Sound accompaniment over the radio. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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In order not to disturb others in the evening, the soundtrack of television programs is listened to through headphones connected to the phone jack of the TV. But in this case, the connecting wires create inconvenience. For wireless listening, you can use a transmitter-receiver system operating at radio frequencies, in the IR range or due to inductive coupling. If the reader has a small-sized VHF FM receiver, then it is advisable to use the radio band.

It is not difficult to make a low-power FM transmitter for broadcasting sound within the same room, and descriptions of such devices have already been given in Radio. The simplest transmitter runs on just one transistor [1]. It is easier to establish and more stable in operation a transmitter assembled according to a multivibrator circuit on two transistors [2]. By introducing a third transistor, it was possible to simplify the circuit and improve the signal quality. The scheme of the improved transmitter is shown in fig. 1. It contains a generator on transistors VT1 and VT2, connected according to the multivibrator circuit with a frequency-setting circuit L1C3, and a controlled current source on a field-effect transistor VT3.

When an audio signal is applied to the gate of the field-effect transistor, the resistance of the transistor channel will change, and the current through the multivibrator and the frequency of its generation will change accordingly. Thus, frequency modulation (FM) of the emitted signal is carried out. Tuning to the center frequency is done by tuning capacitor C3. The transmitter operates at a frequency of 87,9 MHz, which is authorized for wireless microphones and similar devices.

The antenna is a piece of wire about 80 cm long. The transmitter power is small, but it is enough to receive a signal within the apartment. The transmitter is powered by one galvanic cell with a voltage of 1,5 V and consumes a current of no more than 1 ... 1,5 mA. The sound signal to the transmitter is fed from the line output of the TV (from the "AUDIO" output or from the "SCART" jack). The required frequency deviation is set by resistor R1.

All parts of the device are mounted on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 2.

The second metallized side is connected to the common conductor of the first side in several places along the edge of the board. The board can be placed in a plastic case of a suitable size. The following parts are applicable in the device: transistors VT1, VT2 - KT368A, VT3 - KP303, a tuned capacitor C3 - KT4-25, KT4-35, the rest - KLS, K10-17. Variable resistor - SPZ-4vm (with a switch), it can be replaced by any trimmer, for example, SPZ-3, SPZ-19, and a separate small-sized switch of any type. The inductor is wound with wire PEV-2 0,8 on a mandrel with a diameter of 3,5 mm, it contains 7 turns with a tap in the middle.

The adjustment comes down to setting the required frequency of the transmitter by the capacitor C3 and the frequency deviation by the resistor R1, while the volume of the reception of broadcast radio stations and the sound should be approximately the same. If the variable resistor R1 is with a switch, then its handle must be provided with a mark corresponding to the normal frequency deviation. The radius of reliable reception can be selected by changing the length of the antenna.

However, the radio channel also has certain disadvantages. These are possible interference with radio or television reception from neighbors, interference from radio stations. The probability of interference increases due to the low frequency stability of transmitters of simple designs. For example, if there are many broadcasting stations operating in the VHF band, then the frequency instability of the transmitter can lead to convergence of the frequencies of the transmitter and one of the radio stations. In this case, there will be mutual interference in their reception.

To eliminate this disadvantage, the transmitter frequency must be stabilized by a quartz resonator. But then the problem arises of obtaining the required frequency deviation (up to 50 kHz), which provides the necessary loudness during reception. It can be solved by frequency modulation of a crystal oscillator operating at a relatively low frequency of a few megahertz, and then multiplying the frequency.

A diagram of a transmitter operating on this principle is shown in Fig. 3. It contains a 3H amplifier on a VT1 transistor, a quartz-frequency oscillator based on logic elements DD1.1, DD1.2, a short pulse shaper on elements DD1.3, DD1.4, a C5C4L2L3C6 bandpass filter, to the output of which an antenna is connected. If it is necessary to increase the output signal level, the circuit can be supplemented with a power amplifier based on the VT2 transistor.

The device works as follows. The audio signal from the "AUDIO" output of the TV is fed to the input of the 3H amplifier, where it is amplified to an amplitude of 1 ... 2 V. This signal is fed to the varicaps VD1 and VD2, which are connected in series with a quartz resonator and provide frequency modulation. An inductor L1 is also connected in series with the quartz resonator, with which you can adjust the frequency of the generator.

Rectangular pulses from the output of the generator (pin 6 DD1.2) are fed to the elements DD1.3 and DD1.4, in addition, pulses from the output DD1.4 are received at the input DD1.3. Due to the delay of pulses in the element DD1.3, antiphase pulses arrive at the input DD1.4 with a small time delay (units of nanoseconds). As a result, short pulses of negative polarity are formed at the output of DD1.4, which contain harmonic components with relatively high levels.

With the help of a band-pass filter, the desired spectral component is selected, which enters the antenna. To obtain an output frequency of 87,9 MHz, an 8000 kHz quartz resonator was used. The oscillator frequency was set to approximately 1 kHz using the L7991 coil, and the filter isolated the 11th harmonic of the signal 7991 x 11 \u87900d 20 kHz, while the remaining components were suppressed by more than 100 dB (XNUMX times in power).

With this choice of frequencies, in order to obtain the required maximum deviation, it is necessary to ensure the frequency deviation of the master oscillator 50: 11 = 4,5 kHz, which is quite realistic. The filter can also be adjusted to other harmonic components that fall into the VHF broadcasting range. The signal level at the filter output is enough to listen to the signal within a room or a small apartment. When an output power amplifier is installed, the output signal voltage increases by about 10 times (in the author's version it was 400 mV).

The following parts can be used in the device: transistor VT1 - KT3102 with any index, KT312V, polar capacitor - K50-6, K50-35, trimmer capacitors - KT4-25, KT4-35, the rest - KLS, K10-17, trimmer resistor - SPZ-19, constant - MLT, S2-33. Coil L1 is a choke DM-0,4 or DM-0,1 with an inductance of up to 10 μH, the remaining coils are wound with wire PEV-2 0,8 on a mandrel with a diameter of 4 mm and contain 8 turns each (tap from the 1,5th turn ), the distance between the axes of the coils L2 and L3 is 12 mm.

The transmitter is mounted on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in fig. 4. One side of the board is used as a common wire. The conclusions connected to it are passed into the holes indicated by light circles and soldered to the foil. The rest of the leads are soldered to the contact pads without drilling holes.

The establishment of the transmitter begins with the setting of the DC mode of the transistor VT1. By selecting the value of the resistor R2, a voltage of approximately 2,5 V is set on the collector. Then, by selecting the inductance of the coil L1, the required generator frequency is set. Trimmer capacitors tune the circuits to the frequency of the selected harmonic. The device is powered by a stabilized source, the current consumption is approximately 25 mA. In the absence of a small-sized receiver, it can be made according to the schemes given in the literature [3-5].

literature

1. Bobrov O. Sound accompaniment - on the radio. - Radio, 2001, No. 7, p. 56.
2. Nechaev I. Sound accompaniment - without wires. - Radio, 1998, No. 10, p. fifty.
3. Gerasimov N. Dual-band VHF receiver. - Radio, 1994, No. 8, p. 6 - 8.
4. Makarov D. VHF receiver - in a pack "MARLBORO". - Radio, 1995, No. 10, p. 41-43.
5. Semenov B. A simple VHF receiver. - Radio, 1996, No. 10, p. 22, 23.

Author: I. Nechaev, Kursk

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