Measurement of the frequency of signals with a large period. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When measuring the frequency of signals with a long period, in order to obtain high accuracy and the possibility of tracking the dynamics of the process, it is necessary to calculate the frequency from the value of the period between two adjacent signals from the sensor. The frequency value is obtained by dividing some constant by the current value of the period simultaneously with the measurement process, which is important when studying signals with changing frequency, such as heart rate. The ability to observe the process of arrhythmia is a very useful feature.

In [1], a measurement method is proposed based on the principle of piecewise linear approximation of the graph of the function y=a/x, where there is no division, but subtraction on the segments of the graph approximation, which gives a significant absolute measurement error. The method proposed in this article allows you to directly perform the division operation, thereby obtaining greater accuracy.

The proposed method is based on the principle of a counter with a variable capacity. A functional diagram of the construction of such a device is shown in Fig.1.

Fig.1. Functional diagram

The operation of dividing a/x proceeds as follows. The counter U2 records x the number of pulses coming from the generator G1 in one period between two adjacent signals from the sensor. Register U4 remembers this value for the duration of the count. The device U1 generates a burst of pulses, equal in number to the value a, and transmits it to the counter U3. If the codes at the outputs of the counter U3 and the register U4, where the code of the number x is written, coincide, a short positive pulse will appear at the output of the element U5, which will reset the counter U3. Thus, the capacity of the counter will be determined by the code of the number x. This cycle will continue until the pulse train from device U1 ends. The number of pulses received at the output of the U5 element during the counting time will be the desired value of a / x.

The pulse repetition rate is calculated using the formula F = 60/Tp, where Tp is the period in seconds between two pulses. The lower measurement limit is determined by the maximum period value equal to (2n - 1) * t, where (2n - 1) is the maximum counter capacity, and dt is the period measurement discreteness, equal to 1/fG1. The number of pulses generated by device U1 is 60fG1.

Fig.2. Schematic diagram (click to enlarge)

One of the options for the circuit implementation of the proposed method is shown in Fig. 2 at n = 7 and dt = 0,01 s. Consider the operation of the device at Tp=1s.

When a positive pulse is received at the input, a short negative pulse is generated at the output of element DD1.3, which switches the RS-trigger on elements DD4.2 and DD4.3, and through the inverter DD1.4 sets the counter DD5 to zero. When a high level appears at the output of DD4.3, the generator will start working on the elements DD1.1 and DD1.2, and the recording in registers DD7 and DD8 will also stop, where the value of the previous period will be saved. At a frequency of 102,4 Hz generator elements DD2.1, DD2.2 and DD2.3, this value is 102.

The pulse train shaper works as follows. The pulses from the generator on the elements DD1.1, DD1.2 are fed to the inputs of the counters DD3 and DD6. When the counter DD3 reaches the value 6144, a negative pulse will appear at the output of the element DD1.4, which will switch the RS flip-flop to its original state, and it, in turn, will stop the generator. The filling time of the counter DD3 to the value of 6144 will determine the counting time.

Thus, the output of the counter DD6 will receive a series of 6144 pulses. When the counter reaches state 102, the outputs of all elements DD9 and DD10 ("EXCLUSIVE OR") will appear at level 0, and at the outputs of elements DD11 - levels 1. At the output of element DD2.4, a logic zero level will be generated, which will set through the inverter DD4.4 the counter to its initial state, after which writing to it will continue. Therefore, for 102 pulses that came to the counter, one reset pulse is formed, and for 6144 - 60 such pulses. The operation of a counter with a controlled capacity is considered in detail in [2].

The lower limit of measurement is 49 pulses per minute. The upper limit will be determined by the count time. With a generator frequency on the elements DD1.1, DD 1.2, equal to 120 kHz, the counting time is 0,05 s. The accuracy and limits of measurement depend on the capacity of the device and the discreteness of the period measurement, which makes it possible to use this device in a wide frequency range.

To tune the device to the input, a signal with a frequency of 1 or 0,5 Hz should be applied and the appropriate readings of the display device should be set by selecting the resistor R6. If, when calculating the device, it becomes necessary to obtain the number of pulses in a series described by more than two binary digits, instead of the DD4.1 element, it will be necessary to use the "AND-NOT" multi-input element.

Using this method, it is also possible to build devices for dividing one sequence of pulses into another.

In the proposed version of the circuit solution, the DD9 - DD11 microcircuits can be replaced by two K561IP2 microcircuit packages. Two or three blocking capacitors with a capacity of 0,01 microfarads (not shown in the diagram) should be installed in the power supply circuits of microcircuits.

Literature

1. Chekin V. Measuring the frequency of signals with a large period. - Radio, 1990, No. 6, p. 57-59.
2. Psurtsev V. Counters with asynchronous reset. - Radio, 1984, No. 1, pp. 33, 34.

Author: I.Kostryukov

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