FM Transmitter Modulation

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

  1. Ag565

    Ag565 Thread Starter New Member

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    Sep 8, 2012
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    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

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    Could you post a schematic?
  3. MikeML

    MikeML Senior Member

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    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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