RU2101714C1 - Device which measures frequency of low- frequency signals - Google Patents

Abstract

FIELD: radio engineering instruments. SUBSTANCE: device has operational amplifier 1, reference voltage source 2, operational amplifier 3, digital-to-analog converter 4, measuring unit 5, counting pulse generator 6, time selector 7, pulse counter 8, memory unit 9, JK flip-flop 10, C flip-flop 11, differentiating circuit 12, input line, control line with corresponding connections. EFFECT: possibility to design low-frequency meter. 2 dwg

Description

 The invention relates to electrical radio engineering and can be used as a low-frequency frequency meter.

 A device for measuring the frequency of electrical vibrations [1] is known based on counting pulses of an unknown frequency during an exemplary time interval. The disadvantage of such a device for measuring frequency is unacceptably long measurement time in the low frequency region, which limits the performance of devices that implement this method of measurement.

 A known converter of frequency into an analog signal [2] based on the formation of two pulses of a reference duration, averaging mismatch pulses, the duration of which is equal to the time during which the pulses of the reference duration have the same logical level, and the first pulse of the reference duration is formed from the moment the signal conversion period begins, a second pulse of a reference duration is generated from the moment the second signal conversion period begins.

 The disadvantages of such a converter include the complexity of signal processing.

 Closest to the proposed device is a frequency measurement device [3] based on an intermediate conversion of the period of the signal under study and the reference time interval into constant voltages, followed by finding the ratio of these voltages so that the output value of the result of the transformations is proportional to the frequency of the signal under investigation.

 The disadvantage of such a device should be considered low speed, due to the initial conversion of time intervals to constant voltage and the subsequent determination of their ratio through double integration, as well as the presence of a methodological error arising from the instability of the values (ratings) of the integrator elements that determine the time constant during the preliminary conversion of the period of the measured signal in constant voltage.

 The technical challenge is to increase performance.

 The technical problem is achieved by the fact that in the proposed device, the period of the signal under investigation is converted initially to a digital code, on the basis of which the value of the negative feedback depth of the operational amplifier is formed by digital-to-analog conversion. The output voltage of the operational amplifier is thus proportional to the frequency of the signal under investigation.

 In FIG. 1 shows a block diagram of a device; in FIG. 2 is a timing diagram explaining the operation of the device.

 The frequency meter includes a driver 1, a constant voltage source 2, an operational amplifier (OA) 3, a digital-to-analog converter (DAC) 4, a measuring device 5, a generator 6 of counting pulses (GSI), a time selector 7, a counter 8, and random access memory (RAM) ) 9, JK-trigger 10, C-trigger 11, differentiating chain 12.

 The input of the driver 1 is the input bus of the frequency meter, the output of the driver is connected to the C-input of the JK trigger 10, the inverse output of which through the differentiating circuit 12 is connected to the control input of the RAM 9, and the direct output with the counting input of the trigger 11 and the control input of the temporary selector 7; trigger inverted output 11 combines the J- and K-inputs of trigger 10. The output of the DC voltage source 2 is connected to the non-inverting input of the op-amp 3, the output of the op-amp combines the DAC 4 and the input of the measuring device 5, the output of the DAC is connected to the inverting input of the op-amp 3. The output of the ГСИ 6 through the time selector 7 is connected to the C-input of the counter 8, the output bus of which is connected via RAM 9 to the input control bus of the DAC 4. The control bus of the frequency meter combines the R-inputs of the counter 8 and the triggers 10 and 11.

 The device operates as follows.

