JP2002243778A - Frequency stability inspection device and frequency stability inspection method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a frequency stability inspection apparatus and a frequency stability inspection method capable of easily and quickly inspecting the frequency stability of a signal source such as an oscillator. SOLUTION: A frequency stability inspection apparatus 1 includes a signal Sa having a frequency fa closest to a frequency f0 of a signal S0 output from an oscillator 2 to be inspected among signals Si to Si + m output from a plurality of oscillators 3. And outputs it to the mixer 6. Then, the frequency stability inspection apparatus 1 causes the filter 7 to output the signal SL of the low frequency component of the signal Smix obtained by mixing the signals S0 and Sa output from the mixer 6, and outputs the pulse of the signal SL within a predetermined measurement cycle. From the count value of the counter 8 for counting the number, the signal S in each measurement cycle is determined.
The average frequency of L is calculated, and the frequency stability of the oscillator 2 is determined based on the standard deviation of the average frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency stability inspection apparatus and a frequency stability inspection method, and more particularly to a frequency stability inspection apparatus and a frequency stability inspection method for inspecting the frequency stability of a plurality of oscillators having different oscillation frequencies. About.

[0002]

2. Description of the Related Art Conventionally, there has been known a method of measuring phase noise in order to check the frequency stability of a signal source such as an oscillator. As a method for measuring the phase noise of an oscillator such as an oscillator used in communication equipment that requires high frequency stability, a quadrature phase detection method is generally used. FIG.
FIG. 4 is a block diagram in the case of actually measuring by a quadrature phase detection method.

As shown in FIG. 1, in the quadrature phase detection method, an output of an oscillator 10 to be inspected and an output of a reference oscillator (voltage controlled oscillator) 11 having lower noise than the oscillator 10 are supplied to a mixer 12. At this time, the control voltage VC of the PLL circuit 16 is controlled to adjust the phase difference between the oscillator 10 to be inspected and the reference oscillator 11 to 90 °. Then, the sum of the noise components of the oscillator 10 and the reference oscillator 11 is supplied to the FFT analyzer 14 via the low-pass filter (LPF) 13, and the phase noise of the oscillator 10 to be inspected is measured.

Further, the result analyzed by the FFT analyzer 14, that is, the phase noise characteristic of the oscillator 10 is displayed on a display screen (not shown) of the personal computer (PC) 15. Here, the oscillator 10 to be inspected is shown in FIG.
As an example of the SSB (Single Sid) of the oscillators A and B,
e Band) As shown in the example of the phase noise characteristic curve, the operator can determine from this result that the oscillator A has better phase noise characteristics and higher frequency stability than the oscillator B. As a method other than the quadrature phase detection method, there is a method of directly analyzing the signal of the oscillator to be inspected using an FFT analyzer or a spectrum analyzer. However, the measurement accuracy of the phase noise is low, and high frequency stability is required. Not suitable for measuring oscillators.

In this way, the phase noise is measured by changing the offset frequency, and the personal computer (P
C) By inputting it into 15, a phase noise specific curve of the oscillator 10 can be displayed. Here, as shown in FIG. 6 as an example of the SSB phase noise characteristic curves of the two oscillators A and B, the operator can see from this result that the oscillator A has better phase noise characteristics than the oscillator B, It can be determined that the degree is high. As a method other than the quadrature phase detection method, there is a method of directly analyzing the signal of the oscillator to be inspected using an FFT analyzer or a spectrum analyzer. However, the measurement accuracy of the phase noise is low, and high frequency stability is required. Not suitable for measuring oscillators.

[0006]

However, in the quadrature phase detection method, it is necessary to adjust the reference oscillator 11 so that the phase difference with the oscillator 10 to be inspected becomes 90 degrees. There was a problem. Furthermore, since expensive equipment such as an FFT analyzer and a spectrum analyzer had to be used, it was not suitable as a method for inspecting all mass-produced oscillators. On the other hand, since there are oscillators of various oscillation frequencies in mass-produced oscillators, it is convenient if there is a device capable of inspecting the frequency stability of all the oscillators in a short time.

SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency stability inspection apparatus and a frequency stability inspection method capable of easily and quickly inspecting the frequency stability of a signal source such as an oscillator.

