Refinement of the frequency meter control unit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In N. Kovalev's article "Frequency meter control unit" ("Radio", 1996, No. 3, pp. 55, 56), a good idea was given to combine an electronic switch and a frequency meter control unit and options for implementing this idea were described. And yet, a certain disadvantage of the node, in my opinion, is the lack of a mode for measuring the period of pulses. Such a mode is necessary, for example, when measuring signals of very low frequency, when using various attachments to the frequency meter, in which the repetition period of the output pulses is directly proportional to the value of one or another measured physical quantity (capacitor capacitance, for example).

This shortcoming can be easily corrected - it is enough to use a two-input adder modulo 3.1 EXCLUSIVE OR instead of the DD1 inverter (see Fig. 2 in the above article) and replace the measurement mode switch with another one - with three groups of switching contacts for five positions (5P3N).

Such a replacement allows you to release one input (D2) of the DD1.1 multiplexer (see the diagram shown here), to which you need to apply a low logic level pulse with a duration equal to the period Tx of the measured signal (code name Fx / 2).

The measured signal Fx is supplied to the upper input of the DD3.1 element according to the scheme, and the lower one is used to control the element. If the control input is connected to a common wire (in position 5 of the SA1 switch), then a low logic level will be set on it and the DD3.1 element will work as a repeater of the signal received at the upper input. At a high level at the control input (positions 1 - 4 of the SA1 switch), the output signal of the DD3.1 element will be the inversion of the signal at the top input.

Thus, the frequency meter control unit with the above changes in positions 1 and 2 of switch SA1 allows you to measure the pulse frequency in the same way as in the article mentioned above, and in position 3 of the switch, low-level pulses with a period equal to period of the measured frequency. During the duration of the pulse, the switch allows the passage of pulses of the reference frequency Fo (from 1.2 Hz to 1 MHz). Their number will determine the counter as the period of the measured signal.

In positions 4 and 5 of the switch SA1, respectively, the measurement of the duration of the high and low logic level pulses takes place. This changes the mode of operation of only the DD3.1 element, and the operation of the multiplexers remains the same (it is described in the above article for positions 3 and 4 of the SA1 switch).

If the device has a free OR element, it is possible to organize the power-up of the Fx/10 frequency divider only in position 1 of the SA1 switch. For this, a node is assembled, consisting of elements DD4.1, VT1, R6, C5.

The same effect will be obtained if the switch 5P3N (SA1) is replaced with 5P4N. The fourth section is included as shown in the diagram by dashed lines.

In the described device, you can do without the SA1.1 section. To do this, you only need to change the inclusion of sections SA1.2 and SA1.3.

The terminals of the moving contacts of these sections "ground". Contacts 1 and 3 connected together (counting from top to bottom according to the diagram) of the SA1.2 section are connected to the input 1 circuit of the DD1 switch, and contacts 1 and 2 of the SA1.3 section connected together are connected to the input 2 circuit of the switch. Section SA1.1 is no longer needed - its role will be played by the lower fixed contact of section SA1.2, which must be connected to the place where the movable contact of section SA1.1 is output.

Author: A. Nemich, Bryansk

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