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

Low Power FM transmitter explanation

  1. May 4, 2012 #1
    Hello everyone,
    Finally I found a useful website where I can find the answers of my questions:)

    I will take signal processing course soon and I thought "building a low power fm transmitter" is a bright idea to train myself to a certain level... Though I need your help and I don't have anybody around me to find out how it goes...

    I found a circuit online and I'm trying to understand every single point of the circuit...from top till toe!!! just building and soldering the elements will do nothing i'm sure!! I need to understand each and every elements, so I thought that if I ask questions to you, maybe you can help me...
    don't tell me that "it is very easy!!" please! I know it is and I just want to understand it as much as the owner does=)

    As far as I understood, first part(with mic and first transistor) is an audio amplifier! How can I measure th output of the amplifier?

    I guess second transistor is used for frequency modulation! I think I need an immediate assistance on that part :rolleyes:

    Tank circuit oscillates at the rezonant frequency (i'll use 22pF varicap) and 0.118μH will give me 98.70Mhz output!!

    So, since we know that a human voice frequency is in between 60Hz-4000Hz, what is the procedure in my circuit step by step??

    Any answer or comment will save my life=)))) Thanks for your concerns!!!

    Attached Files:

  2. jcsd
  3. May 4, 2012 #2
    The easiest way would be with an oscilloscope. If you don't have an oscilloscope you could try to use a meter but the signal will probably be too small to see with a meter. (Put the meter on AC, connect the leads to the collector of the first transistor and ground, and speak into the microphone.

    What do you need assistance with?

    If 22pF is the maximum value of your varicap, you may want to add a 15 pf cap in parallel to reduce the tuning range and put 98.7 MHz closer to the center of the capacitor's tuning range.


    Any answer or comment will save my life=)))) Thanks for your concerns!!![/QUOTE]
  4. May 4, 2012 #3
    Thanks, I tried and experinced the change in value of collector output of the 1st transistor.

    As I tried to explain above, I know that the first transistor part is for audio amplification. And the 2nd transistor (the place where i will modulate my frequency) uses the audio signals to shift my carrier frequency ie. if I have 98.70Mhz oscillation frequency from the tank circuit, it will be shifted by the transistor just before it's radiated via aerial wire. (I don't know how much it will be shifted, this is the point where i lose my track :cry: )

    If there is something missing or wrong or completely wrong, please correct me... :uhh:

    I told you that I'm losing my point after a while=)) I hope i am not beyond redemption :frown:
  5. May 4, 2012 #4
    I don't know of any easy, cheap way of measuring deviation. You could listen to your signal on an FM radio and compare the volume of your signal with the volume of other FM stations. If you have access to a radio lab, there are instruments like a measuring receiver that you might use.

    The person who designed this circuit may have tested it and found the deviation to be about right.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook