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oscillator circuit

Discussion in 'General Electronics Discussion' started by chino_drk, May 4, 2014.

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  1. chino_drk


    May 4, 2014
  2. duke37


    Jan 9, 2011
    The quick answer is that I do not know !

    T1 is the oscillator which is an amplifier with a frequency selective component in the positive feedback. In this case the selection is done by the crystal. However there are two feedback paths, one through the crystal and one through C2 to the emitter. Normally only one or the other would be used. If the feedback was through C2 then the crystal would be connected from base to ground.
    I have not worked out the resonant frequency of L1 which could be tuned to 27MHz or just an RF choke. In any case, there should be a capacitor to bypass the +9V supply.

    I presume that the line coming in from the bottom has a 30Hz signal on it which alters the biasing on T2 so altering the gain and producing an amplitude modulated output. No defined bias level is given.
  3. LvW


    Apr 12, 2014
    Yes - there are two feedback paths - however, the path with the crystal acts as a NEGATIVE feedback (because base and collector are in anti-phase).
    Therefore, my understanding is as follows:
    Positive feedback via C2 establishes the oscillation condition (but not "very sharp" at the desired frequency).
    Hence, we have a sharp negative feedback path which inhibits the oscillation - except at the frequency where negative feedback does not exist: When the crystals impedance is very large (parallel resonant frequency). This gives the resulting oscillation frequency.
    chino_drk likes this.
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