JPH081448B2 - Alan dispersion measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to an Alan dispersion measuring device (also referred to as an Alan dispersion measuring device or an Alan dispersion measuring device).

This Alan dispersion measuring device is a device that measures very small frequency fluctuations of an oscillator with high accuracy.In the field of telecommunication measurement, it is a frequency stability evaluation device for a heterodyne communication device using microwaves or a microwave oscillator for distance measurement. Moreover, in the field of optical measurement, it is used as a highly stable frequency stability evaluation device for laser light sources. Further, it is also used for evaluation of frequency stability of a laser light source for optical heterodyne communication, which is being studied as a communication system of the next generation.

[Prior Art] Alan dispersion is one index representing the frequency stability of an oscillator, and is generally defined as follows.

As shown in FIG. 4, first, in a signal source whose frequency changes temporally, τ time average value _k of the frequency is sequentially obtained. Average value of τ time of two consecutive frequencies ₁
When the square of the sample standard deviation of about _2, However, the Alan variance is _{obtained by} calculating this for N average values _k and averaging them. It is represented by. Where _τ is the nominal frequency of the source at τ time.

Generally, to express frequency stability, σ (τ), that is,
The square root of equation (2) is used. Alan dispersion is
As shown in the equation (2), it is expressed by a function of two variables of the averaged frequency data number N and the integration time τ, and as a definition, the measurement idle time, that is, _{k in} FIG.
The time at the boundary of the _{k + 1} integration region is required to be zero.

FIG. 5 is an embodiment showing a configuration of a conventional Alan dispersion measuring instrument, in which an input signal Sa is converted into a rectangular wave signal Sb by a detector 1 having a waveform shaping function. A method is used in which the signal Sb is counted by the frequency counter unit 2, data is temporarily stored in the memory 3, and then the Allan variance value is calculated by the data processing unit 4.

More specifically, the rectangular wave signal Sb that has been rectangularized through the detector 1 is input to the counter 5 in the frequency counter unit 2, and the number of rising signals is counted and accumulated.
Count value f _{c of the} number of rising signals accumulated in this counter 5
Is sent to the latch circuit 7 every τ seconds by the latch signal Sc having the period τ seconds set in the counting timing signal generator 6.

Here, the counter 5 for counting the frequency of the rising signal
Continuously counts the frequency of the rising signal of the rectangular wave signal Sb regardless of the latch signal Sc, and therefore, between the counting of the time τ and the counting of the next time τ, the Alan variance is defined. , A method that does not cause free time is taken.

However, as shown in FIG. 6, when this rising signal is counted, one count is missed due to a timing difference at the start or end of counting of the set time τ, or a rounding error of ± 1 due to over-counting. It was inevitable that the occurrence of.

This rounding error determines the stability measurement limit of the Alan dispersion measuring instrument, and its value is Becomes Where f _L is the nominal frequency of the input signal Sa for 1 second. For example, when f _L is a frequency input of 5 MHz, the stability below the Alan variance square root of 2 × 10 ⁻⁷ cannot be measured in the integration time of τ = 1 second.

The followings are known as documents relating to the Alan dispersion measuring device.

(1) "Real-time measuring device of laser frequency stability" Shiio,
Otsu, Tayuki: IEICE Technical Report OQE80-52 (198
0) (2) "Prototype of real-time measuring device for laser frequency stability"
Shiio, Otsu, Tayuki: IEICE Transactions VOL.J64-C NO.
3 (1981) PP.204-208 (3) "Performance evaluation of frequency offset lock system"
Kato, Kubogi, Otsu: Frequency Control and Application of Laser / Atomic Oscillator, 2nd Symposium Proposal (1987) [Problems to be Solved by the Invention] The present invention reduces the rounding error of ± 1 count generated in the prior art. It is an object of the present invention to provide a measuring device for Alan dispersion that improves the frequency stability measurement limit that is limited by the rounding error.

[Means for solving the problem]

The present invention solves the above problems by using the following technical means.

