FM modulation ckt..

Discussion in 'Homework Help' started by viip, Apr 27, 2009.

  1. viip

    Thread Starter Member

    Apr 16, 2009
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    here i have attached a FM modulation ckt. The Cc and Lc are for carrier wave generation and the modulating signal is provided.But still i couldnt get how frequency is getting modulated?Can u please explain this whole ckt...if possible from the scratch??
     
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  2. Ron H

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    Apr 14, 2005
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    The collector-base capacitance varies with voltage. It acts as a varicap (varactor) to change the resonant frequency of the tank circuit as the audio changes the base voltage.
     
  3. viip

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    Apr 16, 2009
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    hey..then whats the need of the three 10 pf capacitors just before antenna....which are providing some sort of feedback?? and as u said about base collector capacitance then why such thing is changing only in this ckt...we have almost similar ckts for oscillator and tuned amplifier.Then why B-C capacitance doesn't changes there or show some effect there??please explain in bit more detail..:)
     
  4. Ron H

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    All three 10pF caps affect the resonant frequency of the tank. The C-E cap provides the positive feedback needed for oscillation. The one across the emitter resistor is probably not necessary.
    B-C capacitance nonlinearity in almost any transistor oscillator will affect the harmonic distortion of the output to some extent. The amount of distortion will depend on the circuit, and on the ratio of fixed capacitance to C-B capacitance.
     
  5. viip

    Thread Starter Member

    Apr 16, 2009
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    ohh i got it....the modulating voltage will change the B-C junction capacitance which will cause the overall tank circuit capacitance to change and hence the generated frequency,which will be the FM wave...Just two more doubts...
    1.) you said that the three 10 pf cap are for affecting the resonant frequency...but are we not complicating the ckt by adding these three,I mean we could have changed the frequency using simply a DRB :eek: ?? Or are there any other benefits of using these cap :confused:?? I think one is the for oscillation...am i write?
    2.) In the ckt the feedback is given to modulating signal also....wont it affect the source??
    [Please dont get frustrated by my silly doubts ]
     
  6. Ron H

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    What is a DRB?
     
  7. viip

    Thread Starter Member

    Apr 16, 2009
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    oops I wanted to tell about variable inductance for changing the resonant freq.
     
  8. viip

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    Apr 16, 2009
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    please nyone clear the doubt....
     
  9. viip

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    Apr 16, 2009
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    RON H????????
     
  10. viip

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    Apr 16, 2009
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    Please ny1 reply to query.....:(...
     
  11. Audioguru

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    Dec 20, 2007
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    Usually the oscillator is tuned with a trimmer capacitor. The inductor is just a few turns of wire with an air core at this very high frequency.
    The FM (frequency modulation) is performed by modulating the collector-base capacitance of the transistor.

    The circuit has too many capacitors so I removed two and changed one to be grounded. The capacitor from the collector to the emitter provides positive feedback for oscillation to occur.

    The modulating signal is very low (audio) frequencies applied to the base of the transistor. The transistor oscillator is a common-base type with the 330pF capacitor grounding the base at 100MHz so the 100MHz signal does not feed into the source.
     
  12. Ron H

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    This forum is not my job. Sometimes, I have more pressing things to do, like a doctor appointment, grocery shopping, etc.:eek:
     
  13. viip

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    Apr 16, 2009
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    I never said its ur job:D:D....just asking for help....may be i forgot please...:p
    Hey Thanks man...I never knew the FM is done using BC junction capacitance variation.
     
  14. viip

    Thread Starter Member

    Apr 16, 2009
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    Got it...thanks...i will try it again in lab...
     
  15. KL7AJ

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    Although this circuit will work, it will be far from ideal. It will A.M. modulate the signal about as much at it will F.M. the signal. This can be corrected by following the circuit with a limiter or a class C amplifier.

    eric
     
  16. Audioguru

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    An FM radio ignors any AM on an FM transmission.

    The very simple circuit performs poorly and is a child's toy:
    1) Its frequency changes if something moves toward it or moves away and also changes as its battery voltage runs down.
    2) It does not have pre-emphasis (treble boost) like all FM radio stations have so its sound received on an FM radio (they all have de-emphasis) will sound muffled like your stereo with its treble tone control turned all the way down.
     
  17. Ron H

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    Why don't you post your FM transmitter circuit?
     
  18. KL7AJ

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  19. Ron H

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  20. KL7AJ

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