embodiment, instead of using a large external device such as a general-purpose device, the components in the measurement system, that is, the DSP and the A / D converter are used simply. It is very convenient because the frequency can be detected.

FIG. 2 shows an asynchronous calibration circuit according to the present embodiment using the above-described frequency detection circuit. The asynchronous calibration circuit 29 is used in a linear IC tester or the like, and is a calibration circuit for the signal generation circuit 20 that generates an arbitrary AC signal and is a premise of a measurement signal applied to the DUT. The signal generation circuit 20 converts the digital data read from the latch circuit 21 into a digital signal in synchronization with the clock CLK0.
An arbitrary quantized analog signal is obtained by conversion by the A-converter 22, and an arbitrary frequency component is extracted from the analog signal by the filter 26, thereby obtaining an AC signal having a desired frequency and voltage level required for measurement. Signal.

The calibration circuit 29 for calibrating the voltage of the signal generation circuit 20 has a reference AC signal generator 24 for generating a reference AC signal which is set to a constant frequency and can set an arbitrary reference voltage level. This reference AC signal generator 2
4 is not synchronized with the signal generation circuit 20, and therefore, the reference AC signal generated from this is the clock CLK0.
(Reference clock) is generated asynchronously. The AC voltage detection circuit 25 is mainly composed of a DSP, and has a function of obtaining a voltage of an AC signal by FFT processing.

First, the AC voltage detection circuit 25
The reference voltage generated by the reference AC signal generator 24 is obtained as a true value. At this time, instead of being obtained by one measurement,
It is determined by several to dozens of measurements. The true frequency is obtained, and the voltage level at that frequency is detected by a two-stage method. The reference AC signal generator 24 outputs the reference clock C
Since it is asynchronous with LK0, the frequency of the reference AC signal generated therefrom cannot be known from the correlation with CLK0, but must be known as an absolute value. Although the value is an approximate target value, there is a high possibility that the value is deviated, and if the frequency value is deviated, accurate calibration cannot be performed if it is obtained by one measurement. is there.

The true frequency of the reference AC signal is accurately obtained by the FFT processing by the AC voltage detection circuit 25 so that the equation (1) is satisfied, and the reference AC voltage at the true frequency is accurately obtained. Incidentally, the number of times of actual measurement was about 5 times. Then, the switch 27 is switched to determine the voltage of the AC signal to be calibrated generated by the signal generation circuit 20. Since the AC signal to be calibrated is synchronized with the reference clock CLK0, the frequency is known, and thus can be obtained by one measurement. Then, the AC voltage detection circuit 25 compares the reference voltage with the AC voltage to be calibrated, returns the difference to the reference voltage to the D / A converter 22, and corrects the voltage of the AC signal to be calibrated. As described above, the relationship between the true value and the output value of the signal generation circuit 20 is obtained, and calibration is performed.

According to the present embodiment, the true value of the reference AC frequency and the reference AC voltage can be accurately obtained while the reference AC signal generator is asynchronous, so that correct calibration can be performed. Also, by making the reference AC signal generator asynchronous, the relationship with the signal generation circuit system can be cut off and made independent, so that there is no restriction on the signal generation circuit system or measurement system, and the synchronization Since no adjustment circuit or synchronization circuit is required, the burden on hardware is reduced and the degree of freedom in design is increased.

In particular, a linear IC having a CPU capable of digital signal processing in the system as in the first embodiment.
This is particularly effective when the present invention is applied to a tester or the like because there is no additional hardware. In the embodiment of FIG. 2, a case has been described in which the difference between the reference value and the detected value of the AC signal to be calibrated is corrected to perform calibration. However, when the detected reference value is within an allowable range, the AC voltage is Since the normal operation of the detection circuit is guaranteed, correct the difference between the target value and the detected value of the AC signal to be calibrated using this AC voltage detection circuit without using the reference value. You can also. Although the case where the reference AC signal generator has a constant frequency, that is, the case of a monotone has been described, the present invention can be applied to the case of a multitone. Furthermore, the case where the number of the reference AC signal generators is one has been described, but the number may be two or more. In this case, the two or more reference AC signal generators are configured so that the frequency is constant and the voltage is different. It may be set, or set so that the voltage is constant and the frequency is different.

