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Principle:
The transistor T1 is mounted in oscillator. In its collector is injected a signal, which is linked to the frequency of radiodiffusion, generated by a LC circuit with a variable condensater (A variable condensator makes the FM band vary between 88 Mhz and 108 Mhz). The capacity of C3 maintains,of course, the oscillations. The resistances R2 et R3 are used for bridge divider and fix the base's current. The mike's supplying, which can be replaced by any other source, is carried out by the resistance R4. The condensator C4 blocks the continuous component of the frequency's modulator's signal and put it on the transistor T1 's base. Endly We put an antenna on the centre of L in order not to cushion the LC circuit too much. In short an audio signal on the transistor's base makes the frequency generated by a LC circuit,which put a signal on this transistor collector, modulate.
Calculation to determinate the frequency generated by a LC circuit: For an electric oscillator we've got:
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(with w the beat)