RU2617172C1 - Precision digital cymometer - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: proposed digital counter comprising an input square wave generator, a phase detector, the standard frequency generator according to the invention comprises a frequency comparator, a synthesizer provided with square wave generator, an analog-digital converter, a microcontroller and data processing unit, the display and control.

EFFECT: improved accuracy.

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Description

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

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 (not more than a second) reference time intervals and, in addition, are characterized by the presence of 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. - page 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.

Closest to the claimed technical solution is a frequency meter containing a closed auto-control 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], which is taken as a prototype of the invention.

The frequency counter adopted for the prototype consists of a fully digital closed-loop proportional control system with a digital integrator on a binary counter counter. An unknown frequency signal is input to the system. The element of comparison is a phase detector. The output signals of the phase detector control the state of the reversible counter so as to remove the difference in frequency between the unknown signal and the signal received from the output of the binary counter with a controlled division ratio. In turn, the binary parallel code from the output of the reversible counter is output as a measurement result and is simultaneously applied to the control inputs of the counter with a controlled division coefficient, thereby changing its output frequency, and the control system constantly brings it closer to the input unknown frequency. Knowing the division coefficient of a binary counter with a controlled division coefficient 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 prototype has the following disadvantages:

- the use in the work exclusively of a phase detector followed by a low-pass filter, which leads to its operation in a relatively narrow range of input frequencies (less than half an octave) and complete unsuitability 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.

The claimed invention solves the problem of creating a digital frequency meter having improved measurement accuracy and reduced measurement time, as well as operating in an octave or more frequency range.

The problem is solved by the fact that a digital frequency meter is proposed, including an input driver of rectangular signals, a phase detector, a model of a frequency generator, an information processing, indication and control unit, which also contains a frequency comparator, a synthesizer equipped with a driver of rectangular signals, an analog-to-digital converter and a microcontroller moreover, the input of the frequency meter is the input of the input driver of rectangular signals, and the output of the named 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 its output to the input of the shaper of the rectangular signals of the synthesizer, and its first input is connected to the first output of the microcontroller, and its second input is with 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 as its second the output is connected to the input of the control and indication processing means, while the output of the synthesizer rectangular signal generator is connected to the second inputs of the frequency comparator and the phase detector, and the output of the phase detector is connected to the input of the analog-to-digital converter, the output of which is connected to the fourth input of the microcontroller, and the output of the unit information processing, indication and control is connected to the fifth input of the microcontroller.

The information processing, display and control unit may be a computer.

The proposed frequency meter is shown in FIG. 1, where: 1 - input driver of rectangular signals, 2 - frequency comparator, 3 - driver of rectangular signals of the synthesizer, 4 - synthesizer, 5 - phase detector, 6 - generator of reference frequency, 7 - analog-to-digital converter (ADC), 8 - microcontroller, 9 - information processing, display and control unit.

In FIG. 2a and 2b respectively show the output voltage of the phase detector and the output voltage of the frequency comparator when the frequencies do not match on one time axis.

The device operates as follows.

The input of the shaper of rectangular signals 1 receives an unknown frequency Fx. The input driver 1 from the input signal generates rectangular pulses of the same frequency. This is usually a meander. Rectangular pulses are then fed to the first input of the frequency comparator 2 and parallel to the first input of the phase detector 5, also parallel to the second inputs of the frequency comparator 2 and phase detector 5 are rectangular pulses from the output of the square-wave generator of synthesizer 3, the input of which is fed from the output of the synthesizer 4 with an output frequency of Fs (usually a meander). From the output “more” and the output “less” of the frequency comparator 2, the corresponding comparison signals determining the sign of the frequency difference Fx and Fs are fed to the microcontroller 8. From the output of the phase detector 5, the analog signal is converted to digital form in analog-to-digital converter 7 and also received to the microcontroller 8. It should be noted that the signal of the reference frequency F from the generator of the reference frequency 6 is supplied to the input of the synthesizer 4, and the synthesizer is controlled by the same microcontroller 8. In addition, the microcontroller 8 tsya reference frequency to synchronize the timers involved in the formation of timestamps needed to calculate the corrections to the synthesizer frequency to calculate the frequency of the true value. The information processing, display and control unit 9 together with the microcontroller 8 provides operational control of the frequency meter operation, organizes the receipt of data from the analog-to-digital converter and the frequency comparator, the processing of the received data and the indication of the results obtained during the measurements and calculations in the most convenient form. Also, in the information processing, display and control unit 9, the total frequency value is calculated — the synthesizer frequency value and the frequency correction value obtained as a result of the calculation (phase increment for the measured period divided by the value of the measurement time interval).

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

If the input signal goes beyond the operating frequencies of the frequency comparator and the phase detector, as well as the frequencies generated by the synthesizer, you must either divide one of the signals or shift it in frequency to the work area - so that the input frequency Fx is in the frequency range covered by synthesizer, frequency comparator and phase detector.

In the initial state, the frequency value at the output of the synthesizer is set in the middle of the measured frequency range. In parallel with the frequency comparator 2, a phase detector 5 is turned on. 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, the voltage at these outputs is “0”, which corresponds to the “equal” sign (Fig. 2b).

The phase detector 6 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 analog-to-digital Converter 7 sequentially performs several readings of the voltage values at the output of the phase detector 5 to quickly assess the magnitude of the difference in frequencies Fx (unknown frequency) and Fs (frequency synthesized from the reference frequency). If the microcontroller 8 manages to read several times from the analog-to-digital converter 7 the voltage values at the output of the phase detector 5 (in the absence of signals “more” and “less”), then you can evaluate the frequency difference (without knowing the sign of the frequency difference). Knowing the difference of two or several averaged sequentially read phase readings (dF) and the value of the time interval between readings of these values (dt), we can calculate the magnitude of the frequency difference Fx and Fs.

