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Uni Project FM TRnasmitter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by meeral20, Jul 24, 2009.

  1. meeral20

    meeral20 New Member

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    Hello fellow friends
    I need to make under 50mwatts FM transmitter for my uni project.All they gave us is the diagram.I have some idea about how op-amp,capacitors and resister works but when i take a look at the diagram it doesn't make any sense to me so if any1 of u please explain me where from i could begin ma project.thanks.
     

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  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Three stages, audio buffer, RF oscillator (with varicap FM modulation), and an RF amplifer (probably a multiplier as well, as it's tuned).
     
  3. meeral20

    meeral20 New Member

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    thnkx but how i define audio buffer,RF oscillator and an RF amplifier???

    And wea frm i kan begin ma calc???:)
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I imagine you are supposed to research it, not just ask other people to do it for you - these forums aren't a free cheating service.
     
  6. meeral20

    meeral20 New Member

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    i knw!! im nt tryna cheat or anythin,i just dont knw wea to begin frm thts awll. anywae thnkx for help ae.:)
     
  7. Hero999

    Hero999 Banned

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    D1 is upside down, it's a varactor diode and should reverse biased in order to work.
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    So it is - well spotted.
     
  9. marcbarker

    marcbarker New Member

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    Nice transmitter, better than average. Looks like a commercial design for retail. Maybe the manufacturer can provide free technical support for their ripped-off circuit?

    The varicap looks the correct polarity to me. Don't forget it's an RF circuit.
     
  10. Hero999

    Hero999 Banned

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    No, it should be reverse biased, I think you're confusing it with a normal diode.

    Here's an example of how a varicap should be used.

    [​IMG]
    Varactor or varicap Diode :: Electronics and Radio Today

    Why should that make any difference?
     
  11. marcbarker

    marcbarker New Member

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    It is reverse-biased. Don't forget it's an RF circuit.

    The 'should-be-done-this-way' example that was posted has too many extra parts and is 'not in the spirit' of a cheapest 'built down to a price' design :) which is one that uses the minimum of parts to do a job, with acceptable performance. These 'extra parts' do provide 'better' circuit performance (theoretcially), but on the other hand they are more circuit connections to go wrong.
     
    Last edited: Jul 24, 2009
  12. Mikebits

    Mikebits Well-Known Member

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    I thought varactors worked in reverse bias region? How is it reversed biased?
     
    Last edited: Jul 24, 2009
  13. Hero999

    Hero999 Banned

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    Whether it's RF or not makes no difference to the DC voltage across the diode.

    All right you tell me, in the original circuit, what's DC voltage across the diode? Please annotate the schematic.

    It's a different design but it illustrates the correct way of using a varicap.
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The varicap diode has NO DC bias. So it will work with either polarity to ground.
     
  15. Hero999

    Hero999 Banned

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    Correct, there is no bias.

    A varicap won't work properly without a slight negative bias.

    I discovered this myself when messing around with a varicap VCO.
     
  16. marcbarker

    marcbarker New Member

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    Yes, no doubt.

    The OP posted varicap gets its bias. Analyse the circuit.
     
    Last edited: Jul 24, 2009
  17. marcbarker

    marcbarker New Member

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    Because of the clever way this RF circuit works.
     
    Last edited: Jul 24, 2009
  18. Hero999

    Hero999 Banned

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    Where does it get its negative bias from?

    The cathode is connected to 0V and R6 connects the anode to 0V. If anything there's a slight positive bias from the tiny gate leakage current from Q1.

    If there's something I've missed, then you tell me.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The vari-cap diode has no bias voltage.
     
  20. marcbarker

    marcbarker New Member

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    negative bias...

    Why so many so 'sure' about this ? :)
    I still say there is reverse bias.

    Shall I go with the majority and 'say I was wrong' ? Or shall I prove it ?


    Nah!!! I'll leave it open...... :D
     
    Last edited: Jul 24, 2009
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you let the preamp blast a very high signal then the vari-cap diode will rectify the signal and develop some bias. The audio coupling capacitor C9 might be able to hold the bias for a moment until the next audio peak.
     

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