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Discussion in 'Electronic Basics' started by Ken O, Aug 3, 2006.

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  1. Ken O

    Ken O Guest

    HI all,

    I was looking at some RF circuits. I have a question concerning the
    capacitor c4 in the folowing diagram:
    Apparently this capacitor keeps the carrier oscillation ( C3 L1 ) from
    decreasing , or by keeping its voltage steady. Theoretically we would not
    need that. But with heat loss we do, I do not understand is HOW the C4
    capacitor do that, that is keeping a constant voltage for the oscillator.


  2. Tom Biasi

    Tom Biasi Guest

    The circuit is not very conventional. He spikes the tank with a LED pulser
    Why don't you direct your questions to Tony?
    Post on his forum regarding this circuit.

  3. Andrew Holme

    Andrew Holme Guest

    Normally, single transistor VHF transmitters have an emitter resistor.
    Here's an example

    The transistor operates in common base mode, with the 4.7pF capacitor
    providing feedback from collector (output) to emitter (input).

    Although that tracker circuit shows an earth symbol connected to the
    emitter, the "common" point is really the junction of C2, C3 and L1. Think
    of everything between this point and the emitter as a battery and resistor
    in series and you have the standard circuit.
  4. Ken O

    Ken O Guest

    My question was more with the use of C4. If I remove c4, the circuit
    still operate. But I read on a web page that this capacitor is needed
    real life because there is loss. this capacitor help keeping the
    voltage to
    the tank circuit. I just like to know how it keeps the voltage of L1
    and C3.
    The way I see it, C3 and L1 oscillate at about 90MHz all the time, then
    beep (output of the IC1) turns on the base slowing the oscillation .
    Therefore, c4 theoretically is not needed , right?

  5. Rich Grise

    Rich Grise Guest

    Wrong. It is an integral part of the oscillator. If you remove it, the
    best you will get is some ringing (a damped sine wave) when your timer
    gives it its "ON" pulse.

    Try googling for "oscillator circuit" or so.

    Good Luck!
  6. Andrew Holme

    Andrew Holme Guest

    The transistor is an amplifier. Some energy is fed from the tuned circuit,
    via C4, to the emitter, which is the amplifier's input. The amplifier
    supplies just enough energy back into the tuned circuit to replenish what is
    taken/lost, thereby sustsaining the oscillation. If you remove C4, you
    break the feedback loop, and oscillation ceases.
    The output of IC1 powers the oscillator. When there is no power, there is
    no oscillation.
  7. Andrew Holme

    Andrew Holme Guest

    OK, maybe I take that last sentence back, having looked at the LM3909
    datasheet. If there was sufficient voltage between pins 6 and 4 to keep the
    oscillator going all the time - and I don't know if there is - then it would
    oscillate "all the time" and be both amplitude and frequency modulated by
    the flasher. But, either way, C4 is required.
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