Radio transmitter with FM in the frequency range 1-30 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio transmitter with FM in the frequency range 1-30 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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The device described below can operate in the range of 1-30 MHz with frequency modulation. The radio transmitter is powered by 220 V. The same network is used by the device as an antenna. The diagram of the radio transmitter is shown in the figure.

FM radio transmitter in the frequency range 1-30 MHz

The power supply unit of the device is assembled according to a transformerless circuit. The mains voltage of 220 V is supplied to the chokes Dr1, Dr2 and the quenching capacitor C2, on which the excess voltage is quenched. The alternating voltage is rectified by the VD1 bridge, the load of which is the VD2 zener diode of the KS510 type. Voltage ripples are smoothed out by capacitor C3.

The modulating amplifier is made on a transistor VT1 type KT315. An audio frequency signal is fed to the base of this transistor from an electret microphone with an M1 amplifier of the MKE-3 or M1-B2 "Pine" type. The amplified audio frequency voltage through the resistor R2 is supplied to the VD3 varicap type KB109A, the change in capacitance of which allows frequency modulation.

The master oscillator is made according to the inductive three-point circuit on a VT2 transistor of the KT315 type. The frequency of the generator is determined by the elements of the oscillatory circuit L1, C5, C4, VD3. Feedback is provided through capacitor C7.

The modes of transistors VT1 and VT2 for direct current are regulated by resistors K5 and R4, respectively. The bias voltages of transistors VT1 and VT2 are formed by these resistors and a parametric stabilizer made on a short circuit resistor, an AL4 type VD307 LED and a C8 capacitor. This achieves a higher frequency stability than with normal switching.

The high-frequency voltage, modulated in frequency by an audio signal, from the coupling coil L2 enters the 220 V network through a separating capacitor C1. Capacitor C1 reduces the influence of the mains voltage on the master oscillator.

Inductors Dr1 and Dr2 exclude the penetration of high-frequency voltage through the power circuits. Inductors Dr1 and Dr2 are wound on ferrite rods and contain 100 turns of PEV wire 0,1 mm each. Coils L1 and L2 are wound on a frame with a diameter of 5 mm with a tuning core. For the 27 MHz range, the L1 coil has 10 turns with a tap from the middle, wound with a 0,3 mm PEV wire. The coupling coil L2 has 2 turns of the same wire.

Capacitors C1 and C2 must be designed for an operating voltage of at least 250 V. The KTs407 diode assembly can be replaced by four KD105, KD102 diodes. Instead of the VD2 zener diode, you can use any other one with a stabilization voltage of 8-12 V. The VD4 LED of the AL307 type can be replaced with any LED or two or three silicon diodes connected in the forward direction.

When using standard parts and proper installation, the setting consists in adjusting the frequency of the master oscillator with capacitor C5.

See other articles Section Transmitters.

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