According to the conventional technique, as shown in FIG. 5, an input signal Sa is passed through a detector 1 having a waveform shaping function to be converted into a rectangular wave signal Sb, and the rising edge of the rectangular wave signal Sb is counted by a counter 5. On the other hand, as shown in FIG. 2, the Alan dispersion measuring device of the present invention has a frequency sufficiently higher than the frequency f _L of the input signal Sa.
The rising edge of the output signal of the oscillator 8 that outputs the electric signal of f _R is counted by the counter 5, and the counted value is counted as the wave number of the input signal Sa shaped into a rectangular wave. Latch signal Sc is output from the count timing signal generator 6 when the value reaches the value of (1). The count value of the counter 5 latched in the latch circuit 7 is stored in the memory 3 every time the latch data of the latch circuit 7 is updated. On the other hand, when the latch signal Sc is counted by the measurement data number counter 9 and the count value reaches the number M necessary for calculating the predetermined Allan variance, the measurement data number counter 9 starts data processing. Generate signal Sd. By the data processing start signal Sd, the data processing unit 4 calculates the Allan variance value based on the count value stored in the memory 3. The predetermined numbers n and M will be described later.

By such a method, when the frequency stability of the input signal Sa is measured, the reciprocal of the nominal frequency f _L of the input signal Sa, that is, the period of the input signal Sa is measured as the integration time τ, The frequency fluctuation of the input signal Sa to be measured is measured by counting the output signal of the oscillator 8 as the fluctuation of the integration time τ. The rounding error generated at this time is ± 1 count included in the count value of the oscillator 8 counted by the counter 5, and when this error is converted into the rounding error E with respect to the frequency f _L of the input signal Sa, E = F _L / f _R (4)

Therefore, the stability measurement limit of the Alan dispersion measuring instrument of the present invention, which is limited to the rounding error obtained by the equation (4), is Becomes For example, if the output frequency f _R of the oscillator 8 is 100 MHz
When set to, the stability measurement limit is 1 × 10 ⁻⁸ when τ = 1 second. This value is an order of magnitude higher than the stability measurement limit of 2 × 10 ⁻⁷ when f _L = 5 MHz in the conventional technique.

〔Example〕

The block diagram shown in FIG. 1 is a basic first embodiment relating to the Alan dispersion measuring device of the present invention, and FIG. 2 is a block diagram of the second embodiment according to the present invention.

The input signal Sa is detected by the detector 1 having a waveform shaping function, and converted into a rectangular wave signal Sb having the same repetition frequency f _L as the input signal Sa. Further, an electric signal having an oscillation frequency f _{R that} is sufficiently higher than the frequency f _{L of the} input signal Sa is output from the oscillator 8 independently of the input signal Sa.

The counter 5 constantly counts and accumulates this input signal Sa, and this count value is latched in the latch circuit 7 by the latch signal Sc which is the output signal of the count timing signal generator 6. The output timing of the latch signal Sc from the counting timing signal generator 6 is such that the rising edge of the rectangular wave signal Sb obtained by shaping the waveform of the input signal Sa by the detector 1 is counted by the counting timing signal generator 6 and the cycle of the input signal Sa is calculated. The latch signal Sc is output for each n count corresponding to the minimum time unit τ _min of the set integration time τ for measuring the set Alan variance.

That is, when the frequency f _L of the input signal Sa is 5 MHz and the minimum integration time τ _min is to be measured from 1/1000 seconds, the input signal Sa is counted in the number of 5000 waves of the rectangular wave signal Sb whose waveform is converted.
Since the time range is 1/1000 seconds, n is 5000. The count value of the counter 5 latched by the latch circuit 7 is
Each time the latch is updated, it is stored in the memory 3.

The latching of the count value by the latch circuit 7 and the operation of storing the count value are such that the count value of the measurement data number counter 9 for counting the latch signal Sc is M (the maximum integration time τ _max for measuring the Allan variance is minimum). It is determined by how many times the integration time τ _min can be collected and configured, and how many times the maximum integration time τ _max is measured to calculate the Alan variance). Therefore, M is Is.

