JP2010096608A - Method and device for inspecting frequency stability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automated frequency stability inspection device of high speed and high accuracy. <P>SOLUTION: This frequency stability inspection device includes a clock source 5, a first oscillation means 7 outputting a frequency slightly different from that of an output signal of a measuring object 9, a first mixing means 8 outputting a difference signal between the output signal of the measuring object 9 and an output signal of the oscillation means 7, and a first count means 10 for measuring the frequency of the difference signal output from the mixing means 8. Further, this device includes a second oscillation means 12 outputting a frequency based on the clock source 5, a reference signal source 14, a second mixing means 13 outputting a difference signal between the output frequency of the signal source 14 and the output frequency of the oscillation means 12, a second count means 16 for measuring the difference signal output from the mixing means 13, and a computation means 11 for computing based on the outputs of the first and second count means 10 and 16. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a frequency stability inspection method and an inspection apparatus for a piezoelectric device such as a crystal resonator, and more particularly to an improved frequency stability inspection method that improves measurement accuracy and can measure an object to be measured at an arbitrary frequency, And a frequency stability inspection apparatus.

In recent years, high frequency short-term stability is required for crystal resonators used in high-speed transmission devices such as GPS and digital communication. The short-term stability of the frequency represents the fluctuation (irregular fluctuation) of the output frequency within a short time period.
Patent Document 1 discloses a method for measuring the drifting (fluctuation) characteristics of a crystal resonator.
Is a block circuit diagram thereof. The measurement circuit 30 includes a crystal resonator 31 whose drift characteristics are to be measured, a transistor oscillation circuit 32 configured to oscillate at the resonance frequency of the crystal resonator 31, a sweep voltage generator 33, and a non-contact type. Current probe 34, amplifier 35, vector voltmeter 36, and amplifier 37.
A DC voltage that changes over time is generated by the sweep voltage generator 33, and the DC voltage is applied to the base of the transistor of the oscillation circuit 32 to sweep the base voltage. Then, a non-contact current probe 34 is coupled to one terminal of the crystal unit 31 to detect the crystal current flowing through the crystal unit 31, and the current is amplified to an appropriate level by the DC amplifier 35. The non-linearity of the change in current is measured with the vector voltmeter 36. On the other hand, the oscillation output of the oscillation circuit 32 is amplified to an appropriate level by the amplifier 37, and then the change of the oscillation frequency is measured by a frequency counter (not shown).
Using the measurement circuit 30 and changing the crystal current between 0.4 mA and 0.6 mA and measuring the oscillation frequency of the oscillator, it is possible to detect a crystal resonator in which a frequency jump, that is, a drift occurs. Have been disclosed.
JP-A-9-54129

However, in the drift characteristic measuring method disclosed in Patent Document 1, a synthesizer and a multiplier circuit are used as shown in the block diagram of the specific example. For this reason, there is a problem that noise generated from a synthesizer or a multiplier circuit is mixed to deteriorate the S / N ratio and the measurement accuracy is deteriorated.
The present invention has been made to solve the above problems, and in particular, it can measure the short-term stability of the frequency of an object to be measured at an arbitrary frequency, shorten the measurement time, and improve the measurement accuracy. The object is to provide a frequency stability inspection device.

SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1] A first measurement step of measuring a beat frequency generated when an output signal of a device under test is mixed with a frequency slightly different in oscillation frequency from the output signal of the device under test, a reference signal, A second measurement step of measuring a beat frequency generated when a frequency slightly different in oscillation frequency from the reference signal is mixed; and a plurality of the first samples sampled at a predetermined time interval
And a calculation step of calculating the frequency stability from the second beat frequency.

According to the frequency stability inspection method as described above, by having two measurement steps,
This has the effect of speeding up the measurement and greatly improving the measurement accuracy.

[Application Example 2] The frequency stability inspection method according to Application Example 1, further including a feedback step of transmitting a calculation result of the calculation step to the first and second measurement steps.

According to the frequency stability inspection method as described above, it is possible to automate the measurement, and it is possible to increase the measurement speed and greatly improve the measurement accuracy.

Application Example 3 A first oscillating means for outputting a clock source, a frequency that is slightly different from an output signal of the device under test based on the clock source, an output signal of the device under test and the first signal First mixing means for outputting a difference signal generated when the output signal of the oscillation means is mixed;
First counting means for measuring the frequency of the difference signal output from the first mixing means, second oscillating means for outputting a frequency based on the clock source, a reference signal source for outputting a reference frequency, A second mixing means for outputting a difference signal generated when the output frequency of the reference signal source and the output frequency of the second oscillating means are mixed; and the frequency of the difference signal output from the second mixing means. A frequency stability inspection apparatus comprising: a second counting unit that measures the above and a calculating unit that calculates based on outputs of the first and second counting units.

