FM Radio Receiver
This simple fm radio receiver
circuit consists of a regenerative rf stage, TR1, followed by a two of
three-stage audio amplifier, TR2 to TR4. In some areas 3 stages of audio
amplification may not be necessary, in which case TR3 and its associated
components can be omitted and the free end of capacitor C5 connected to the
collector of TR2.
Radio Receiver Circuit
Diagram
The critical part of the fm
radio receiver is the first stage, TR1/VC1, where the wirings must be kept as
short as possible. Coil L1 is formed by winding 8 turns of 1mm (20 swg) enameled
copper wire on a 6 mm diameter former, which is then removed. After that L1
should be stretched carefully and evenly to a length of about 13mm.
Transistors List
TR1 = BF199
TR2 = TR3 = TR4 = BC547
TR1 = BF199
TR2 = TR3 = TR4 = BC547
The tunning capacitor VC1 is
one of the two fm sections of a miniature fm transistor radio with built-in
trimmers (VC2). The “earthy” end (moving vanes and spindle) is connected to the
22pF capacitor C1. The value of the rf choke L2 is not critical, anything from 1µH
to 10µH being suitable.
The output is suitable for
ordinary earphones connected in series to provide an impedance of 64Ω.
Tuning-in the fm radio
receiver
To operate the radio receiver,
potentiometer VR1 must first be advanced slowly (towards the end of the track
connected to battery positive) until, at about the half-way point, a sudden
slight increase in background noise will be heard, indicating the onset of
oscillation. It then should be backed off, very slowly, until oscillation just
stops; it then should be possible to tune in some stations.
The correct frequency range of
87 MHz to 108 MHz can be obtained by adjusting VC2 at the high frequency (108
MHz) and slightly stretching or squeezing together the turns of coil L1 at the
end (87 MHz).
