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FM Transmitter Circuit (Please Explain working)

  1. Jan 4, 2010 #1
    Hi.I have got this circuit for a low cost short range FM transmitter but I am unable to understand the working and functionality of the circuit.Please help me in understanding the working.What I know is that its working as a VCO(voltage controlled oscillator) when input (through electret mic) is applied.

    Attached Files:

  2. jcsd
  3. Jan 4, 2010 #2
    The top transistor, inductor and two caps form a Colpitts type oscillator that generate the actual radio waves. The lower transistor is just an audio amplified current source that pulls the bias conditions of the oscillator which will AM modulate it in theory, but FM will still be detected by a radio due to overdriving the oscillator which can generate alot mess, and one among may FM modulation.
  4. Jan 4, 2010 #3

    AM Modulation...??
    Please sir elaborate a bit more...cant understand...!!
  5. Jan 4, 2010 #4
    In AM modulation, the amplitude of the carrier frequency is varied, in FM the frequency is slightly varied to convey any data. In case of audio transmission, the amplitude varies with audio in AM, and frequency varies with audio in FM.

    The circuit that you provided modulates input in AM primarily, but because the transistors are overdriven, alot of ugly harmonics will be generated, and many of them will be FM modulated which can be heard in FM radio.

    And alot of times FM radios are poorly designed, and will actually pick up AM modulation in the FM band due to similar reasons. So this circuit is bound to work.
  6. Jan 10, 2010 #5
    Thanks for the reply sir...
    also please tell that whether the circuit will produce NBFM(narrow band FM) or WBFM(wide band FM)...??
    and how can i match the antenna...??
    Like a senior of mine was saying that a simple 6-inch wire is enough to act as an antenna...?? As i have not studied da course on "antennas" yet...!!
    How to do "impedance matching" for antenna with the circuit...??
    If i want to increase the range of transmission...?? can that just be done by merely increasing the antenna length...??
  7. Jan 11, 2010 #6

    please Help...!! :(
  8. Jan 11, 2010 #7
    It will produce NBFM because the transistor doesn't have enough capacitance to pull too much frequency. A wideband VCO will have varactor diodes which change alot of capacitance with input voltage, and the changed capacitance changes the resonant frequency.

    A simple whip antenna up to a meter will do fine. For more range, a more powerful amplifier is needed. Impedance matching is used to improve performance, but this circuit is just a standalone oscillator which probably generates some interference and output is not low pass filtered.
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