According to the frequency stability inspection apparatus as described above, there is an effect that it is possible to cancel the drift due to the temperature variation of the clock source and to increase the speed and accuracy of the frequency stability measurement.

Application Example 4 A first oscillating means for outputting a clock source, a frequency that is slightly different from an output signal of the device under test based on the clock source, an output signal of the device under test and the first signal First mixing means for outputting a difference signal generated when the output signal of the oscillation means is mixed;
First counting means for measuring the frequency of the difference signal output from the first mixing means, second oscillating means for outputting a frequency based on the clock source, a reference signal source for outputting a reference frequency, A second mixing means for outputting a difference signal generated when the output frequency of the reference signal source and the output frequency of the second oscillating means are mixed; and the frequency of the difference signal output from the second mixing means. Second counting means for measuring, third counting means for measuring the output frequency of the object to be measured, computing means for computing based on outputs of the first, second and third counting means, A frequency stability inspection apparatus comprising: a transmission unit that feeds back a control signal based on a calculation result of the calculation unit to the first and second oscillation units.

According to the frequency stability inspection apparatus as described above, there is an effect that the measurement can be automated. In addition, there is an effect that it is possible to increase the speed and accuracy of frequency stability measurement.

Application Example 5 The frequency stability inspection apparatus according to Application Example 3 or 4, wherein a high-frequency fundamental wave piezoelectric vibrator or a surface acoustic wave vibrator is used as the clock source.

According to the frequency stability inspection apparatus as described above, a high-purity signal, that is, a frequency with a good S / N ratio can be obtained from the clock source, so that the measurement accuracy of the frequency stability is greatly improved. is there.

[Application Example 6] The frequency stability inspection apparatus according to any one of Application Examples 3 to 5, wherein the reference frequency is supplied from an external device.

According to the frequency stability inspection apparatus as described above, the frequency stability inspection apparatus can be reduced in size and price because a reference signal source is not built in and a high stability and high purity frequency is supplied from the outside. There are advantages.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic block circuit diagram showing a configuration of a frequency stability inspection apparatus 1 according to an embodiment of the present invention.
The frequency stability inspection apparatus 1 includes a clock source 5 having an oscillation frequency Fc using a high-frequency fundamental wave piezoelectric vibrator (HFF) or a surface acoustic wave vibrator (SAW vibrator) as an oscillation element, and oscillation of the clock source 5 The first distributor 6 that distributes the frequency Fc and the output frequency Fc output from one of the first distributors 6 are used as the clock Fclk, and the output signal Fd of the device under test (DUT) 9 is the oscillation frequency. Is a first oscillation means that outputs a slightly different composite frequency
(Digital direct synthesis oscillator, frequency division ratio d1) 7 and the first mixing that outputs a difference signal (frequency IF1) generated when the output signal Fd of the device under test (DUT) 9 and the output signal of DDS7 are mixed And a frequency counter 10 as first counting means for measuring a difference signal (frequency IF1) output from the mixer 8.

Furthermore, the output frequency Fc output from the other of the first distributor 6 is used as the clock Fclk, and the second oscillating means for outputting the synthesized frequency is a DDS (digital direct synthesis oscillator, division ratio d2) 12; A reference signal source (OCXO) 14 that outputs a reference frequency Fs, a second distributor 15 that distributes the output frequency of the reference signal source 14, one output frequency Fs of the second distributor 15, and the DDS 12 Difference signal (frequency IF2) generated when mixed with synthesized frequency
And a frequency counter 16 as second counting means for measuring a difference signal (frequency IF2) output from the mixer 13.
Furthermore, an arithmetic unit (CPU) 11 is provided which is an arithmetic means for inputting the output frequency of the frequency counters 10 and 16 and calculating based on the frequency. The frequency counter 10,
16, the other output frequency Fs of the second distributor 15 is supplied as a reference frequency.