[0008]

According to the present invention, there is provided a frequency stability inspection apparatus for measuring a frequency variation of a signal output from an arbitrary signal source, which outputs reference signals having different frequencies. A plurality of reference signal output means, a plurality of reference signals output by the plurality of reference signal output means, a signal selection means for selecting and outputting one reference signal, and a signal output from the signal source. A mixer that mixes and outputs a reference signal output by the reference signal selection means, a filter that passes a low-frequency component of an output signal of the mixer, and a filter that is included in each measurement cycle for each predetermined measurement cycle. Result output means for calculating an average frequency of the output signal, calculating and outputting a standard deviation of the calculated average frequency.

According to this configuration, the average frequency of the signal output from the filter in each measurement period is calculated for each predetermined measurement period, and the standard deviation of the calculated average frequency is calculated. Start and standard deviation calculations can be performed quickly.

The present invention also relates to a frequency stability inspection apparatus for measuring a frequency variation of a signal output from an arbitrary signal source, wherein a plurality of reference signal output means for outputting reference signals having different frequencies are provided. Signal selecting means for selecting and outputting one of the plurality of reference signals output by the reference signal output means, a signal output from the signal source, and a reference signal output from the reference signal selecting means. And a filter that passes a low-frequency component of an output signal of the mixer, and a standard deviation of an average frequency of a signal output from the filter in each measurement cycle for each predetermined measurement cycle. And a result output means for converting the calculated standard deviation into a value of SSB (Single Side Band) phase noise and outputting the value.

[0011] According to this configuration, the standard deviation of the average frequency of the signal output from the filter in each measurement period is calculated for each predetermined measurement period, and the calculated standard deviation is used as the SSB (Single Side Band) phase. By converting to a noise value and outputting it, the start of measurement and the calculation of the standard deviation can be quickly performed, and the SS which has been conventionally used can be obtained.
The pass / fail judgment can be made based on the reference value for the pass / fail judgment of the B phase noise.

According to the present invention, there is provided a frequency stability inspection method for measuring a frequency variation of a signal output from an arbitrary signal source.
A selection step of selecting a reference signal having a frequency closest to the frequency of the signal output from the signal source; a signal output from the signal source; and a mixed signal obtained by mixing the reference signal selected in the selection step. An extraction step of extracting a signal of a low-frequency component, and calculating an average frequency of the extracted signal in each measurement cycle for each predetermined measurement cycle,
A result output step of calculating and outputting the standard deviation of the calculated average frequency.

According to this method, the average frequency of the low frequency component of the mixed signal of the output signal of the signal source and the reference signal in each measurement cycle is calculated for each predetermined measurement cycle, and the average frequency of the calculated average frequency is calculated. By calculating and outputting the standard deviation, it is possible to quickly start the measurement and calculate the standard deviation.

According to the present invention, there is provided a frequency stability inspection method for measuring a frequency variation of a signal output from an arbitrary signal source.
A selection step of selecting a reference signal having a frequency closest to the frequency of the signal output from the signal source; a signal output from the signal source; and a mixed signal obtained by mixing the reference signal selected in the selection step. An extraction step of extracting a signal of a low-frequency component, and calculating a standard deviation of an average frequency of the extracted signal in each measurement cycle for each predetermined measurement cycle, and calculating the calculated standard deviation as an SSB (Single Sid).
e Band) a result output step of converting the phase noise into a value and outputting the result.

According to this method, the standard deviation of the average frequency of the low frequency component of the mixed signal of the output signal of the signal source and the reference signal within each measurement cycle is calculated for each predetermined measurement cycle, and the calculated value is calculated. Standard deviation is SSB (Single Side Band)
By converting the phase noise into a value and outputting it, the start of the measurement and the calculation of the standard deviation can be performed quickly, and the pass / fail of the SSB phase noise, which has been conventionally used, is determined based on the reference value. A determination can be made.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

(1) Embodiment (1-1) Configuration of Embodiment FIG. 1 is a block diagram of a frequency stability inspection apparatus according to an embodiment of the present invention. This frequency stability inspection apparatus 1
It comprises an oscillator 2 to be inspected, a plurality of oscillators 3 having different oscillation frequencies, a power supply 4 for driving these oscillators 2 and 3, a switching circuit 5, a mixer 6, a filter 7, a counter 8, and a microcomputer 9. . Here, since the frequency stability inspection apparatus 1 aims at determining the frequency stability of a plurality of types of oscillators,
The oscillator 2 to be inspected is not limited to a specific type of oscillator. For example, any of a low frequency oscillator, a medium frequency oscillator and a high frequency oscillator may be used.

