Network low-frequency radio transmitter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Network low-frequency radio transmitter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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The above devices (1, 2) radiate high frequency vibrations to the network using the network wires as an antenna. But there are also devices that operate in the low-frequency range (50-300 kHz) and use the power grid or telephone line as a communication channel. Such radio transmitters have increased secrecy, since they practically do not radiate signals into the surrounding space. An example of signal transmission in the low frequency range is three-program wire broadcasting, where 2 and 3 programs are transmitted at frequencies of 78 kHz and 120 kHz, respectively, using amplitude modulation. AC-powered devices can transmit information over it for a long time to any point in the building and even beyond it.

A diagram of one of these devices is shown in the figure. To transmit information, frequency modulation and a carrier frequency of 95 kHz are used.

(click to enlarge)

The device is powered from the network through a transformerless power supply. Excess mains voltage is quenched by capacitor C1. The reduced voltage is rectified by a VD1 diode bridge of the KTs407 type. The short circuit resistor and capacitor C4 form a smoothing filter that prevents carrier frequency oscillations from penetrating into the power supply circuit of the device. The voltage is limited to the required value by the zener diode VD2 type KS520. This voltage is used to power the power amplifier. The voltage taken from the parametric stabilizer on the resistor K6, the zener diode VD3 and the capacitor C7 is used to power the device.

The audio frequency signal taken from the M1 microphone of the M1-B2 "Pine" type is amplified by a single-stage amplifier based on a VT2 transistor of the KT315 type.

The FM modulator is a voltage controlled square wave generator. It is assembled on a DD1 chip of the K561LA7 type. The initial (in the absence of audio frequency voltage) the pulse repetition rate of the generator is set equal to 95 kHz using a tuning resistor R10. When the AF voltage is supplied from the divider R9, R10, the generator pulse frequency begins to change, i.e., it is modulated by the AF voltage.
The modulated oscillations are fed to a power amplifier assembled on a VT1 transistor of the KT315 type. The load of this transistor is the transformer T1. The primary winding of the transformer together with the capacitor C2 form an oscillatory circuit tuned to the carrier frequency. In this oscillatory circuit, square-wave pulses are converted into a sinusoidal signal, which eliminates the appearance of side harmonics in the output signal. From winding 2 of transformer T1, the carrier frequency signal through capacitors C1 and C3 enters the 220 V AC network. Such a signal must be received on a special receiver (see below).

The device uses resistors of the MLT-0,125 type. Resistor R10 any small. Capacitors C1 and C3 must be designed for an operating voltage of at least 250 V. Zener diodes VD2 and VD3 can have a stabilization voltage of 18-24 V and 6-12 V, respectively. Chip DD1 can be replaced by K176LA7, K564LA7, K1561LA7.

Transformer T1 is wound on a ring ferrite core K12x7x600 mm grade 100NN. The primary winding contains 0,1 turns of PEV 20 wire, the secondary winding - 0,15 turns of wire in insulation with a diameter of 0,3-XNUMX mm. The core of the transformer is insulated with varnished cloth or fluoroplast. The windings are also separated by a layer of insulation.

It is better to start setting up using a 30 V DC voltage source, the positive wire of which is connected to point A (the device is not connected to the network!). Check the voltage at the zener diodes VD2 and VD3. Then, the base of the transistor VT2 is shorted to a common wire and, by selecting the resistance of the resistor R10, the generator frequency on the DD1 chip is set to 95 kHz (controlled by an oscilloscope or a frequency meter on the resistor R2). By selecting capacitor C2, an undistorted sinusoid is obtained on the collector of transistor VT1. After that, remove the jumper from the base of the transistor VT2 and make sure that there is frequency modulation.

ATTENTION! When setting up and operating devices with transformerless AC power, you must follow the rules and safety measures, since the elements of these devices are energized at 220 V.

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