Encyclopedia of radio electronics and electrical engineering / radio reception

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Automation of the receiver setup processes is one of the most urgent tasks for devices installed on moving objects, where the operator's attention should not be distracted from the process of moving in space. Here is another solution that can be applied both in newly developed designs and in existing ones with electronic tuning methods.

Currently, receivers with electronic tuning to the frequency of the received station are widely used. The use of this method has undoubted advantages over mechanical tuning, and above all, this is due to the ease of use of the receiver, especially for portable ones - when driving in a car, the need to tune in to the station of interest,

without looking at the scale, etc. The use of specialized microcircuits in which the electronic tuning function is implemented (for example, TDA7088T microcircuits from Philips) is not always convenient. They are intended mainly for operation only in the VHF-2 range (88 ... 108 MHz), they have insufficiently high sensitivity. And, most importantly, scanning can only occur in one direction (up the range), and when the upper limit is reached, a forced reset must be performed. And the last, and according to the author, this is the most significant drawback, when receiving stations with a low signal level, a spontaneous "jump" often occurs to a station with a higher signal level.

In view of the foregoing, an electronic tuning unit was developed, in which the noted negative points were eliminated.

Device functionality:

- formation of a range signal;

- formation of tuning voltage;

- automatic scanning up and down on both VHF bands with the possibility of short-term listening to stations;

- Capturing and holding the selected station.

The electronic tuning device, the diagram of which is shown in fig. 1 works as follows. When the supply voltage is applied, the multivibrator, made on the elements DD1.1, DD1.2, starts to work, a pulse is formed at the output of the inverter (element DD1.4), which sets the triggers DD3.1, DD3.2 to the zero state and writes information from the inputs of the preliminary installation of counters DD4 - DD6 on their outputs, as a result of which the first subrange is selected and the minimum voltage is set at the output of the DAC device (chips DD7 and DA2). The lower 10 digits of the output code of the counters determine the voltage at the DAC output, and the upper two - the range, so that when selecting or searching for the required station, the transition from range to range occurs automatically in a cycle, i.e. preselection of the range is not possible.

(click to enlarge)

When you press the increase button SB3 ("+") or the decrease button SB2 ("-") of the tuning frequency, the RS-trigger on the DD2.1 chip generates the corresponding signal to increase or decrease the output code of the reversible counters, and the DD3.1 trigger generates a signal allowing the operation of a single vibrator (element DD1.3) through the integrating circuit R14C6. The signal from its output goes to pin 2 of the DD1.1 element and disables the operation of the multivibrator.

The output voltage is fixed after the appearance and reliable capture of the input radio signal by the PLL system of the tracking demodulator, which is included in the functional blocks of the receiver chip (K174XA34). Such operation of the receiver is provided as follows. The appearance of a radio signal close to the tuning frequency is accompanied by a change in voltage on the K174XA34 chip (pin 2) of the receiver, the envelope of this voltage is allocated by the VD1C1R3 circuit and converted by the DA1 comparator to a log level. 1, supplied through the separating circuit C3R5 to the information input of the trigger DD3.1. At its output, this signal appears after the next clock pulse. However, the prohibition of multivibrator generation will occur with a delay determined by the time constant of the R14C6 single vibrator circuit. In addition, the same circuit will suppress random short-term operation of the comparator from random interference. As a result, at the capture stage, the natural frequency of the tunable oscillator of the PLL system is "stretched" to the frequency of the input signal.

The source follower on the field-effect transistor VT1 eliminates the influence of the circuits of the electronic tuning unit (auto-search) on the tuner and shifts the level of the constant component coming from the receiver chip.

When you press the button SB1 "Scan" (scan), the receiver tuning frequency will begin to change depending on the previously set state of the RS-trigger DD2.1, i.e., decrease or increase. At the same time, at the output of the one-shot (RS-trigger on the DD2.2 element), a pulse is generated that is applied to the counting input of the DD3.2 trigger, as a result of which a log appears on its direct output. 1. At the output of the element DD1.3, a log signal will be generated. 0 and the multivibrator will block. The state of the counters and, accordingly, the setting voltage will be fixed.

As the capacitor C5 discharges to a voltage level below the log. 1 at the output of DD1.3 there is a log signal. 1, allowing the operation of the multivibrator. The processes of capturing the radio signal and setting the frequency in the PLL system in this case are the same as those described earlier, however, the hold mode is interrupted after 5 s if the "Scan" button is not pressed again, and scanning continues further until the next radio signal appears. Log state duration. 1 in time is approximately 5 s and is determined by the elements R12, C5. Fixing the program of the desired radio station during the search is carried out by pressing the SB1 "Scan" button again.

The setting indication device is made on the elements VT2, HL1, R23, R24. In scan mode, the LED flickers, when the radio station is acquired, the LED is on constantly.

The design of the block is shown in fig. 2.

The device can be used with any receivers, the functional microcircuit of the radio channel of which incorporates a station capture indicator, i.e. with almost all modern microcircuits (K174XA34, KS1066XA1, etc.).

Literature

1. Dahin M. Receivers with automatic tuning. - Radio, 2001, No. 6, p. 33, 34.
2. Potachin I. Refinement of a pocket receiver. - Radio, 2001, No. 10, p. 13, 19.

Author: S.Bashirov, Moscow

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