The plurality of oscillators 3 are reference oscillators having high frequency stability. Preferably, the frequency difference from the plurality of oscillators 2 to be inspected by the frequency stability inspection apparatus 1 is [k
[Hz] -order signals Si to Si + m. Oscillator 2
For example, an oscillator using bipolar transistors Q1 and Q2 as shown in FIG. 2 or a CMOS oscillator having an inverter IV constituted by a MOS transistor as shown in FIG.

The switching circuit 5 selects one of the signals Si to Si + m output from each oscillator 3 (hereinafter, referred to as a signal Sa) based on a switching control signal SE supplied from the microcomputer. And output. Hereinafter, the frequency of the signal Sa will be described as a frequency fa. The mixer 6 mixes and outputs the signal S0 of the frequency f0 and the signal Sa output from the oscillator 2 to be inspected. Therefore, the mixer 6 outputs a signal Smix having the frequency (f0−fa) and the frequency (f0 + fa).

The filter 7 allows only the low frequency component of the signal Smix output from the mixer 6 to pass,
The frequency (f0 + fa) is cut and the frequency (f0-f
The signal SL of a) is output. The counter 8 counts and outputs the number of pulses of the signal SL, and resets the count value based on a reset signal SRE output from the microcomputer 9. It is preferable to use a counter such as a so-called double counter which can continue counting without idle time even if it is reset.

The microcomputer 9 controls the entire frequency stability inspection apparatus 1 to automatically measure the frequency accuracy of the oscillator 2 to be inspected. Further, the microcomputer 9 calculates the average frequency of the signal SL within each average measurement time τ for each predetermined average measurement time τ based on the count value of the counter 8, and calculates the average based on the difference between the adjacent average frequencies. The standard deviation σy (τ) of the frequency is calculated, and the determination result based on the calculation result is displayed on a display screen (not shown). Further, the microcomputer 9 outputs a control signal for instructing start and end of the measurement when measuring the frequency accuracy of the oscillator 2.

(1-2) Operation of Embodiment Next, the operation of the frequency stability inspection apparatus 1 will be described. First, in the frequency stability inspection apparatus 1, when an operator instructs to start measurement, the microcomputer 9
Supplies the power of the power supply 4 to the oscillators 2 and 3 and
And 3 are driven. In addition, the microcomputer 9
Are the signals Si〜 output from each oscillator 3 by the switching circuit 5.
Frequency f0 of Si + m output from oscillator 2 to be measured
Is selectively output to the mixer 6 as the reference signal of the frequency fa closest to.

In this case, the microcomputer 9 selects the signal Sa to be output to the mixer 6 based on input data such as the value of the frequency of the oscillator 2 or the model name input by the operator. When the oscillator 2 to be inspected has a function of communicating identification data of the oscillator, the microcomputer 9 automatically selects the signal Sa to be output to the mixer 6 based on the identification data received from the oscillator 2. You may.

Next, the microcomputer 9 first resets the counter 8 by the reset signal SRE.
The reset processing is performed at the same time as the count value of the counter 8 is repeatedly taken in every predetermined average measurement time τ. Thus, the microcomputer 9 causes the counter 8 to count the number of pulses of the signal SL within each average measurement time τ without any idle time, and sequentially obtains the counted number of pulses. At this time, the microcomputer 9
Calculates the average frequency Y of the signal SL within each average measurement time τ based on the acquired number of pulses of the signal SL, stores the average frequency Y within the not-shown memory, and calculates the average frequency Y within the current average measurement time τ First, a division value X obtained by dividing the square of the difference from the average frequency Y within the previous average measurement time τ by 2 is calculated and stored in the memory.

Specifically, the division value X is calculated by the following equation (1). In the equation (1), Yk + 1 is the average frequency within the current average measurement time τ, and Yk is 1
This is the average frequency within the previous average measurement time τ.

(Equation 1)

When the number of times of calculation of the divided value X reaches a predetermined number of times M, the microcomputer stops the above-described acquisition of the count value and the arithmetic processing of the divided value X, and stops the M number of times stored in the memory. The standard deviation σy (τ) of the average frequency Y of the signal SL is calculated by calculating the average value of the division value X of the signal SL and calculating the square root thereof. That is,
The standard deviation σy (τ) is calculated by the following equation (2).

