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[question] quite specyfic FM modulator; request for opinion

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Antey, Apr 18, 2005.

  1. Antey

    Antey New Member

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    Hello Everybody :)

    So here I am again with request for opinion - this time about another "so-called" design, which is FM modulator (not transmitter !).
    As far as I know, my project may seem trivial, my solutions may be found as unproffessional and even funny.

    The idea is to make use of PM modulator (attached) which will have integrator connected at it's input and - voila ! - this should result in frequency modulation.

    The problem is that the original circut ( found @ http://sound.westhost.com/doppler.htm#2.1 ) is only demonstration which runs @ 1kHz carrier with 50 Hz modulating signal.
    What I need is 100 MHz carrier, with 50 kHz deviation and modulation index 15. Because it's NOT going to be a real-life circut - ONLY SIMULATION - so I figured out that it should be enough to select proper OpAmp [EL2045], and adjust other vaules to get the simulation (in SIMetrix) working.

    So here are the questions :
    - does it have any chance to run at all ?
    ( I think it has, the output spectrum looks promising...but I have trouble with transients which create enormous peaks @ 0 Hz )
    - if my approach looks acceptably what should I change in circut ?

    Of course, any other suggestions shall be much appreciated :)



    Best Regards,
    Peter
     

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  2. JimB

    JimB Super Moderator Most Helpful Member

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    Peter

    The circuit you have found is not particularly good to use at high frequencies. The author of the original article used it to test an idea at LOW frequencies.

    If you want to produce Frequency Modulation (FM) at 100Mhz, I would suggest any LC oscillator, and use a varicap diode across the tuned circuit.

    If you wish to produce Phase Modulation (PM), and you want 50khz deviation, you will need to run your phase modulator at a much lower frequency, (say about 1 Mhz), and use several multiplier stages to get to 100Mhz.

    JimB
     
  3. Antey

    Antey New Member

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    I know that the cirut included is just a model - and that's no problem for me. My specific problem is that i don't need real life cirut - only working simulation (and please don't ask, why it's so stupid ).

    From all what I read, i see that LC+varicap is a must.
    I have few such circuts, and I'll do my best.

    But... what problems could appear while trying to move that cirut up ?

    Anyway - Thank you very much for showing interest in this strange problem :)
     
  4. dave

    Dave New Member

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  5. JimB

    JimB Super Moderator Most Helpful Member

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    The op amp EL2045 has a gain-bandwidth product of 100 to 200Mhz (depends on supply voltage), I think it will struggle at 100Mhz to work correctly in this circuit.

    However, you are only doing a similation of a phase modulator, The op-amp is not doing anything useful as far as the modulation process goes, it is just a buffer.

    What I suggest you try is to leave out the opamp and the 10uF capacitor, scale the 56nF capacitor to suit 100Mhz and use your simulated spectrum analyser at the junction of the scaled 56nF cap and the FET.
    If you are lucky, you will see a small amount of phase modulation.

    JimB
     
  6. Antey

    Antey New Member

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    :) I'll try that. Meanwhile I also found other circuts.
    Thank You once again.
     

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