How this simple rc circuit works

Discussion in 'Wireless & RF Design' started by chenziqing, Jan 16, 2012.

  1. chenziqing

    Thread Starter New Member

    Jan 16, 2012
    2
    0
    Hi,all
    Here is a brief description of my project, I would like to know the thoery behind how this simple RF circuit works mathematically.(especially the function of BJT)Thank you.

    Following is the hyperlink of the schematic.
    http://www.futurekit.com/2009/manual/future/eng/PDF_FK7/FK702E-1.pdf
    The board is intended to be working at 94.9 Mhz, however the signal i measured at collector, base and emitter of the bjt are all 1khz in frequency. Is this supposed to be right?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    No, it should be operating at the high frequency. The circuit is an oscillator (a Colpitts type I believe) with the BJT providing the amplification required for the oscillations to occur. A Google search will tell you more.

    How are you measuring the frequency? Are you using a 10:1 probe?
     
  3. chenziqing

    Thread Starter New Member

    Jan 16, 2012
    2
    0
    Hi, thank you so much for answering me! The 1KHz is not real frequency which means it is just used as a experimental purpose. The actual input is something like human voice. I just wanna know how does this BJT mix the 2 signal (human voice and carrier signal which is generated by the tank circuit)?
    Thanks very much again!
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    The BJT's collector-base capacitance varies slightly with the change in bias caused by the input signal. This capacitance change slightly alters the oscillation frequency, giving FM modulation of the carrier.
     
  5. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    @OP

    I deleted your second post, as it was written in a language other than English, which is forbidden in the AAC forums.

    If it contained crucial information, please repost it in English.

    Thank you.
     
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