Simple FM receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Simple FM receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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For a long time, a huge number of circuits of simple HF receivers were published in magazines, but there was nothing similar for the VHF band. We will try to correct this omission. The circuit was originally designed as part of a school project.

scheme a simple FM receiver is shown in the figure. It consists of a TR1 regenerative RF stage and a TR2-TR4 two- or three-stage audio amplifier. In some areas with reliable reception, there is no need for a three-stage VLF, in this case, the TR3 transistor and the corresponding components can be omitted, and the free terminal of the capacitor C5 can be connected to the collector of the TR2 transistor.

(click to enlarge)

The most critical part of the circuit is the first stage, TR1 / VC1, here the connections should be made with as short conductors as possible. Coil L1 contains 8 turns of 1mm (20 SWG) enamelled copper wire on a 6mm mandrel. After winding, the coil should be stretched to a length of 13 mm for further adjustment.

Tuning capacitor VC1 is used from a two-section capacitor with a trimmer from a pocket receiver, one section is used. The "mass" of the capacitor VC1 is connected to the capacitor C1, the capacitance of which is 22pF. The inductance of the RF inductor L2 is not critical and can be in the range of 1 μH ... 10 μH.

The output stage is designed to work on headphones from the player, the windings of which are connected in series to obtain a resistance of 64 ohms (connect to the outermost and ring contact of the plug).

For settings Receiver potentiometer VR1 must be slowly rotated (towards the output connected to the "+" power supply) until, somewhere in the middle, there is a sharp increase in noise, signaling the beginning of generation. After that, the potentiometer should be slightly turned in the opposite direction, very slowly, so that the generation stops. Now you can tune in to a radio station. The frequency range 87MHz ... 108MHz should be set using the VC2 trimmer at the upper limit (108MHz) of the range, and by compressing / stretching the L1 coil at the lower limit (87MHz).

The receiver has been tested in various locations in three different countries, including England, and has always been able to receive several radio stations at good volume.

Author: Francis Hall, Meinershagen, Germany

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