Reactance FM modulator

Discussion in 'The Projects Forum' started by PIgeral, Sep 16, 2010.

  1. PIgeral

    Thread Starter New Member

    Sep 8, 2010
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    the only thng i need help from my circuit is the values of each component. i'm designing a FM modulator that will operate on 89.6MHz. i only have 100uH for the RF choke (L1 & L2). if this value of this RF choke are not suitable for this circuit and how can i make my own RF choke. i also want to know how the audio transformer works in this circuit. you can click the link below to see the circuit.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    First of all that isn't any sort of RF transmitter circuit, matter of fact with the audio transformer and the cap values I'm not really sure what its intended use was.

    Are you wanting a simple FM transmitter you can broadcast through? If so, what you're wanting to do is rather simple and hand winding coils is a lot easier than it sounds. If you search for "wireless FM transmitter" you should come up with a ton of working circuits and design ideas.
     
  3. PIgeral

    Thread Starter New Member

    Sep 8, 2010
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    what i want is the modulator circuit and i come up with this but i'm not really sure about the components values that i used now..i tested and the output is not FM or not the expected output..m asking if the values of the RF choke is correct wrong.. and even the audio transformer that i used..how it work in this circuit. anyways thanks for stealing some of your time here n your comment will higly appreciated.
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    What do you mean by modulator? Modulation is done on the RF portion of an FM transmitter and the circuit you have drawn doesn't make much sense to me at the moment, nor did it the first time I looked at it.

    I'm about late for a meeting.
     
  5. PIgeral

    Thread Starter New Member

    Sep 8, 2010
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    ok than..my circuit is a reactance modulator but the thing is no values of the components. my assignments is to design a modulator circuit. just the modulation part of the FM transmitter and if you know a simple circuit for modulation that have the values of the component and you can send it so i can have a look at it. your response is highly appreciated. thank you
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    To put this in the middle of the 88-108 MHz FM broadcast band, the inductor (shown as the primary of the "audio transformer") needs to be about 300 nanohenries. So don't use an audio transformer, use an air core coil in stead. Take your output from the collector of Q2 but don't load it down.

    The RF chokes can be anything over a few microhenries; 10 uH should work fine. Get rid of the 10 uf capacitor and the varying (modulation) voltage on the collector of Q1 will case the collector-to-base capacitance to vary, which will result in frequency modulation of the oscillator.

    Use a 1000 pf ceramic capacitor in parallel with C4.

    You will have to experiment with the value of the inductor to get the thing to oscillate where you want it because in a physical circuit, strays are bound to be very significant.
     
  7. PIgeral

    Thread Starter New Member

    Sep 8, 2010
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    well, thanks DickCappels for your time to stop by and i appreciated very much. anyways..you mean that i will not need to put the capacitor with 10uF (C1) in my circuit?

    i remove the capacitor (C1) connected from the base of Q1 to the collector and my results is something like AM signal instead of Fm. what about the value of the capacitor connected between the collector of Q1 to collector of Q2? is it possible that to cuase the result in AM signal and what value should be used?
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    Omit the capacitor from the base to collector of Q1. With a 2.2 uf capacitor between the collector of Q1 and the collector of Q2, you will have a combination of AM and FM. To greatly reduce the amount of AM change the value of the capacitor; I suggest 100 pf is enough.
     
  9. PIgeral

    Thread Starter New Member

    Sep 8, 2010
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    ok..thanks..now i have the FM signal..so whats the function of this capacitor? the one connected between the collectors.why the we omit the capacitor?
     
  10. pranav0091

    New Member

    Oct 12, 2010
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    It seems to me that the capacitor between the two collectors has two purposes;
    1) DC blocking: the capacitor sees that the dc voltages from the collector of Q1 does not affect the biasing of Q2.
    2) Frequency filtering: The capacitor acts as an impedance to lower frequency signals thereby preventing them from reaching the collector of Q2.

    I am not very sure about the second point though.
     
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