Frequency meter as a generator of fixed frequencies. Scheme, description

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Frequency meter as a generator of fixed frequencies. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The addition of a frequency meter, made on several ten-day counters with a quartz oscillator, with one multiplexer, allows you to get from it a set of up to eight fixed frequencies, for example, from 1 Hz to 10 MHz.

In amateur radio publications, digital frequency counter circuits using K155 (K555) series microcircuits have been repeatedly published. It must be assumed that now radio amateurs still use such devices. The prefix proposed for repetition will make it possible to obtain a generator of fixed frequencies from such a frequency meter.

The essence of this refinement is to use the block of exemplary frequencies of the frequency meter as a source of pulsed signals of fixed frequencies. As a rule, such a block consists of a quartz-stabilized pulse generator and an N-stage frequency divider. Taking signals directly from the output of the generator and after each stage of the divider, we have a set of N + 1 frequencies (in this case, the condition N + 1 < 8 must be satisfied). If, for example, the generator generates pulses with a frequency of 10 MHz, and the division circuit consists of counters-dividers by ten, we have a grid of the following frequencies: F1 = 10 MHz, F2 = 1 MHz, F3 = 100 kHz, F4 = 10 kHz, F5 = 1KHz, F6=100Hz, F7=10Hz, F8=1Hz. The proposed prefix provides transmission to its output (and simultaneously to the input of the frequency meter) of pulses of one of the frequency grids. The electrical circuit of the attachment is shown in fig. 1.

Frequency meter as a generator of fixed frequencies

The device works as follows. When the power is turned on, the output of the RS-trigger (pin 3 DD1) is set to a high level. The counter DD2, included in the divisor circuit by 16, is set to an arbitrary state, for example, to zero. The number "zero" in the code 1-2-4-8 is transmitted from the output DD2 to the inputs of the address selector and the input of the OE multiplexer DD3. This combination provides signal transmission from the D0 input to the direct output of the DD3 chip. Thus, the input of the frequency meter receives a signal with a frequency F1, which is a test signal for checking the operation of the frequency meter.

To transfer the device from the generator mode to the frequency counter mode, you need to press the SB1 button (momentary button). In this case, an odd number is set at the output of the counter DD2 and a high level is applied to the input of the OE multiplexer DD3. It puts the output of the multiplexer in a high-impedance state (Z-state), which allows the frequency counter to measure the frequency of external signals. The next time you press the SB1 button, the device switches back to the generator mode and a signal with a frequency F2 is fed to the input of the frequency meter. And so on, along the "ring": if there is an even number at the control inputs DD3 (low level at the input OE), the output of the multiplexer is connected to one of the inputs; if odd (high level at the OE input), then the output of the multiplexer is in a high-impedance state.

The printed circuit board of the device and the location of parts on it are shown in Fig. 2. The printed circuit board is made of one-sided foil fiberglass (getinax) 1,5 ... 2 mm thick. The prefix is powered by the power supply of the frequency meter.

Frequency meter as a generator of fixed frequencies

It should be noted that the signals from the output of the multiplexer can be fed to the input of the frequency counter, bypassing the input square waveformer, since these signals already have TTL levels. In the event that the reference frequency block has a number of division steps less than 7, it is necessary to make some changes to the device circuit. For example, when using a set of four frequencies (F1 - F4), it is possible to change the scheme in such a way that each of the signals F1 - F4 is applied simultaneously to two inputs: F1 - to inputs D0, D4; F2 - on D1, D5; F3 - to inputs D2, D6; F4 - to inputs D3, D7.

Since the prefix board is small, it fits in the case of the frequency meter itself, in the immediate vicinity of the reference frequency generator. No changes are required to the design of the frequency meter, with the exception of the front panel, on which the SB1 button and the "Exit" jack are installed.

Author: V. Klepalchenko, st. Giaginskaya, Adygea

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