[0027]

(Equation 2)

As a result, the microcomputer 9
After taking the count value of the counter 8 m times, that is,
The standard deviation σy (τ) of the average frequency Y of the signal SL can be calculated immediately after the lapse of the average measurement time τ × m from the start of the measurement.

Next, the microcomputer 9 determines whether or not the calculated standard deviation σy (τ) is equal to or less than a predetermined reference value, and if determined to be equal to or less than the reference value, the frequency stability satisfies the reference value. Is displayed on the display screen, and when it is determined that the frequency stability is equal to or higher than the reference value, the fact that the frequency stability is equal to or lower than the reference value, that is, a defective product is displayed on the display screen. Then, the microcomputer 9 stops supplying power of the power supply 4 to the oscillators 2 and 3 after a lapse of a predetermined period unless a re-measurement instruction is input after displaying any determination result.
The driving of the oscillators 2 and 3 is automatically stopped. When measuring the oscillator 2 continuously, the supply of the power of the oscillator 3 does not have to be stopped.

Here, the standard deviation of the average frequency Y of the signal SL, instead of the standard deviation of the average frequency of the signal S0 of the oscillator 2, is determined for the following reason. That is, if the frequency fluctuation of the signal S0 is measured from the signal SL having a lower frequency than the signal S0, the rate of change of the average frequency is correspondingly increased, and the measurement accuracy of the frequency fluctuation is substantially improved. For example, when the frequency of the signal S0 (the frequency of the carrier) is 20 [MHz] and the frequency of the signal SL (the frequency of the carrier) is 0.1 [MHz], the measurement accuracy can be substantially increased by about 200 times. it can.

(1-3) Effects of the Embodiment Therefore, the frequency stability inspection apparatus 1 does not need to adjust the reference-side oscillator 3 so that the phase difference with the oscillator 2 to be inspected becomes 90 °. Therefore, the measurement of the frequency fluctuation of the oscillator 2 to be inspected can be started immediately, and the standard deviation σy (τ) can be calculated immediately after the measurement is completed. Can be determined. According to the experimental results of the inventors, the measurement time of the present apparatus can be 1/12 to 1/30 as compared with the case of the quadrature phase detection method, and the measurement time can be greatly reduced. Further, the frequency stability inspection apparatus 1 includes a reference side oscillator 3
, The frequency fluctuation of the oscillator to be inspected can be accurately measured over a wide frequency range without changing the reference-side oscillator or changing the oscillation frequency. The reference side oscillator 3 is automatically switched by the switching circuit 5, so that the reference side oscillator 3 after the inspection of a certain type of oscillator, which has been conventionally performed, is completed.
The labor of the operator who replaces the battery can be saved. Also,
In the frequency stability inspection apparatus 1, since the frequency fluctuation is calculated from the count result of the counter 8, the measurement can be easily performed without using expensive equipment such as an FFT analyzer and a spectrum analyzer. Thus, the frequency stability inspection apparatus 1 can determine the frequency stability of the oscillator easily and in a short time, and is optimal as an apparatus for inspecting all mass-produced oscillators.

(2) Modified Example (2-1) First Modified Example In the above-described embodiment, the case where the frequency stability is directly determined from the standard deviation σy (τ) calculated by the frequency stability inspection apparatus 1 is described. Was. By the way, it is known that the scale for judging the frequency stability is roughly classified into a “frequency domain” and a “time domain”, and has conventionally been used as one of the criteria for judging the frequency stability of an oscillator. SS
The B phase noise indicates frequency stability in the frequency domain, whereas the standard deviation σy (τ) calculated by the frequency stability inspection device 1 of the present invention indicates frequency stability in the time domain. On the other hand, there is a known method of converting the frequency stability scale from the frequency domain scale to the time domain scale by a mathematical analysis that converts the expression based on the autocorrelation function into the expression based on the power spectrum density using the Fourier transform. Although not described in detail, the relationship with the standard deviation σy (τ) can be expressed by the following equation.

[0033]

(Equation 3)

Here, Sy (f) is the power spectrum density, and Sy (f) is obtained by calculation from the value of SSB phase noise. Therefore, in the present invention, the standard deviation σ calculated by the frequency stability inspection apparatus 1 using the equation (3) is used.
By calculating the value of the SSB phase noise from the value of y (τ), it is possible to determine the frequency stability based on the conventional reference value for determining the quality of the SSB phase noise.

