FM Transmitter Circuit

Discussion in 'Wireless & RF Design' started by kapildevkumar a, Sep 21, 2016.

  1. kapildevkumar a

    Thread Starter New Member

    Jun 5, 2015
    I would like to know how a FM Transmitter Circuit works exactly. I came across many answers while searching but I would like to understand from basics(like how current flows, explain each thing part by part - Purpose of each component)..
    I would be very much happy if some one could help me out.

    Any FM transmitter Circuit merely even this
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010

    This answer is about the circuit in your post. Scroll down a little to find it.
    Use Wikipedia to research "miller capacitance", to discover how the oscillator is modulated.
    Hint: this is not a simple circuit. It is using a single transistor to perform many different functions that could be more easily explained using a more conventional three transistor design.
    Sensacell likes this.
  3. radiohead

    Active Member

    May 28, 2009
    This is as basic explanation as I can... R2 4.7K resistor is a bias resistor that provides a voltage drop so the microphone can function properly. Audio from the microphone has an AC waveform and is fed directly to Q1 base. C1 .001uF is a feedback capacitor that filters some harmonics (spurious frequencies). Q1 has two jobs... 1) audio amplifier and 2) Radio Frequency (RF) oscillator. As the RF oscillator is the primary function, it has a low value capacitor C2 4.7pF across the collector and emitter. C2 will charge and discharge at such a rapid rate, it will generate RF. The tank circuit made from L1 and VC1 controls C2 charge/discharge rate, giving you the transmit frequency. R1 is a current-limiting resistor that will prevent the transistor from getting hot and to limit the RF power output. C3 is a bypass capacitor that will (sort-of) compensate for battery resistance and stabilize battery voltage variances. RF is sent out the antenna connected to Q1 collector. Personally, I would add a loading capacitor about 10pF between Q1 collector and the antenna for stability. Does this help explain the circuit?
  4. ian field

    Distinguished Member

    Oct 27, 2012
    Years ago I made a 6MHz FM transmitter (UK PAL sound intercarrier)..

    A 74LS629 VCO running at 12MHz fed a JK flip flop to divide by 2 and balance the mark space ratio - it fed a push pull PA stage driving a centre tapped tank coil.

    Most people use a reactance stage to modulate bug type band II transmitters.