Frequency coding system. Encyclopedia of radio electronics and electrical engineering

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When building a radio warning system, for example, for radio signaling or in other similar cases, a fairly simple and reliable digital frequency coding system can be used. The essence of this principle lies in the fact that a rectangular pulse generator is installed on the transmitter, which generates pulses of a certain frequency. These pulses are fed to the modulator and through the communication channel are fed to the input of the decoder, which is a simplified digital frequency meter whose task is to convert the frequency into some digital code, which is then compared with the code set at the output of the frequency meter. And if there is a match, a logical unit appears at the output of the decoder.

A schematic diagram of the simplest encoder is shown in Figure 1. This is a conventional multivibrator on logical inverters. It generates pulses of some specific "code frequency" (eg 4200 Hz). Pulses from its output must be fed to the input of the communication channel, for example, to the radio transmitter modulator.

Fig. 1

The decoder circuit is shown in Figure 2. As mentioned above, this is a simplified frequency counter that converts the frequency at its input into some binary number at the output of register D4. In this case, the frequency 4200 Hz corresponds to the code "1010" (10).

Fig. 2

The frequency meter consists of a key device on the element D1.3, a reference frequency generator on the elements D1.1 and D1.2, a control device on the counter D3, a working counter D2 and a memory cell - D4.

Pulses from the output of the receiving device (pre-formed to the MOS logic level) are fed to the input D1.3. In the initial state, both counters D2 and D3 are set to zero, so there is a logical zero at the output of D3. This zero goes to pin 13 D1.3 and this element opens. Pulses pass through it to the input of the working counter D2. At the same time, the reference frequency pulses from the output of the multivibrator on the elements D1.1 and D1.2 are fed to the counter D3. As soon as this counter counts up to 32, a unit appears at its output and the measurement time interval ends. Element D1.3 closes, then the measurement result is written to register D4, and then, with the arrival of the first positive pulse from the multivibrator through the logical element "AND" on the diodes VD1 and VD2, a unit arrives at both inputs "R" counters D2 and D3. The circuit returns to its original position, and a certain binary number is set at the output of register D4, corresponding to the frequency received at the input. In this case, the selected frequency is 4200 Hz, when it arrives at the input of the decoder, the binary number "1010" is set at the output of the register. Frequency recognition is performed by a simple decoder on element D1.4 and diodes VD3-VD7. Two tires are obtained - one connected to the resistor R3, the other to R4. We need the code number "1010" which means that there should be ones on pins 12 and 8 of D4, and zeros on pins 2 and 10. The diodes are installed in such a way that when a code number arrives, they are all closed. In this case, the output will be high (through resistor R3). If the number does not match the code, at least one diode will be open and the output will be zero.

The disadvantage of any digital frequency meter is that there is always an error in the last digit, which is determined by the discreteness and accuracy of the measurement time interval. The non-synchronization of the operation of the control device with pulses of the measured frequency leads to such errors. In order to minimize such errors, the lower two bits of the D2 counter (weight numbers "1 and 2") are not used, because it is in these that errors are most likely.As a result, the entire frequency range (for a reference multivibrator frequency of 3200 Hz) divided into 15 values ​​from 600 Hz to 6200 Hz in steps of 400 Hz This is also convenient in that it allows the frequency of the transmitter's encoding multivibrator to deviate within some small limits.

With the frequency of the multivibrator on the elements D1.1 and D1.2 equal to 3200 Hz, the decoder registers 15 frequencies that correspond to the following output codes at the output D4:

If the frequency of the multivibrator on D1.1 and D1.2 is changed, these frequencies will also change. Thus, the number of codes can be increased.

If at the output, instead of a decoder on element D1.4, two decoders of the K561ID1 type are installed, it is possible to organize a 15-command telecontrol system. Transmit commands by changing the frequency of the transmitter multivibrator (Figure 1), for example, by switching resistors R1 tuned to different frequencies (according to the table).

Setting. It is necessary to set the frequency of the multivibrator on the elements D1.1 and D1.2 (Figure 2) equal to 3200 Hz, by selecting the nominal R1, controlling the frequency by the frequency meter. Then you need to select one of the code frequencies (according to the table) and tune the transmitter multivibrator to it (Figure 1) by selecting R1, also using a frequency meter. Then you need to install the diodes VD3-VD6 in such a way that, with a code corresponding to the selected frequency, all these diodes are closed and the output has a high logic level.

Literature

1. M. Nazarov "Digital frequency indicator". Radio #3 1984, pp. 29-30.

Author: Kozhanovsky S.D.

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