Having received the difference value of two frequencies, you can change the frequency at the output of the synthesizer, 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.

Further, it is possible to obtain different results.

If the steepness of the saw at the output of the phase detector has decreased (theoretically - by half), then there has been a frequency shift in the desired direction, and now it is known in which direction and how far from the unknown frequency the synthesizer and the difference itself are located.

If there was a movement in the opposite direction from an unknown frequency (the steepness of the saw at the output of the phase detector increased, or the analog-to-digital converter does not have time to measure, and a signal is generated from the output of the frequency comparator), then the magnitude and sign of the frequency difference are known. Then 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 frequency Fx and Fs.

You can also sequentially approach the synthesizer to an unknown frequency according to some law.

To reach the precision level of frequency measurements, it is necessary several more times to sequentially take measurements of the tilt of the saw 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 position of the saw at the output of the phase detector or with a minimum tilt - everything is determined by the discreteness changes in the frequency of the synthesizer, temporary and temperature instability, as well as phase noise of the applied components.

Here it is always necessary to keep in mind that during frequency jumps you can get into the non-working zone of the phase detector and constantly monitor this state. It is also necessary to constantly ensure that the results of measurements with a phase detector are carried out on one of the slopes of the saw, and if there is a transition to another slope of the saw or through it, it is necessary to make measurements again.

The described option works with a relatively small difference in frequency between the signals Fx (unknown) and Fs (synthesizer output). With a large difference in frequency, the microcontroller may not have time to make several measurements on one tilt of the saw, then the frequency comparator starts working, forming its own impulses for comparing the frequencies Fx and Fs. These are impulses Fx greater than Fs, or impulses Fx less than Fs. These pulses also indicate that the saw slope at the output of the phase detector may have changed and that phase measurements need to 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 synthesizer output towards the signal Fx, bringing it closer to it, for example, by the method of successive approximations. From 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 have a higher priority, since the analog-to-digital converter and microcontroller have lower speed and do not have time to work out this difference in frequency. In addition, these pulses indicate that the phase detector is operating in an undesirable zone and it is best to return the middle of the linear characteristic by changing the phase or frequency of the synthesizer. As the synthesizer frequency Fs approaches the frequency of the unknown signal Fx, the pulses at the output “more” and at the output “less” become less frequent (they are proportional to the difference frequency), and the rate of change of the saw at the output of the phase detector is less and less, eventually approaching horizontal. At the same time, the ADC channel for phase measurement has more and more time to work. At the final measurement site, it is desirable to take measurements in the middle of the linear region of the phase detector.

In practice, the frequency comparator 2 and the phase detector 5 operate in series. First, a frequency comparator operates (the difference in frequency is large - up to hundreds of megahertz), then a phase detector (the difference in frequency is several kilohertz or less, reaches a value of several microhertz in the limit). At the end of the measurement (the maximum accuracy of setting the frequency of the synthesizer has been achieved), by changing the phase or short-term changes in the frequency of the synthesizer, the operating point of the phase detector is established - in the middle of its linear characteristic, and the synthesizer fixes its state. Now, at the output of the phase detector, a phase drift is seen between the unknown signal and the signal from the output of synthesizer 4, the frequency of which we know quite accurately. This phase drift after passing through the analog-to-digital converter 7 is fed to the microcontroller 8 and then fed to the information processing, display 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 dФ / dt), since according to the phase drift graph, it is easy to calculate the frequency correction equal to the magnitude of the phase change during the measurement. To clarify the sign of the correction, it is necessary to increase or decrease the frequency of the synthesizer by several minimal steps of the synthesizer and relate the deviations of the frequency and phase in sign. You can 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.

Tests have shown that the device provides an accuracy of measuring the frequency of input signals for 1 s to values better than 1⋅10 ^-13 (Allan deviation), and for an interval of 100 s, Allan deviation was 2⋅10 ^-15 with a working area of input frequencies from fractions of megahertz to hundreds of megahertz. The experiment used a DDS synthesizer based on the AD9912 chip.

Claims (2)

1. A digital frequency meter, including an input driver of rectangular signals, a phase detector, a model of a reference frequency, an information processing, indication and control unit, characterized in that it also includes a frequency comparator, a synthesizer equipped with a driver of rectangular signals, an analog-to-digital converter and a microcontroller, moreover, the input of the frequency counter is the input of the input driver of rectangular signals, and the output of the named driver is connected to the first input of the frequency comparator and the first input the phase detector house, called the frequency comparator, has two outputs connected to the first and second inputs of the microcontroller, the synthesizer is connected by its output to the input of the rectangular signal generator of the synthesizer, and its first input is connected to the first output of the microcontroller, and its second input to the first output of the generator reference frequency, called the reference frequency generator its second output is connected to the third input of the microcontroller, and the named microcontroller its second output is connected to the input of the means control and indication processing, while the output of the synthesizer rectangular signal generator is connected to the second inputs of the frequency comparator and the phase detector, and the output of the phase detector is connected to the input of the analog-to-digital converter, the output of which is connected to the fourth input of the microcontroller, and the output of the information processing and indication unit and control is connected to the fifth input of the microcontroller. 2. The digital frequency meter according to claim 1, characterized in that the information processing, display and control unit is a computer.

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