[0039]

According to the present invention, the following effects can be obtained.

(1) According to the asynchronous calibration circuit of the first aspect, high-accuracy calibration can be performed while adopting a simple configuration of non-synchronization.

(2) According to the frequency detection circuit of the second aspect, by adopting a simple method of sequentially narrowing the measurement range and narrowing the measurement points, high-precision and high-speed frequency detection is performed. Can be easily performed.

[Brief description of the drawings]

FIG. 1 is a step diagram showing a process of narrowing down a frequency to be detected according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a signal generation circuit including an asynchronous calibration circuit of the present invention.

FIG. 3 is a configuration diagram of a frequency detection circuit according to the present embodiment.

FIG. 4 is a configuration diagram of a signal generation circuit including a synchronous calibration circuit according to a conventional example.

FIG. 5 is a voltage level characteristic diagram of an AC signal obtained by FFT obtained when a sampling frequency is shifted around a detected frequency f ₀ .

FIG. 6 is a configuration diagram of an analog unit of a linear tester exemplified as a system having a DSP.

[Explanation of symbols]

Reference Signs List 20 signal generation circuit 21 latch circuit 22 D / A converter 23 synchronization adjustment circuit 24 reference AC signal generator 25 AC voltage detection circuit 26 filter 27 changeover switch 29 calibration circuit 31 AC signal generator of frequency f ₀ 32 A / D conversion Unit 33 oscillator 34 digital signal processing unit

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields investigated (Int. Cl. ⁷ , DB name) G01R 23/02 G01R 19/25 G01R 23/16 G01R 35/00

Claims (2)

(57) [Claims] A calibration circuit for calibrating a voltage level of an AC signal to be calibrated output in synchronization with a clock, wherein a reference AC signal for calibrating a voltage level of the calibrated AC signal is generated asynchronously with the clock. A reference AC signal generator, and an AC voltage detection circuit that samples each of the AC signal to be calibrated and the reference AC signal, and performs digital signal processing on the sampled value to detect each voltage value of the AC signal. The voltage level of the AC signal to be calibrated is detected by the voltage detection circuit, and the voltage level of the reference AC signal generated from the reference AC signal generator is detected as in the following a to e. The difference between the reference value and the detected value of the AC signal to be calibrated, or the difference between the target value and the detected value of the AC signal to be calibrated. A non-synchronous calibration circuit characterized in that the correction is made. a. An asynchronous reference AC signal generated from a reference AC signal generator is input to the AC voltage detection circuit, and a sampling measurement section including a corresponding frequency on a sampling frequency corresponding to the frequency of the AC signal is set. Select some sampling frequencies that define the section, b. By sampling and processing the AC signal at the selected sampling frequency, a voltage level indicating the proximity to the corresponding frequency is obtained for each sampling frequency; c. Comparing the obtained voltage levels, a sampling point having a small proximity is excluded from the measurement section, and a narrow section defined by the remaining sampling points is set as a new measurement section, and d. The sampling frequency is converged by repeatedly selecting the sampling frequency within the new measurement section as described above, obtaining the voltage level at that frequency, and further narrowing the next measurement section, e. The voltage level of the reference AC signal at the converged sampling frequency is set as a reference value. 2. A frequency detection circuit for detecting a frequency of an AC signal whose approximate value is known, wherein a sampling measurement section including a corresponding frequency on a sampling frequency corresponding to the frequency of the AC signal is set. By selecting a number of sampling frequencies that define the area, sampling the AC signal at the selected sampling frequency and processing the voltage, a voltage level indicating the proximity to the corresponding frequency is determined for each sampling frequency. Compare the sampling points with the smaller proximity to the measurement section, remove the narrowed section defined by the remaining sampling points as a new measurement section, and select the sampling frequency again in the new measurement section as described above. To find the voltage level at that frequency and further narrow the next measurement interval By repeating the operation, converges the sampling frequency, the frequency detection circuit being characterized in that so as to determine the true frequency of the corresponding frequency to the AC signal from the converged sampling frequency.