Simple FM transmitter help!

Discussion in 'Wireless & RF Design' started by nickerst, Dec 4, 2010.

  1. nickerst

    Thread Starter New Member

    Dec 4, 2010
    2
    0
    I just made a simple fm transmitter and now I am trying to understand the different components that go into it. I am mainly working on the right half of the circuit. I think that L2 and C5 are acting like an oscillator, C6 is just keeping it oscillating. But I have no idea what the diode is doing, or the base and collector parts, or the purpose of C4, C7, and C8. Thank you in advance for your help!

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  2. radiohead

    Active Member

    May 28, 2009
    474
    31
    L2 and C5 is a tank circuit. The tank circuit controls the frequency of the Colpitts oscillator that is comprised of Q2, C6, L2 and C5. C6 is a feedback capacitor that produces a modulated signal. R6 is an emitter bias resistor. Typically, you would need the 220 ohm for 3 volt operation, 470 ohm for 9-12 volt operation. These little transmitters work just fine at 3 volts. What's the current consumption of this circuit? I'm guessing around 30mA or so.
     
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  3. nickerst

    Thread Starter New Member

    Dec 4, 2010
    2
    0
    Oh, that makes a lot more sense! The current consumption is about that, yes. Is there any significance to the series combination of L1 and R6 to bias the emitter? Also, why is the base is being biased by R5 and C3? I have never seen this before. I have always seen the usual emitter follower with two resistors...
     
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