Please explain this circuit of a coil-less fm transmitter

Discussion in 'The Projects Forum' started by Asmita Nayak, May 17, 2015.

  1. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    It would be of great help if anyone could explain in detail how each component in the circuit works.[​IMG]
     
  2. ScottWang

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    Aug 23, 2012
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    Are you the same person with DEBPARNA MAJUMDER?
    This forum only allowed one person one ID, if you are the same person, please decide which ID that you would like to reserved ?
     
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  3. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    No. Debparna Majumder and I are classmates working on the same project.
     
  4. ScottWang

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    Aug 23, 2012
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    Ok, thank you.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I would NEVER build the given transmitter as it will transmit EVERY 10.7 MHz.

    Bertus
     
  6. ScottWang

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    Aug 23, 2012
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    10.7Mhz, that is a little strange, I was played 88~108Mhz in Fm and remote control with 27Mhz.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You will hear ALL harmonics created by the 10.7 Mhz filter used in the oscillator section.
    The 9th and 10th harmonic can be heard in the FM band at 96.3 and 107 Mhz.
    BUT the 8th harmonic will fall below the FM band 85.6 Mhz, wich could be in the aircraft band.

    Bertus
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Not all circuits you find on the internet are good ones. This one is illegal, a square wave should never be used to broadcast.

    The ceramic resonator is filling the role of inductor in this circuit.
     
  9. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    There is a frequency multiplier after the oscilltor. The transmitted frequency will be 96.3 Mhz.
     
  10. ScottWang

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    Aug 23, 2012
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    So you have to divided by 9 for the audio, does the audio still can keep the normal sound ?
     
  11. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    There's some noise but the audio can be heard.
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a freqiuency multiplyer does not devide the modulating frequency, it multiplys it. probably not enough since there is noise too. and using a varicap to fm modulate a 10.7 ceramic filter isnt going to fm the signal much.
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    Normally the FM transmitter modulate the frequency 88~108Mhz with audio sound that it assuming the frequency as 300Hz to 5Khz used a small microphone.

    If you multiple the transmitter's frequency 9 time from 10.7Mhz to 96.3Mhz, then the audio frequency will be also multiple 9 times from 2.7Khz to 45 Khz, so that's why I said that you need to divided by 9 to back to the original frequency of audio sound, is this used digital or linear to divide the frequency that I'm interested in this.
     
  14. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    Can anyone explain how the CMOS inverter N2 works as an oscillator in this circuit?
     
  15. DickCappels

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    Aug 21, 2008
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  16. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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    Its a project I'm working on.
     
  17. Asmita Nayak

    Thread Starter New Member

    May 17, 2015
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  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    There is no frequency multiplier. The 4069 gates merely act as buffers and current boosters for the 10.7 MHz square-wave. A square-wave output in theory contains an infinite number of odd harmonics of the fundamental frequency (the ninth harmonic being 96.3 MHz), so you will splatter RF interference across a wide band. See Bertus' post #7.
    Not much. Positive feedback from the output to the input of an amplifier turns it into an oscillator.
     
  19. JWHassler

    Member

    Sep 25, 2013
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    The difference is that "oscillators don't and amplifiers do."
     
  20. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Don't think so. This is FM, so the deviation and modulation bandwidth will increase by 9 times, but the rate of deviation will be the same. The carrier will transition from one instantaneous frequency to the next at the original audio rate.

    ak
     
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