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Workings of an FM receiver?

Discussion in 'Radio and Communications' started by Robin Mitchell, Jan 24, 2015.

  1. Robin Mitchell

    Robin Mitchell New Member

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    Hi all,

    I understand why AM is hated over FM so I have decided to start on the construction of an FM transmitter and reciever. I understand exactly how the FM transmitter works but I cant understand entirely how an FM reciever works. How is a vary frequency generated into a LF signal? Does passing the signal through a high pass filter block the FM wave when its frequency drops and reduce the filters reactance when the incomming FM waves frequency increases?

    All the best,
    MitchElectronics
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Google Ratio Detector and/or Discriminator
     
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  3. Robin Mitchell

    Robin Mitchell New Member

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    Hi MikeMl,

    I did just that and found some fantastic methods! Really amazed :D

    But none of them explain a typical FM reciever like this one:
    [​IMG]

    Unless this is using a simple slope detector?

    All the best,
    MitchElectronics
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    AfaIk, that is a super-regen AM receiver that might detect wide-band FM (poorly) if tuned off to the side of the station...
     
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  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Not an FM receiver at all, never mind a 'typical' one.
     
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  7. Robin Mitchell

    Robin Mitchell New Member

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    I find that type of slope detection fantastic! Personally it is quite amusing that an AM receiver can be used to pick up FM too
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A real FM receiver demodulates the frequency deviations of the transmitted signal and ignors any amplitude fluctuations.
    A super regen receiver slope detects the signal that changes the frequency deviation into amplitude fluctuations. Static, click and pop radio interference noises create amplitude fluctuations that the super regen radio plays perfectly!
    A real radio has many tuned circuits so that there is no interference from adjacent frequency stations. A super regen receiver has only one tuned circuit so it usually plays a few stations at the same time.


    A real FM receiver is very sensitive to pickup weak distant stations and is not overloaded by strong local stations. But my Sony Walkman portable FM radio has a local-distant switch. My cheap clock radio is not sensitive.
    A super regen receiver can be made fairly sensitive so it picks up medium strength stations but then it is overloaded by strong local stations, or it can be not sensitive so it is not overloaded by strong local stations but then does not pickup distant stations.

    the horrible super regen FM receiver you found has DC current in the headp[hones that can damage them and/or cause distortion.
     
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  9. Robin Mitchell

    Robin Mitchell New Member

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    Wow audioguru! Nice response!

    I see that if the amplitude changes with the non real FM reciever then the output would also increase. Would clipping work?

    All the best,
    Robin
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes, then the output produces all the snaps, crackles and pops (mainly from lightning) of amplitude interference like an AM radio does.
    You can reduce the level of the amplitude interference by clipping it down to the level of the FM modulation but it will still be as loud as the FM program you want. On a half-decent FM radio you will not even hear any amplitude interference.
     
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  11. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    FM demodulation has a SNR discriminator threshold when limited thus the signal level is used to control Muting when the SNR is too low or the tuner is on edge or between channels.
    THe basic workings today use Crystal ceramic filters for both IF at 10.7MHz and 455kHz
     

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