As shown in FIG. 4, the standard deviation σy
(Τ) and the standard deviation based on the actual measurement value of the SSB phase noise−
It is also possible to obtain a conversion formula from a characteristic curve (approximation formula) of SSB phase noise. FIG. 4 is a logarithmic scale. Therefore, in the present invention, the value of the standard deviation σy (τ) calculated by the frequency stability inspection device 1 is easily converted into the value of the SSB phase noise based on this conversion formula or the conversion table obtained from this conversion formula. It is also possible to determine the frequency stability based on the reference value of the conventional SSB phase noise quality determination.

(2-2) Second Modification In the above embodiment, the case where the present invention is applied to the device for determining the frequency stability of the oscillator has been described.
The present invention is not limited to this, and can be widely applied to an apparatus that determines or measures the frequency stability of a signal source that outputs a signal of a constant frequency, without being limited to an oscillator.

(2-3) Third Modification In the above embodiment, the case where the average frequency of the signal SL within each average measurement time τ is calculated based on the count value of the counter 8 has been described. Instead of
The average frequency may be determined using a time interval analyzer. In this case, expensive equipment is used, but the average frequency of the signal SL within each average measurement time τ is repeatedly calculated with the predetermined average measurement time τ, and the standard deviation is calculated based on the difference between adjacent average frequencies. Is calculated, so
The effect of calculating the standard deviation in a short time is maintained. Also,
The calculation may be performed using a frequency counter system incorporating a computer.

(2-4) Fourth Modification In the above-described embodiment, the case where the standard deviation of the average frequency is calculated based on the difference between the adjacent average frequencies has been described. The standard deviation of the average frequency may be calculated based on the difference between the average frequency and the average value. The standard deviation σy (τ) in this case is calculated by the following equation (3).

[0039]

(Equation 4) Here, Yaver is the average value of Yk, and the following equation (5)
Is calculated by

(Equation 5)

[0040]

As described above, the present invention can determine the frequency stability of an oscillator easily and in a short time, and is most suitable as an apparatus for inspecting all mass-produced oscillators.

[Brief description of the drawings]

FIG. 1 is a block diagram of a frequency stability inspection apparatus according to an embodiment of the present invention.

FIG. 2 is an example of a circuit diagram of an oscillator.

FIG. 3 is an example of a circuit diagram of an oscillator.

FIG. 4 is a characteristic curve diagram showing a relationship between a standard deviation calculated by the frequency stability inspection device and SSB phase noise.

FIG. 5 is a block diagram for explaining a quadrature phase detection method.

FIG. 6 is a diagram showing SSB phase noise characteristic curves of two oscillators.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Frequency stability test | inspection apparatus, 2, 10 ... Oscillator to be inspected 3, 11 ... Oscillator (reference signal output means), 4 ... Power supply, 5 ... Switching circuit (Signal selection means) 6, 12 ... ... Mixer 7 ... Filter 8 ... Counter 9 ... Microcomputer (signal selection means, result output means, reset means, standard deviation calculation means). 13: low-pass filter, 14: FFT analyzer, 15: personal computer, 16: PLL circuit, Q1, Q2: bipolar transistor, IV: inverter.

Claims (14)