Here, a high-frequency fundamental piezoelectric vibrator (HFF) 40 used as an oscillation element of the clock source 5.
The surface acoustic wave transducer (SAW transducer) 50 will be briefly described.
FIG. 4A is a plan view showing the configuration of the high-frequency fundamental piezoelectric vibrator (HFF) 40, and FIG.
) Is a cross-sectional view taken along the line Q-Q. An excitation electrode 45a is disposed on the flat side of a piezoelectric substrate 41, for example, an AT-cut quartz substrate, having a recess 42 in the center of one main surface, and the excitation electrode 45b is opposed to the excitation electrode 45a on the recess side. Form. The excitation electrodes 45a and 45
The lead electrodes 46a and 46b extend from b to the end of the piezoelectric substrate 1 and are connected to pad electrodes 47a and 47b formed in the annular surrounding portion to constitute the HFF 40. Note that a full-surface electrode may be provided on the recessed side 42 to eliminate electrode connection failure at the boundary portion between the annular surrounding portion and the thin portion that is the vibrating portion.
By thinly processing by etching and making the recess 42 to be a fundamental vibration part, a high frequency can be achieved, and a high frequency fundamental wave piezoelectric vibrator up to several hundred MHz has been put to practical use.

FIG. 5 is a plan view showing a configuration of an example of a surface acoustic wave vibrator (SAW vibrator) 50. An IDT (comb-shaped) electrode 52 is disposed on one main surface of the piezoelectric substrate 51 along the propagation direction of the surface wave, and grating reflectors 53a and 53b are disposed on both sides of the IDT electrode 52 to provide a SAW vibrator. 50. A surface wave is excited by the central IDT electrode 52, and vibration energy of the surface wave excited by the grating reflectors 53a and 53b on both sides is reflected to the central part and confined to constitute a vibrator. The resonance frequency of the AT cut HHF 40 is inversely proportional to the thickness of the vibration part, whereas the resonance frequency of the SAW vibrator 50 is inversely proportional to the gap between the adjacent electrode fingers. It is.

The first oscillating means (DDS) 7 and the second oscillating means (DDS) 12 are DDS function generators. DDS is an abbreviation for direct digital synthesizer (digital direct synthesis oscillator). FIG. 6 is a block diagram showing the configuration of the DDS.
The DDS includes an accumulator (address calculator) 61, a waveform memory 62, a DA converter 63, and a low-pass filter LPF 64.
The frequency setting data is input to the accumulator 61 through the data input terminal 65 and the clock signal Fclk is input through the clock input terminal 66, and the output setting data is output at the rising or falling timing of the clock signal Fclk. The input data is cumulatively added. In other words, an address is generated by an accumulator (address calculator) 61 that obtains a phase in real time using the clock signal Fclk.
Here, assuming that the number of bits of the accumulator 61 is p, when the accumulated value of the accumulator 61 exceeds 2 ^p , the accumulation operation is continued with the excess as an initial value. This accumulator 6
1 output setting data (accumulated value data) is input to the waveform memory 62. The address of the waveform memory 62 corresponds to the phase of the waveform.
The waveform memory 62 is preliminarily written with sine wave digital data, and addresses are designated by the output data from the accumulator 61, and sine wave data corresponding to the address designation is output. The sine wave data is input to the DA converter 63 and converted into an analog signal by the DA converter 63. Since this analog signal has a staircase waveform that changes at the clock frequency Fclk, the analog signal is smoothed by the low-pass filter LPF 64 and output through the output terminal 67.
Here, when the clock frequency of the clock signal is Fclk and the input data is N, the output frequency Fout is Fout = (N / 2 ^p ) Fclk.
The DDS has features that the frequency resolution can be increased and the output frequency can be switched at high speed.

Using the output frequency Fd of the device under test (DUT), the output frequency Fs of the reference signal source (OCXO), the mixer, and the frequency counter, the output frequency Fd and the output frequency Fs are input to the mixer. The output difference signal df (df = | Fd−Fs |) can be measured with a frequency counter to determine the short-term frequency stability of the DUT. However, it is not practical to always prepare an OCXO close to an arbitrary output frequency of the DUT.
Although the frequency short-term stability can be obtained using a frequency synthesizer with a PLL instead of OCXO, the purity of the output signal of the frequency synthesizer is orders of magnitude worse than that of a crystal oscillator. Come.
When a DDS (Digital Direct Synthesis Oscillator) is used, the clock requires a frequency several times the output frequency of the DDS. When a general crystal oscillator is used as a clock, the output frequency of the crystal oscillator is multiplied and used. In addition to deteriorating the S / N ratio of the signal, the multiplier circuit superimposes a low frequency on the output. These noise components degrade the signal purity and become jitter, which causes variations in measured values.
In any case, since the short-term stability of the reference signal is worse than that of the DUT, it is difficult to measure the true frequency fluctuation state of the DUT with such a configuration.

