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Modulating Synthesisor to FM

Discussion in 'Radio and Communications' started by electricguru, Jul 15, 2010.

  1. electricguru

    electricguru New Member

    Jul 15, 2010
    I'm having trouble broadcasting my synth on the FM band. I have tried around 100 circuits, but am have been bummed quite a few times. I had one going pretty well on my beadboard but after I soldered it on a circuit board, it did not work. Any ideas or troubleshooting tips I would very greatful for.

    Also, I wondered if you knew the equation using the coil and cap values for the emitting band frequency of the modulator.

    Last edited: Jul 19, 2010
  2. RCinFLA

    RCinFLA Well-Known Member

    Jul 23, 2009
    FLA, USA
    Not sure if I got your question correct. Do you mean FM modulate a synthesized RF source?

    There are a number of ways to do this. Probably the simplest is to modulate the reference source and the RF VCO in matched proportions such that the phase detector sees no net effect. The frequency movement of reference and VCO at the phase detector exactly equal.

    You have got to take into account the various divider chain of each. The matching will change with RF freq setting as the VCO divider ratio is changed. If VCO freq range is small it does not require readjusting modulation level on VCO. If range is wide there is usually a look up table to adjust the modulation level on VCO versus RF frequency setting of synthesizer.

    If you only modulate the VCO, as the modulation frequency gets inside the loop bandwidth of the PLL the PLL will cancel out the applied VCO modulation. You will lose the low frequencies of the modulation.

    You can make the PLL loop bandwidth very low (<30 Hz) but then you have to have a very mechanically stable VCO as any vibration will likely modulate the VCO.

    The other extreme is to design a synthesizer with a very wide loop bandwidth and digitally modulate the frequency setting of synthesizer. This is somewhat complicated and is best done with a DSP or at least a microprocessor to handle the digital modulation and center frequency setting.
    Last edited: Aug 18, 2010

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