How to design a Varactor FM Modulator?

Discussion in 'The Projects Forum' started by electrogirl, Mar 9, 2011.

  1. electrogirl

    Thread Starter Member

    May 2, 2010
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    Hi, In our basic electronics class we were asked to design a Varactor diode FM Modulator. Each of us were assigned with a different carrier frequnecy in our FM modulator project. The carrier frequency assigned to me was fc = 1 MHz. I know tha principles of frequency modulation and on how a varactor diode can be used to varry the oscillating frequency.

    What is not clear to me at this point is on what component values should I used in order to fulfill what was assigned to me. That is the resistor values, the capacitor values, the inductor values and on what type of varactor diode am I going to use.

    Any help is very much appreciated.
    Go to the link to see the circuit of a Varactor FM Modulator.
    http://www.tutorvista.com/physics/frequency-modulation-circuit
     
    Last edited: Mar 30, 2011
  2. electrogirl

    Thread Starter Member

    May 2, 2010
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    up...up....upp
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
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    The data sheet for any particular Varicap will show you a curve of voltage input vs. resultant capacitance. Roughly speaking you have to factor the min and max capacitance into your resonant freq equations.
     
  4. electrogirl

    Thread Starter Member

    May 2, 2010
    47
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    up...up update
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    If you keep doing this the Mods are going to spank you..:rolleyes:
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    There is no need to do the bumping.
    It can take more than 24 hours for an answer, as this is a worldwide forum.

    Read this PDF for more information on varicap diodes:
    Variable Capacitance Diodes

    This is a link from the diode types page of the EDUCYPEDIA:
    Diode types

    Bertus
     
  7. electrogirl

    Thread Starter Member

    May 2, 2010
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    Sorry but I didn't get it...I'm just a newbie please do understand ^^,
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The posted circuit will not work as L1 will short the DC voltage needed for the varicap.

    Bertus
     
  9. electrogirl

    Thread Starter Member

    May 2, 2010
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    is that so?I found that circuit in a book...but anyway how about this,
    this circuit uses a crystal oscillator to generate the carrier frequency
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    That circuit will work, as the crystal will act as a series LC circuit.
    The two resistors connected to the varicap will provide the needed DC voltage to set the base capacitance.

    Bertus
     
  11. electrogirl

    Thread Starter Member

    May 2, 2010
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    How do I figure it out what component values to use?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Why do you want to modulate with FM in the middle of the AM broadcast band??
     
  13. electrogirl

    Thread Starter Member

    May 2, 2010
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    huh?what do you mean?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Your circuit is at 1MHz. The AM broadcast band in North America (I don't know where you are) is from 0.54MHz to 1.6MHz.
     
  15. electrogirl

    Thread Starter Member

    May 2, 2010
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    But I'm not from north america( ^^,)
    Can you help me figured out those component values?
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    You should ask your teacher to teach the details about radio circuits.
     
  17. electrogirl

    Thread Starter Member

    May 2, 2010
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    He wont tell T_T
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Your first circuit (LC) should work if you add a DC blocking cap between the cathode of D1 and the junction of L1 and C1. This cap will be very large in comparison to your tank caps. A 1000pF should work OK.
     
    Last edited: Mar 25, 2011
  19. Audioguru

    New Member

    Dec 20, 2007
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    Never mind talking about the politics of why your teacher doesn't teach anything in your country. Do gardening instead of electronics.
     
  20. electrogirl

    Thread Starter Member

    May 2, 2010
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    can you give me more details...
     
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