RU2679930C1 - Precision digital frequency meter - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: invention relates to measurement technology and can be used in radio engineering, electrical engineering, metrology and other industries for precision measurement of the frequency of sinusoidal signals, frequency and phase deviations from the nominal value, time intervals, as well as to obtain statistical parameters characterizing the frequency stability for different periods of time. Precision digital frequency meter is proposed, which includes an input shaper of rectangular signals, a phase detector, a frequency comparator, a synthesizer equipped with a shaper of rectangular signals, a generator of exemplary frequency, the first analog-to-digital converter, a microcontroller, an information processing, indication and control unit, which further comprises a second high-frequency analog-to-digital converter, a random access memory and a means of controlling an operational memory.EFFECT: technical result of the implementation of the claimed solution is the creation of a digital frequency meter having an increased accuracy of single phase and phase noise measurements, as well as an increased frequency measurement accuracy.4 cl, 4 dwg

Description

The invention relates to measuring equipment and can be used in radio engineering, electrical engineering, metrology and other industries for precision measurement of the frequency of sinusoidal signals, deviations of the frequency and phase from the nominal value, time intervals, and also to obtain statistical parameters characterizing the stability of the frequency for different periods of time .

A fairly large number of digital frequency meters of various designs are known, the operation of which is based on the counting of pulses arising in the reference time interval. Such frequency meters have low accuracy at small (no more than one hundred seconds) reference time intervals and, in addition, have a period of time called “dead”, such as a digital frequency meter containing a port for receiving a pulse of the measuring period, a port for receiving a reference signal, a circuit synchronization, counter, read register [US Patent No. 4984254].

A digital frequency meter is known, including an input signal receiving port that converts an input signal into a sequence of counting pulses, a first counter equipped with a first reading register, a pulse receiving port for the measuring period and a first synchronization circuit through which the output of the pulse receiving port for the measuring period is connected to the control input of the first register reading device, it also contains an exemplary generator that generates exemplary pulses, a second counter equipped with a second reading register, a processing means and an indicator In addition, the inverter and the second synchronization circuit, while the input signal receiving port is connected through the second synchronization circuit to the clock input of the first counter, the reference generator is connected to the clock input of the second counter, with the clock input of the second synchronization circuit and through the inverter with the clock input of the first circuit synchronization, the control input of the second read register is connected to the output of the first synchronization circuit, and the outputs of each counter are connected through their respective read registers with the processing means and indicator and [RF patent №2210785]. A complex circuit has been introduced in this frequency meter: a second error pulse shaper, a second duration measurement channel, and a mismatch selection circuit. The disadvantage of this frequency meter is the complexity of its design.

Known receiver-comparator with a phase detector, including a receiving device, a phase measuring system and a reference generator [Handbook of radio measuring instruments. Ed. B.C. Nasonova, Volume 2 "Measurement of frequency, time and power. Measuring generators" - M: Soviet Radio, 1977. - p. 9, Fig. 1.5]. This comparator receiver allows you to get a very narrow system bandwidth and high output signal-to-noise ratio. The uncertainty of the sign of the frequency deviation, the very narrow operating frequency range and the long measurement time are its disadvantages.

A known frequency meter containing a closed auto-regulation system, which includes a phase detector, a reference frequency generator, a binary reversible counter, the state of the outputs of which is a binary representation of the measurement result, a binary counter with a controlled division ratio [US Patent No. 4,144,489]. This frequency meter has several disadvantages, namely:

- the use of an exclusively phase detector followed by a low-pass filter determines its operation in a relatively narrow range of input frequencies (less than half an octave) and is completely unsuitable as a frequency meter in a wide range of input frequencies due to the presence of a low-pass filter at the output of the phase detector;

- the presence of a negative feedback error proportional to the signal interferes with speed with high accuracy, as a result of which it is necessary to choose lower accuracy with high speed or, conversely, low speed with higher accuracy;

- the presence of a single source of an error signal - a phase detector - excludes the possibility of working in the octave range or more, since it can lead to an error several times.

Closest to the claimed technical solution is a digital frequency meter including an input driver of rectangular signals, a phase detector, a model of a frequency generator, an information processing, indication and control unit, a frequency comparator, a synthesizer equipped with a driver of rectangular signals, an analog-to-digital converter and a microcontroller [RF Patent No. 2617172], which is taken as a prototype of the invention.

