Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to design fm transmitter

  1. May 25, 2009 #1
    I want to know how to design fm transmitters. That is how to select appropriate values of the components. I need help
  2. jcsd
  3. May 25, 2009 #2


    User Avatar
    Science Advisor

    This is a really, really broad question. Also, it should be in the Electrical Engineering or regular Engineering subforums. Since you're asking about specific component values, do you have a design that you're already using?
  4. May 25, 2009 #3
  5. Jun 28, 2009 #4
    I want to know only how to make a coil for specific output frequency? what is a crystal? How to make a crystal and how to use it?
  6. Jun 28, 2009 #5


    User Avatar
    Science Advisor

    There is a formula for designing tuned circuits.

    L (inductance of coil in microhenries) = 25330.3 / ((Frequency in MHz) * (Frequency in MHz) * (Capacitance in pF))

    So, to make a coil to resonate with 50 pF at 25 MHz you would need an inductance of 0.81 microhenries.
    In practice, this would be a small coil maybe 10 mm in diameter with 6 turns of wire on it and you might make the capacitor partly variable to tune the circuit and achieve resonance.

    There are many inductance calculators on Google. They usually give different results for small coils but better ones for larger coils.

    Regarding crystals.... you always have to buy these. There are large factories that make them but you would not be able to make them at home. Old computers always have a few crystal oscillators in them.

    See the following site for quite a good article about quartz crystal oscillators.
  7. Jun 28, 2009 #6
    It not really possible to wideband FM a crystal oscillator. Free running oscillators will FM easily but do drift. It is possible to have a phase locked loop which holds the centre frequency steady but allows deviation. Some broadcast FM transmitters deviate a stabilised 10.7 Osc (required to remain within a few hundred Hz by the authorities) and then mix up to the final frequency.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook