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FM transmitter not transmitting

  1. Jan 31, 2015 #1
    I studied the working of the fm transmitter and end up making my own. The first part is amplifier and second (right) part is colpitts oscillator.
    The circuit works good on simulator. The output is AM modulated wave lying in fm band so it could be detected by radio easily but the circuit is not transmitting in real.

    I can't understand where have I gone wrong.

    I have read that these high frequency circuits don't work on breadboard. So I made it on pcb. Can I make it on breadboard also?
    Here is the circuit I made
  2. jcsd
  3. Jan 31, 2015 #2


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    Staff: Mentor

    Welcome to the PF.

    Please upload attachments instead of linking to slow-loading websites. I still do not see the circuit.

    Also, the discussion in that link implies that the circuit is an AM transmitter, and contains some clumsy misinformation about RF receivers. I'll move this thread to the EE forum for better views. Please upload the circuit so we can help explain it to you. Also, please be aware that experimenting with RF transmitters can be illegal if you transmit too much power. Most hobby project circuits are designed to keep the TX power low and within regulatory limits, but some others are not...
  4. Feb 1, 2015 #3


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    I could see a circuit on that link you gave, but it was so small nothing was identifiable

    when you do as Berkeman asked and up load the image to here .....
    make sure its large enough to read and it has ALL the component values on it

  5. Feb 1, 2015 #4
    Transmitters are to the developing EE what fire was to ancient man. The presented circuit is not good for a beginner. It is crude and building at 100MHz is demanding.

    This circuit is not as glamorous, but I recommend it as a start. This oscillator starts well, can be heard easily on an AM radio, and is guaranteed by design not to be capable of sourcing more than 10mw. It should pop up near the bottom of the AM band (~590 kc).
    Use a voltmeter to measure the voltage at C3 - It will typically be over 2.5 volts when not oscillating and drop below 2 volts when oscillating. touching the junction of L1 and C1 with a screwdriver and your finger will likely cause it to stop oscillating - giving you a means to test.

    R4 varies the amount of feedback to the oscillator. Increasing it's value will reduce the distortion and give you a purer signal. You can test this by going up on the radio dial and looking for the distortion at twice it's transmitting frequency.
    Alternately, if you have a good bias voltage at C3, but it doesn't appear to oscillate, you can increase the feedback by decreasing the value of R4. Again, you will know when it's working because of the lower voltage on C3.

    Okay, this is basic and necessary to ensure that you don't get fined playing with this design!
    - Do not increase the supply voltage
    - Do not change the values of R1, R2, or R3
    - Do not intentionally place the circuit or it's output into a position to intentionally interfere with other services
    - Do apologize and graciously concede to dismantle the circuit should anyone, including your kid sister, complain of it's effects
    - Do not construct an antenna that can focus the transmitted energy (this is near impossible without using your entire yard and a lot of wire)

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