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AM Transmitter using Colpitts Oscillator

Discussion in 'Radio and Communications' started by Frank Casey, Dec 13, 2017.

  1. Frank Casey

    Frank Casey New Member

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    I am experimenting with AM transmitters, I found this Colpitts oscillator circuit and have found the right inductor which produces a frequency of about
    1,246,458 Herz which is what I want (in the middle of the AM frequency band).

    I cannot figure out where to connect the audio input.

    I tried connecting audio to the base of the transistor and also to the output, neither works.

    Can anyone assist? I am an RF electronics newbie.

    Colpitts-osc-01.gif
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    You need to vary +9V node in the circuit above from ~2Vmin to ~ 16Vmax in step with the audio waveform. That is called high-level Amplitude Modulation. It takes an audio amplifier to do that. When used that way, the amplifer is called a "modulator". That's what it takes to make an AM signal.

    Playing with the bias on the base of the oscillator transistor might vary the frequency of the oscillator, and at best, produce a crude form of Frequency Modulation...
     
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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