a capacitor in FM modulator

Discussion in 'Wireless & RF Design' started by baby_1, Mar 29, 2012.

  1. baby_1

    Thread Starter Member

    Jun 3, 2011
    39
    0
    what does the capacotir (120pf)do?

    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,536
    This is a common base configuration on the transistor RF design. The capacitor grounds any RF signals, but is low enough in value it is fundamentally invisible at audio frequencies.
     
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  3. baby_1

    Thread Starter Member

    Jun 3, 2011
    39
    0
    hello and thanks for your response
    could you tell me about how can we calculate this capacitor value according this circuit?
     
  4. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    |Xc| = 1/(2*pi*f*C)

    since frequencies of carrier and audio signal differ greatly, you can use any value in quite wide range, for example 100pF - 2.2nF will do.
     
  5. BlackCow

    Active Member

    May 11, 2009
    65
    1
    Just in case the OP doesn't understand what this is this formula gives you the reactance of a capacitor at a given frequency in Ohms. Reactance is like resistance but only AC signals.
     
  6. BlackCow

    Active Member

    May 11, 2009
    65
    1
    Perhaps the OP doesn't know what reactance is?

    This is this formula gives you the reactance of a capacitor at a given frequency in Ohms.

    Reactance is like resistance but only for AC signals.
     
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