The specified frequency counter operates as follows. An unknown frequency signal is input to the system. Comparison elements are a frequency comparator and a phase detector. A frequency comparator is used as an initial coarse comparison element. A phase detector is used at the final, most accurate, comparison site. Using the information about the comparison result, the microcontroller controls the synthesizer so as to remove the difference in frequency between the unknown signal and the signal received from the output of the synthesizer, constantly bringing it closer to the input unknown frequency. Knowing the frequency code recorded in the synthesizer and the value of the reference frequency, it is easy to obtain the value of its output frequency, and if the control system has equalized these frequencies, then the conditional value of the input unknown frequency is obtained at the output.

The disadvantages of the prototype include the use in the work at the final stage exclusively of a phase detector followed by a low-pass filter, which leads to a limitation of the spectrum of output frequencies of the phase detector, and hence the accuracy of measuring single outliers of phase noise.

The claimed invention solves the problem of creating a digital frequency meter having increased accuracy of single measurements of phase and phase noise, as well as increased accuracy of frequency measurement.

The problem is solved by the fact that a precision digital frequency meter is proposed, including an input rectangular signal shaper, a phase detector, a frequency comparator, a synthesizer equipped with a square wave shaper, a reference frequency generator, a first analog-to-digital converter, a microcontroller, an information processing, indication and control unit, which further comprises a second high-frequency analog-to-digital converter, random access memory and ope control means a memory device, wherein the input of the frequency meter is the input of the input driver of the rectangular signals and the first input of the second analog-to-digital converter, the output of the driver is connected to the first input of the frequency comparator and the first input of the phase detector, the frequency comparator has two outputs connected to the first and second inputs microcontroller, the synthesizer is connected by its output to the input of the shaper of rectangular signals of the synthesizer, the first input of the synthesizer is connected to the first the output of the microcontroller, and the second input of the synthesizer is connected to the first output of the reference frequency generator, called the reference frequency generator with its second output connected to the third input of the microcontroller, and the named microcontroller is connected by its second output to the input of the information processing, indication and control unit, the output of the rectangular signal generator of the synthesizer connected to the second inputs of the frequency comparator, phase detector, second analog-to-digital converter and operational control means by a blinking device, the output of the phase detector is connected to the input of the first analog-to-digital converter, the output of which is connected to the fourth input of the microcontroller, the output of the information processing, indication and control unit is connected to the fifth input of the microcontroller, the output of the second analog-to-digital converter is connected to the first input of random access memory wherein the second input of random access memory is connected to the first output of the control means of random access memory, the first output of the operas A random access memory device is connected to the sixth input of the microcontroller, and the second output of the random access memory is connected to the first input of the RAM control means, the second output of the random access control means is connected to the seventh input of the microcontroller, and the third output of the microcontroller is connected to the third input of the random access control means device. The information processing, display and control unit may be a computer. As a phase detector, a frequency-phase detector can be used. The first, second, and seventh inputs of the microcontroller may be interrupt inputs.

The proposed precision digital frequency counter is shown in FIG. 1, where:

1 - input driver of rectangular signals;

2 - frequency comparator;

3 - shaper of rectangular signals of the synthesizer;

4 - synthesizer;

5 - phase detector;

6 - reference frequency generator;

7 - the first analog-to-digital converter (first ADC);

8 - microcontroller;

9 - block information processing, display and control;

10 - the second high-frequency analog-to-digital converter (second ADC);

11 - random access memory (RAM);

12 - control tool for random access memory (RAM control tool);

Fx is the unknown frequency;

Fs is the frequency of the signal from the output of the synthesizer (synthesized from the reference frequency).

In FIG. 2a, 2b and 2c respectively show the output voltage of the phase detector, the output voltage "Fx is greater than Fs" of the frequency comparator, the output voltage "Fx is less than Fs" of the frequency comparator. Simplified in a pair, we further call their signals “more” and “less”.

The device operates as follows.

At the input of the shaper of rectangular signals 1 and the first input of the second ADC 10, a signal of unknown frequency Fx (presumably a sinusoidal shape or a limited sinusoid) is received. The input driver 1 from the input signal generates rectangular pulses of the same frequency. This is usually a meander. Rectangular pulses then go to the first input of the frequency comparator 2 and in parallel to the first input of the phase detector 5. Also, parallel pulses to the second inputs of the frequency comparator 2, phase detector 5 and the second ADC 10 come from the output of the square-wave generator of synthesizer 3, the input of which a signal is output from the output of the synthesizer 4 with the output frequency Fs (usually a meander).