When the frequency of the clock is controlled using the PLL, the phase noise characteristic of the clock is deteriorated as compared with the case where the frequency is not controlled. Therefore, in the present invention, instead of controlling the frequency of the clock source 5, as shown in FIG. 1, a DDS7-mixer 8-counter 10 and a DDS12-mixer 13-
It was conceived that the frequency fluctuation of the clock source 5 was corrected by the calculation by the arithmetic unit (CPU) 11 by measuring with two measurement systems including the counter 16. By using the calculation means, the clock source 5 is not required to have strict aging characteristics or temperature characteristics, and the HFF,
Alternatively, it is possible to use an oscillator specialized for phase noise using a SAW vibrator as an oscillation element. As a result, the measurement accuracy of the short-term frequency stability can be greatly improved.

First, frequency short-term stability is measured using one of two measurement systems. That is, the frequency division ratio d1 of the DDS 7 shown in FIG. 1 is appropriately set so that the output frequency (Fc / d1) is slightly different from the output frequency Fd of the DUT 9. DDS7 output frequency (Fc /
The frequency IF1 (= Fc / d1-Fd) of the difference between d1) and the output frequency Fd of the DUT 9 is measured by the counter 10, and this frequency is input to the arithmetic unit (CPU) 11. In this way, by using an HHF or SAW vibrator as the oscillation element for the clock source 5, both the multiplier circuit and the PLL are required, so that measurement without deterioration in measurement accuracy is possible.

However, drifts such as temperature fluctuations of the clock source 5 are superimposed on the measurement result of the counter 10. Therefore, this drift is compensated by using the other of the two measurement systems. That means
The frequency division ratio d2 is set so that the output frequency (Fc / d2) of the DDS 12 is slightly different from the output frequency Fs of the OCXO 14. The frequency (Fc / d2
) And the frequency Fs, the frequency IF2 (= Fs−Fc / d2) is output, the frequency is measured by the counter 16, and the value is input to the CPU 11. If a sufficiently stable oscillator is used for the OCXO 14, the frequency variation of IF2 is the drift of the clock source 5 itself.
Therefore, if the short-term frequency stability of IF2 measured by the counter 16 is subtracted from the short-term frequency stability of IF1 measured by the counter 10, the true short-term frequency stability of the DUT 9 can be obtained.
In the frequency stability inspection apparatus 1 of FIG. 1, the example in which the reference signal source (OCXO) 14 is incorporated is shown, but it is not always necessary to incorporate the reference signal source (OCXO) 14, and a reference frequency with high purity (good S / N ratio) is externally provided. You may supply. By receiving the reference frequency from the outside, there is an advantage that the frequency stability inspection apparatus can be reduced in size and price.

In general, the arithmetic processing function of the CPU 11 is used to perform the Allan dispersion processing on the count signals output from the counters 10 and 16 to obtain the short-term frequency stability. As is well known,
Allan dispersion is an objective evaluation defined in the time domain for short-term stability characteristics of output frequency of piezoelectric devices, etc., and continuously vibrating piezoelectric devices can achieve the same frequency over a period of time. Among the scales indicating the degree, that is, the frequency stability, this is a scale of the frequency stability particularly in the time domain. Specifically, the Allan variance is obtained by continuously performing periodic measurements a plurality of times over a period to obtain a plurality of frequency measurement values, and the difference between the temporally adjacent values of these frequency measurement values (frequency ) Is calculated by dividing the sum of squares of 2 by 2 (m-1) to calculate the square root (m is the number of samples of the frequency difference).

FIG. 2 is a schematic block diagram showing the configuration of the frequency stability inspection apparatus 2 according to the second embodiment of the present invention. The difference from the frequency stability inspection apparatus 1 of the first embodiment shown in FIG. 1 is that a third distributor 20, a counter 21 as a third counting means, and control signal lines 22, 23 are provided.
This is a point added to the frequency stability inspection apparatus 1 of the first embodiment. That is, the output frequency Fd of the DUT 9 is input to the third distributor 20, one of the output frequencies Fd of the third distributor 20 is input to the mixer 8, the other is input to the counter 21, and the output frequency of the DUT 9 Fd is measured and the measured value is input to the CPU 11. Further, the CPU 11 is connected to the DDS 7 and DDS 12 through control signal lines 22 and 23, respectively. The counter 21 as the third counting means has an OCXO
Fourteen reference frequencies Fs are supplied via the second distributor 15.

