FM modulation circuit

Discussion in 'General Electronics Chat' started by BkraM, Nov 30, 2014.

  1. BkraM

    Thread Starter New Member

    Jan 19, 2014
    Hi All,

    I'm looking at a 1 transistor FM transmmitter circuit.
    But I fail to understand how the audio signal is modulated.
    I think I recognise the colpitts oscillator in the circuit (1-30pF 5pF and L1).


    I would think that to alter the oscillating frequency (fm modulating) the capacitance of the oscilating circuit has to change with the incoming audio signal. Is this done by the internal capacitance of the 2N3904??
    How exactly does this work?

  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    the audio changes the bias on the transistor, gibing a mixture of am and fm. the base to collector capacitance changes with bias.