The frequency comparator 2 has two outputs: the output “more” generates a rectangular logic signal when the input frequency Fx is greater than the frequency of the synthesizer Fs, and the output is “less”, respectively, generates a signal when the frequency Fx is less than the frequency of the synthesizer Fs.

From the output “more” and the output “less” of the frequency comparator 2, the corresponding frequency comparison signals determining the sign of the frequency difference Fx and Fs are fed to the first and second inputs of the microcontroller 8 (which may be interrupt inputs). At the output of the phase detector 5, an analog signal is formed, which is converted to digital form in the first ADC 7 and then fed to the microcontroller 8.

It should be noted that the input of the synthesizer 4 receives a signal of the reference frequency F from the generator of the reference frequency 6, and the control of the synthesizer 4 is performed by the same microcontroller 8. In addition, the reference frequency (lower than the frequency F to performance level of the used microcontroller) for synchronizing timers involved in the formation of time stamps necessary for calculating the correction to the frequency of synthesizer 4 to calculate the frequency Fx.

From the output of the shaper of rectangular signals of the synthesizer 3, the signal Fs also arrives at the second (synchronizing, or clock) inputs of the second ADC 10 and the control means of the RAM 12. The latter, in turn, organizes the recording of information received from the output of the second ADC 10 along the edges of the signal Fs, in the random access memory 11 in order to subsequently make this information available to the microcontroller 8.

The information processing, indication and control unit 9 together with the microcontroller 8 provides operational control of the frequency meter operation, organizes the receipt of data from the second ADC 10 through the RAM 11 and the corresponding RAM control tool 12, as well as from the first ADC 7 and the frequency comparator 2, processes the data and an indication of the results obtained during measurements and calculations in the most convenient form. In this block 9, the total frequency value is also calculated (the synthesizer frequency value and the frequency correction value obtained during the calculation: phase increment for the measured period divided by the value of the measurement time interval itself). As a block of information processing, indication and control, a computer may be used.

The microcontroller 8 is used to control the measurement process in real time.

In the initial state, the frequency value at the output of the synthesizer 4 is set in the middle of the measured frequency range. In parallel with the frequency comparator 2, a phase detector 5 is turned on. The frequency comparator 2 generates rectangular pulses when the phase difference of the compared signals passes through 0 degrees and through 180 degrees either at the output “more” or at the output “less”, depending on the sign of the difference of the compared frequencies. The frequency of these pulses is equal to twice the difference of the compared frequencies. In the interval between these pulses at the outputs "greater" or "less" the voltage is equal to "0" (Fig. 2B, 2C).

The phase detector 5 generates a sawtooth voltage in the interval between pulses at the output of the frequency comparator 2, and it corresponds to the instantaneous phase difference between the compared signals (Fig. 2A). The microcontroller 8, through the first ADC 7, sequentially performs several readings of the voltage values at the output of the phase detector 5 to quickly evaluate the magnitude of the frequency difference Fx and Fs. If the microcontroller 8 manages to read the voltage values at the output of the phase detector 5 several times from the first ADC 7 (in the absence of “more” and “less” signals), then we can estimate the frequency difference (without knowing the sign of the frequency difference). Knowing dΦ (the difference of two or several averaged sequentially read phase samples) and the value of the time interval between readings of these values, we can calculate the magnitude of the frequency difference Fx and Fs. Having received the difference value of two frequencies, it is possible to change the frequency at the output of the synthesizer 4, for example, by half this difference, arbitrarily in one direction or another, again determine the slope of the saw at the output of the phase detector 5 and, accordingly, the difference in the values of the frequencies Fx and Fs.

In this case, it is possible to obtain two different results.

The first is that the steepness of the saw at the output of the phase detector 5 has decreased (theoretically by a factor of two), which means that there has been a frequency shift in the desired direction, and now the magnitude and sign of the difference between the unknown frequency Fx and the frequency Fs at the output of the synthesizer 4 are known.

In another case, there was a movement in the opposite direction from an unknown frequency (the steepness of the saw at the output of the phase detector 5 increased, or the first ADC 7 does not have time to take measurements, and a signal is generated from the output of the frequency comparator 2). Now the magnitude and sign of the frequency difference is known. You can immediately make the shift of the synthesizer 4 in frequency by the amount of the remaining difference in frequency and achieve the minimum difference in frequencies Fx and Fs. You can simply sequentially approximate the frequency of the synthesizer to an unknown frequency according to some law. To reach the precision level of frequency measurement, it is necessary several more times to sequentially measure the saw tilt at the output of the phase detector 5 and approach the constant phase difference between the signals Fx and Fs, which corresponds to the horizontal or with a minimum tilt position of the saw at the output of the phase detector 5 - everything is determined discreteness of the change in the frequency of the synthesizer 4, temporary and temperature instability, as well as phase noise of the applied components. If necessary, averaging or statistical processing of the received signals. It should always be borne in mind that when frequency jumps, you can get into the non-working non-linear zone of the phase detector, and constantly monitor this state. It is also constantly necessary to ensure that measurements with a phase detector are carried out on one of the slopes of the saw; if there is a transition to another slope of the saw or through it, it is necessary to take measurements again.