[Claims] 1. A frequency stability inspection device for measuring a frequency variation of a signal output from an arbitrary signal source, a plurality of reference signal output units outputting reference signals having different frequencies, and the plurality of reference signal outputs. A signal selecting means for selecting and outputting one of the plurality of reference signals output by the means; a signal output from the signal source; and a reference signal output from the reference signal selecting means. A mixer that outputs, a filter that passes a low-frequency component of an output signal of the mixer, and an average frequency of a signal output from the filter in each measurement cycle for each predetermined measurement cycle, and the calculated average And a result output means for calculating and outputting a standard deviation of the frequency. 2. The signal selecting unit according to claim 1, wherein the signal selecting unit selects and outputs a reference signal having a frequency closest to a frequency of a signal output from the signal source from the plurality of reference signals. The frequency stability inspection device according to 1. 3. The result output means, a counter for counting the number of pulses of a signal output from the filter, a reset means for resetting the counter for each measurement cycle, and each count value of the counter immediately before resetting A standard deviation calculating means for calculating an average frequency of a signal output from the filter in each of the measurement periods, calculating a standard deviation of the calculated average frequency, and outputting the calculated standard deviation. The frequency stability inspection device according to claim 1. 4. A frequency stability inspection apparatus for measuring a frequency variation of a signal output from an arbitrary signal source, wherein: a plurality of reference signal output means for outputting reference signals having different frequencies; A signal selecting means for selecting and outputting one of the plurality of reference signals output by the means; a signal output from the signal source; and a reference signal output from the reference signal selecting means. A mixer that outputs, a filter that passes a low-frequency component of an output signal of the mixer, and a standard deviation of an average frequency of a signal output from the filter in each measurement cycle for each predetermined measurement cycle, The calculated standard deviation is calculated as SSB (Single Side Ban
d) a frequency stability inspection apparatus comprising: a result output unit that converts the value into a phase noise value and outputs the result. 5. The apparatus according to claim 1, wherein the signal selecting unit selects and outputs a reference signal having a frequency closest to a frequency of a signal output from the signal source, from the plurality of reference signals. The frequency stability inspection device according to 1. 6. The result output means, a counter for counting the number of pulses of a signal output from the filter, reset means for resetting the counter for each measurement cycle, and each count value of the counter immediately before reset A standard deviation calculating means for calculating an average frequency of a signal output from the filter in each of the measurement periods, calculating and outputting a standard deviation of the calculated average frequency, and a standard deviation-SSB phase noise. 6. The frequency stability according to claim 4, further comprising: a value conversion unit that converts the standard deviation calculated by the standard deviation calculation unit into an SSB phase noise value based on the conversion formula and outputs the value. Inspection equipment. 7. The standard deviation calculating means sequentially calculates a divided value obtained by dividing a square of a difference between the calculated average frequency in the current measurement cycle and the average frequency in the immediately preceding measurement cycle by two. The frequency stability inspection device according to claim 3, wherein the standard deviation of the average frequency is calculated by calculating a square root of an average value of the divided values. 8. The apparatus according to claim 1, further comprising: a determination result output unit that determines a frequency stability of the signal source based on an output result of the result output unit, and outputs a determination result. The frequency stability inspection device according to any one of the above. 9. The apparatus according to claim 1, wherein said signal source and said plurality of reference signal output means are oscillators.
The frequency stability inspection device according to any one of the above. 10. A frequency stability inspection method for measuring a frequency fluctuation of a signal output from an arbitrary signal source, wherein a frequency of a signal output from the signal source is the highest among a plurality of reference signals having different frequencies. A selection step of selecting a reference signal having a close frequency, a signal output from the signal source, and an extraction step of extracting a signal of a low-frequency component of a mixed signal obtained by mixing the reference signal selected in the selection step, Calculating the average frequency of the extracted signal in each measurement period for each predetermined measurement period, calculating and outputting the standard deviation of the calculated average frequency, and outputting the result. Degree inspection method. 11. The result output step includes: a counting step of counting the number of pulses of the extracted signal in each of the measurement cycles; and a counting step of counting the number of pulses of the extracted signal in each of the measurement cycles. A standard deviation calculation step of calculating an average frequency of the extracted signal in each of the measurement periods, calculating a standard deviation of the calculated average frequency, and outputting the calculated standard deviation. The described frequency stability inspection method. 12. A frequency stability inspection method for measuring a frequency variation of a signal output from an arbitrary signal source, wherein a frequency of a signal output from the signal source is the highest among a plurality of reference signals having different frequencies. A selection step of selecting a reference signal having a close frequency, a signal output from the signal source, and an extraction step of extracting a signal of a low-frequency component of a mixed signal obtained by mixing the reference signal selected in the selection step, A result output that calculates a standard deviation of an average frequency of the extracted signal in each measurement period for each predetermined measurement period, converts the calculated standard deviation into a value of SSB (Single Side Band) phase noise, and outputs the value. And a frequency stability inspection method. 13. The result outputting step includes: a counting step of counting the number of pulses of the extracted signal in each of the measurement cycles; and a counting step of counting the number of pulses of the extracted signal in each of the measurement cycles. A standard deviation calculating step of calculating and outputting a standard deviation of the calculated average frequency and a standard deviation-SSB phase noise conversion equation. The frequency stability inspection method according to claim 12, further comprising: a value conversion step of converting the standard deviation calculated in the standard deviation calculation step into an SSB phase noise value and outputting the value. 14. The apparatus according to claim 10, further comprising a determination result output step of determining a frequency stability of said signal source based on an output result of said result output step and outputting a determination result. The frequency stability inspection method according to any one of the above.