That is, τ _min is 1/100 second and τ _max is 10 second,
When the measurement circuit of τ _max is 10 times, the value of M is 10,000. When the count value of the latch signal Sc becomes equal to M, the measurement data number counter 9 outputs the data processing start signal Sd to the data processing unit 4. The data processing unit 4, which has received the data processing start signal Sd, calculates the Allan variance based on the count value data stored in the memory 3. This calculation is performed for a predetermined τ value from τ _min to τ _max , and since the count value stored in the memory 3 is the cumulative value of the counter 5, one cumulative count value immediately before is calculated. Subtract the cumulative value counted in τ _min of
Convert to the value D counted in the time of _min . Since the count value between the given τ values is the sum of τ / τ _min D values, the count value of τ time is calculated from this sum, and N in τ time necessary for calculating the Allan variance of τ time is calculated. Create individual count data.

Since the count value of τ time calculated in this way is the count value of the oscillator 8, the input signal Sa and the oscillator 8 are counted.
The coefficient of f _L / f _R , which is the frequency ratio of, is multiplied by the count value at each τ time to calculate the frequency count value at the τ time of the input signal, and then the Alan variance value is calculated using equation (2). To measure.

〔The invention's effect〕

FIG. 3 shows a comparison of the frequency stability measurement limits of the Alan dispersion measuring instrument according to the prior art and the present invention when the input signal is 5 MHz. In FIG. 3, a is a straight line showing the measurement limit according to the prior art, and b is the frequency of the oscillator according to the present invention.
It is a straight line showing the measurement limit when set to 0 MHz. As shown in the figure, the stability limit due to the rounding error of ± 1 count in the prior art could be improved by about one digit by using the method for reducing the rounding error of the present invention. This limit can be further improved by increasing the frequency of the oscillator.

Further, by combining the present invention with the prior art Allan dispersion measuring instrument, it is possible to realize an Allan dispersion measuring instrument having a wide dynamic range capable of measuring the stability from an oscillator with extremely poor stability to an oscillator with high accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram of the first embodiment of the present invention, FIG. 2 is a block diagram of the second embodiment of the present invention, and FIG. 3 is a comparison of the prior art and the present invention regarding frequency stability measurement limits. FIG. 4, FIG. 4 is a diagram showing a signal whose frequency changes with time, FIG. 5 is a diagram showing a configuration of a conventional Alan dispersion measuring instrument, and FIG. 6 is a diagram showing a rounding error of a conventional Alan dispersion measuring instrument. It is a figure explaining. In the figure, 1 is a detector, 2 is a frequency counter unit, 3 is a memory, 4 is a data processing unit, 5 is a counter, 6 is a count timing signal generator, 7 is a latch circuit, 8 is an oscillator, and 9 is the number of measured data. Counter, Sa is input signal, Sb is rectangular wave signal, Sc
Is a latch signal, Sd is a data processing start signal, a is a straight line showing the frequency stability measurement limit according to the prior art, and b is a straight line showing the frequency stability measurement limit according to the present invention.

Claims (1)

[Claims] 1. A detector (1) for receiving an input signal having frequency fluctuations, shaping the waveform and outputting a binary electric signal, and a frequency signal having a period shorter than the pulse period of the output signal of the detector. An oscillator (8) that outputs a signal, a counter (5) that counts an output signal of the oscillator, and a latch signal Sc when the output signal of the detector is counted and the count value reaches a predetermined number n. Counting timing signal generator (6), a latch circuit (7) for latching the output signal of the counter by the latch signal Sc, and the count value of the counter latched by the latch circuit every time the latch circuit is updated. A memory (3) for storing, and a measurement data number counter (9) for counting the latch signal Sc and outputting a data processing start signal Sd when the count value reaches a predetermined required Alan variance calculation number M, Open the data processing A data processing unit (4) for receiving the signal Sd, processing the data stored in the memory, and calculating the Allan variance value of the input signal having the frequency fluctuation, and measuring the frequency fluctuation of the input signal. Alan dispersion measuring instrument that measures within one wave number.