The described option works with a relatively small difference in frequency between the Fx and Fs signals. With a large difference in frequency, the microcontroller 8 may not have time to make several measurements on one slope of the saw, then the frequency comparator 2 starts working, forming its own pulses for comparing the frequencies Fx and Fs. These signals are “Fx greater than Fs” or “Fx less than Fs”. These signals also indicate that the saw slope at the output of the phase detector 5 may have changed, and phase measurements must be started anew.

Based on these pulses, one can unambiguously decide on the sign of the frequency difference and, accordingly, change the frequency at the output of the synthesizer 4 in the direction of the frequency of the signal Fx, bringing them together using one of the methods of successive approximations. It is best to apply these pulses to the interrupt inputs (first and second inputs) of the microcontroller 8. Using the time difference in the arrival of these pulses, we can estimate the frequency difference Fx and Fs and take a step in the frequency in the right direction by this value, thereby reducing the measurement time. The signals from the output of the frequency comparator 2 have a higher priority, since the first ADC 7 and the microcontroller 8, as a rule, have lower speed and do not have time to work out this difference in frequency. In addition, these pulses indicate that the phase detector 5 is operating in an undesirable zone, and it is necessary to return the middle of the linear characteristic by changing the phase or frequency of the synthesizer 4. As the frequency Fs of the synthesizer 4 approaches the frequency of the unknown signal Fx, the pulses at the output “more” and at the output “less” become less frequent (their frequency is proportional to the difference frequency), and the speed of change of the saw at the output of the phase detector 5 is less and less; while the saw in shape approaches a continuous horizontal line (Fig. 2A). Moreover, more and more often the first ADC 7 manages to work, measuring the voltage at the output of the phase detector 5. At the final measurement site, it is desirable to carry out measurements in the middle of the linear region of the phase detector 5.

In practice, the frequency comparator 2 and the phase detector 5 operate in series. First, a frequency comparator 2 works (the difference in frequency is large - up to hundreds of megahertz), then - phase detector 5 (the difference in frequency is several kilohertz or less, in the limit reaches a value of several microhertz and even fractions of microhertz). At the end of the measurement (when the maximum accuracy in setting the frequency of the synthesizer 4 is reached) by changing the phase or short-term changes in the frequency of the synthesizer 4, the operating point of the phase detector 5 is set - in the middle of its linear characteristic, and the synthesizer 4 fixes its state. Now, at the output of the phase detector 5, a phase drift is seen between the unknown signal Fx and the signal Fs from the output of synthesizer 4, whose frequency we know quite accurately. After passing through the first ADC 7, this phase drift goes to the microcontroller 8 and then to the information processing, indication and control unit 9, where after appropriate processing it is used to observe the phase drift and to calculate the corresponding correction to the synthesizer frequency (by the amount of phase drift for a fixed time interval, dΦ / dt), since according to the phase drift graph, it is easy to calculate a frequency correction equal to the magnitude of the phase change over the measurement time interval. To clarify the sign of the correction, it is necessary to increase or decrease the frequency of the synthesizer 4 by several minimal steps of the synthesizer and relate the deviations of the frequency and phase in sign. You can also observe the so-called "instantaneous frequency value" (which is not available in other digital frequency meters), frequency drift (time derivative of the phase) and, in fact, changes in the phase of the Fx signal itself.

After the voltage at the output of the phase detector 5 is close to constant (the saw is close in shape to a continuous horizontal line) and the synthesizer 4 sets the phase difference value corresponding to the zero phase shift between Fx and Fs, the second ADC 10, RAM 11 and RAM control tool 12 begin to work The measurements are made of the instantaneous value of the input sinusoidal voltage Fx. The number of measurements and the interval between them is set by the microcontroller 8 by recording the corresponding information in the control tool RAM 12.