With the arrival of the Start signal to the control bus, the triggers 10 and 11 are set to zero (Fig. 2, U ₁₀ , U ₁₀ , U ₁₁ ). The short pulse coming from the output of the shaper at time t _{1, the} JK trigger 10 switches to a single state (Fig. 2, U ₁ , U ₁₀ ), thereby allowing the passage of counting pulses from the output of the GSI 6 to the input of the counter 8. At time t ₂ , at the moment of the repeated arrival of the pulse from the output of the former 1 to the C-input of the JK-trigger 10, the latter switches to the zero state (FIG. 2, U ₁ , U ₁₀ ), and the C-trigger 11 to a single (FIG. 2, U ₁₁ ) state. Trigger 10 is blocked by J- and K-inputs (logic zero level) and the signal at the C-input does not lead to a change in the state of the trigger until the next start control signal arrives. At time t ₂ , a short pulse is generated at the input of the differentiating chain 12 (Fig. 2, U ₁₂ ), which records the number of pulses N _x from counter 8 in RAM 9, and the value of N _x is
N _x f • (t ₂ t ₁ ) N _x • T _x (1),
where f is the pulse repetition rate of GSI 6;
T _{x the} period of oscillation of the investigated signal.

где U_вых выходное напряжение ОУ 3;
N разрядность ЦАП.In accordance with the value of N _x at the inverting input of the OS 3 is formed a negative feedback voltage U _oc

where U _{o the} output voltage of the OS 3;
N bit DAC.

In turn, at the output of the OS 3 output voltage U _o can be determined
U _o (U ₁ U _oc ) • K (3),
where U _{1 the} output voltage of the source 2;
To the gain of the differentiating (differential) signal OU 3.

Зная, что значение К для ОУ стремится к бесконечно большому значению, т. е. K _→ ∞, с учетом (1) получаем

где К_пр коэффициент преобразования.Substituting (2) in (3) and solving with respect to U _o , we obtain the following mathematical dependence for the input voltage of the op-amp:

Knowing that the value of K for OA tends to an infinitely large value, i.e., K _ → ∞, taking into account (1), we obtain

where K _pr conversion coefficient.

Equality (5) shows that the value of the measurement result is proportional to the frequency of the measured signal f _x , which is recorded by the measuring device 5.

 Assessment of the technical and economic effect of the invention, given in comparison with the prototype, is as follows.

 1. The output voltage of the op-amp appears almost immediately after the digital code is written to the RAM (excluding the time it took to establish the op-amp, DAC and RAM), the total conversion time is determined by the time of complete oscillation of the investigated signal.

 2. The accuracy characteristics of the device are improved, since errors occurring when converting the period of the signal under study to a constant voltage using an integrator are eliminated.

3. The frequency range of the device is expanded, the methodological error is reduced due to the fact that modern ESL logic can work with frequencies up to 200 MHz; when the counter capacity N _c = 1000, the minimum value of the period of the signal under investigation is 5 μs (corresponds to a frequency of 200 kHz), which is very difficult when using conventional op-amps as an integrator amplifier [4]
This provides a new positive effect.

Using a crystal oscillator as a GSI 6, and as an ADC measuring instrument, the measurement error will be measured mainly by the voltage instability U ₁ . Using source 2 as a reference for the ADC, this error can be minimized, the measurement result will be displayed in digital form.

Claims (1)

 A device for measuring the frequency of low-frequency signals, comprising an input driver, the input of which is connected to the input bus, a counting pulse generator, a time selector, a pulse counter, a reference voltage source, a measuring device, and the output of a counting pulse generator through a time selector connected to a counting input of a pulse counter, characterized by the fact that an operational amplifier, a digital-to-analog converter, random access memory, a JK-trigger, a C-trigger, a differentiating chain are introduced into it ka, a control bus, and the output of the input driver is connected to the C-input of the JK trigger, whose inverse output is connected through a differentiating circuit to the control input of the random access memory, and the direct output to the counting input of the C-trigger and the control input of the temporary selector, inverse output C -trigger is connected to the combined J- and K-inputs of the JK-trigger, the output of the reference voltage source is connected to the non-inverting input of the operational amplifier, the output of which is connected to the reference input of the digital-analog converter the indicator and the input of the measuring device, the output of the digital-to-analog converter is connected to the inverting input of the operational amplifier, the group of information outputs of the pulse counter is connected to the information inputs of the random access memory, the information outputs of which are connected to the information inputs of the digital-to-analog converter, R-inputs of the pulse counter, JK trigger, C -trigger combined and connected to the control bus.

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