In the frequency stability inspection apparatus 2 of the second embodiment, the output frequency Fd of the clock source 5;
The output frequency Fs of the reference signal source (OCXO) 14, the sample number m of Allan variance, etc.
Assume that U11 has been input in advance.
The characteristic of the frequency stability inspection apparatus 2 is that the output frequency Fd of the DUT 9 is first converted to the third distributor 2.
The measurement value is measured by the counter 21 via 0, and the measured value and the pre-input data are stored in the CPU.
11, the division ratios d1 and d2 are obtained, and these values can be automatically set via the control signal lines 22 and 23. If the frequency division ratios d1 and d2 can be set, the frequency short-term stability of the DUT 9 can be automatically measured.
2 also shows an example in which the reference signal source (OCXO) 14 is built in, the example of the frequency stability inspection apparatus 2 in FIG. 2 does not necessarily need to be built in, and supplies a frequency with a good S / N ratio from the outside. May be. The frequency stability inspection apparatuses 1 and 2 according to the present invention are not limited to high-precision measurement of short-term frequency stability, but also high-precision measurement of medium-term or long-term frequency fluctuations (= medium frequency (long-term) stability). Needless to say, it can also be applied.
A feature of the frequency stability inspection apparatus 1 or 2 according to the present invention is that the frequency stability of the object to be measured can be measured at high speed and with high accuracy, and measurement accuracy of 5 × 10 ⁻¹¹ is possible with a gate time of 20 ms. Yes, the measurement speed was increased and the accuracy was greatly improved compared to the conventional measurement method.

1 is a schematic block circuit diagram of a frequency stability inspection apparatus 1 according to the present invention. The schematic block circuit diagram of the frequency stability test | inspection apparatus 2 of a 2nd Example. A conventional measurement block diagram of drift characteristics. (A) is a top view of a high frequency fundamental wave piezoelectric vibrator (HFF), (b) is sectional drawing. The top view of a surface acoustic wave vibrator (SAW vibrator). 1 is a schematic block diagram of a digital direct synthesis oscillator (DDS).

Explanation of symbols

1, 2 ... Frequency stability inspection device, 5 ... Clock source, 6, 15, 20 ... Distributor, 7, 12 ...
Digital direct synthesis oscillator (DDS), 8, 13 ... mixer, 9 ... device under test (DUT), 10
16, 21 ... counter, 11 ... arithmetic unit (CPU), 14 ... reference signal source (OCXO),
22, 23 ... Control signal line

Claims (6)

A first measurement step of measuring a beat frequency that is generated when the output signal of the device under test and the output signal of the device under test are mixed with frequencies having slightly different oscillation frequencies;
A second measurement step of measuring a reference signal and a beat frequency generated when a frequency slightly different in oscillation frequency from the reference signal is mixed;
A calculation step of calculating frequency stability from the plurality of first and second beat frequencies sampled at a predetermined time interval;
A frequency stability inspection method comprising: The frequency stability inspection method according to claim 1, further comprising a feedback step of transmitting a calculation result of the calculation step to the first and second measurement steps. A clock source;
First oscillation means for outputting a frequency slightly different in oscillation frequency from the output signal of the device under test based on the clock source;
First mixing means for outputting a difference signal generated when the output signal of the device under test and the output signal of the first oscillation means are mixed;
First counting means for measuring the frequency of the difference signal output from the first mixing means;
Second oscillating means for outputting a frequency based on the clock source;
A reference signal source that outputs a reference frequency, a second mixing unit that outputs a difference signal generated when the output frequency of the reference signal source and the output frequency of the second oscillation unit are mixed, and
Second counting means for measuring the frequency of the difference signal output from the second mixing means;
Computing means for computing based on outputs of the first and second counting means;
A frequency stability inspection apparatus comprising: A clock source;
First oscillation means for outputting a frequency slightly different in oscillation frequency from the output signal of the device under test based on the clock source;
First mixing means for outputting a difference signal generated when the output signal of the device under test and the output signal of the first oscillation means are mixed;
First counting means for measuring the frequency of the difference signal output from the first mixing means;
Second oscillating means for outputting a frequency based on the clock source;
A reference signal source that outputs a reference frequency, a second mixing unit that outputs a difference signal generated when the output frequency of the reference signal source and the output frequency of the second oscillation unit are mixed, and
Second counting means for measuring the frequency of the difference signal output from the second mixing means;
Third counting means for measuring the output frequency of the device under test;
Computing means for computing based on the outputs of the first, second and third counting means;
A transmission means for feeding back a control signal based on the calculation result of the calculation means to the first and second oscillation means;
A frequency stability inspection apparatus comprising: The frequency stability inspection apparatus according to claim 3 or 4, wherein a high-frequency fundamental wave piezoelectric vibrator or a surface acoustic wave vibrator is used as the clock source. 6. The frequency stability inspection device according to claim 3, wherein the reference frequency is supplied from an external device.

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