Measurements using the first ADC 7 and the second ADC 10 are made at different points in the frequency meter circuit. The first ADC 7 measures the voltage at the output of the phase detector 5, which requires a rectangular waveform generated by the input driver of rectangular signals 1. Thus, the signal entering the first ADC 7 passes through the specified driver of rectangular signals 1 and phase detector 5, having their time delays, for the compensation of which it is necessary to re-establish the magnitude of the phase shift. The second ADC 10 is connected directly to the input of the frequency meter. The frequency of the signals Fx and Fs is the same. Therefore, it is necessary to obtain, by short-term frequency shifts of the synthesizer 4, to set the readings of the second ADC 10, which synchronously measures the input sinusoid, in the middle position (having the greatest steepness of the input voltage change). After that, the control tool RAM 12 organizes the recording of information measured using the second ADC 10, in RAM itself 11. On the same front of the rectangular signal Fs, which runs the phase detector 5, the second ADC 10 is clocked, the previous data is written to RAM 11 with the output of the second ADC 10, and the address of the RAM 11 cell is changed. Thus, the values recorded in the RAM 11 are one clock behind the measured by the second ADC 10. After filling the RAM 11 with data, the control tool of the RAM 12 generates a signal fed to the input of the microcontroller 8 (which may be an interrupt input) to organize the reading of information recorded in RAM 11. It should be noted that the information in RAM 11 can be recorded not every cycle, but, for example, every second, fourth, etc. Based on the results of the information recorded in RAM 11, knowing the shape of the input signal, it is possible to construct a curved line and determine the frequency correction from its slope. By the scatter of points in the curve, one can determine the unit values of the jitter of the input signal. It should also be noted that with a small step of the synthesizer, a sinusoidal input signal and when working in the central part of the sinusoid, the result is close to a straight line.

As a phase detector, a frequency-phase detector can be used.

The claimed device provides greater accuracy compared to the prototype due to the fact that the temporary position of the signals is evaluated by the second ADC with a temporary jitter of less than 100 femtoseconds (for example, AD9467). In the prototype, at least two elements are additionally involved - an input driver of rectangular signals and a phase detector, with its temperature and time drifts. The phase detector chip has a typical jitter value of 200 femtoseconds (for example, MC100ER08 or MC100ER140), and to obtain such results, an individual selection of components is necessary. The input shaper of rectangular signals has a similar jitter, its typical value is 200 femtoseconds. Thus, the total jitter of the prototype is 280 femtoseconds. In addition, in the proposed embodiment, the second ADC can operate at multiple lower clock frequencies, which can significantly improve the thermal regime and the noise environment inside the device.

Claims (4)

1. A precision digital frequency meter including an input driver of rectangular signals, a phase detector, a frequency comparator, a synthesizer equipped with a driver of rectangular signals, a model frequency generator, a first analog-to-digital converter, a microcontroller, an information processing, indication and control unit, characterized in that it also contains a second high-frequency analog-to-digital converter, random access memory and control tool for random access memory, and the input of the frequency driver is the input of the input driver of the rectangular signals and the first input of the second analog-to-digital converter, the output of the driver is connected to the first input of the frequency comparator and the first input of the phase detector, the frequency comparator has two outputs connected to the first and second inputs of the microcontroller, the synthesizer is connected by the output with the input of the shaper of the rectangular signals of the synthesizer, the first input of the synthesizer is connected to the first output of the microcontroller, and the second input is syn a tezator is connected to the first output of the reference frequency generator, the called reference frequency generator with its second output connected to the third input of the microcontroller, and the named microcontroller is connected by its second output to the input of the information processing, indication and control unit, the output of the square-wave generator of the synthesizer is connected to the second inputs of the frequency comparator phase detector, a second analog-to-digital converter and RAM control means, phase det the torus is connected to the input of the first analog-to-digital converter, the output of which is connected to the fourth input of the microcontroller, the output of the information processing, display and control unit is connected to the fifth input of the microcontroller, the output of the second analog-to-digital converter is connected to the first input of random access memory, while the second input random access memory is connected to the first output of the means of control of random access memory, the first output of random access memory is connected with the sixth input of the microcontroller, and the second output of random access memory is connected to the first input of the RAM control means, the second output of the RAM control means is connected to the seventh input of the microcontroller, and the third output of the microcontroller is connected to the third input of the RAM control means. 2. The frequency meter according to claim 1, characterized in that the information processing, display and control unit is a computer. 3. The frequency meter according to claim 1, characterized in that a frequency-phase detector is used as a phase detector. 4. The frequency meter according to claim 1, characterized in that the first, second and seventh inputs of the microcontroller are interrupt inputs.

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