FM Transmitter Modulation

Discussion in 'The Projects Forum' started by Ag565, Sep 11, 2012.

  1. Ag565

    Ag565 Thread Starter New Member

    Sep 8, 2012
    I am designing an FM transmitter as a learning experience and for fun, but I am having some difficulty with FM modulation. I am using a VCO with a Colpitts topology and common emitter configuration. For modulation, I am using a varactor biased at 5V with a 1kHz, 5V amplitude message signal, which should allow the varactor capacitance to vary from 9pf to 19pf. I have an RF choke and DC blocking capacitor to couple the input signal to the oscillator. When I simulate my circuit in Multisim, I am able to measure my center frequency to be around 77MHz on the spectrum analyzer, but I am not seeing any frequency deviation on the output signal. Is the deviation too small for the resolution of the analyzer, which is ~9.6kHz or is my message input too weak once it gets to the oscillator because of the values I have chosen for coupling to the oscillator? I have linked to pictures of my circuit and frequency spectrum output. Thanks for any help!

    Frequency Spectrum Output
    FM Transmitter Circuit
  2. BillO

    BillO Well-Known Member

    Nov 24, 2008
    Could you post a schematic?
  3. MikeML

    MikeML Senior Member

    Oct 2, 2009
    You want ((D1 in series with C5) in parallel with C2). The choke goes on the modulation input side in series with R5.
    Last edited: Sep 11, 2012
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