FM receivers. FM demodulation

Discussion in 'Wireless & RF Design' started by PaulEngineer, Oct 15, 2017.

  1. PaulEngineer

    Thread Starter Member

    Dec 21, 2016
    159
    7
    Hey.

    On these days, i would like to make an FM receiver that works on 27MHz. I had already made a FM transmitter on 27MHz. But the only things i dont understand is the following:
    1) How the FM receivers looks like? I know the AM receiver.
    2) How the FM demodulator is looks like? (I know it have more complex than AM). Where the demodulation happens in FM receiver?

    P.S And the last question. I will give an example: Say we have a transmitter, that the LC tank, resonates at 100MHz. For our example we will take a 33pF and 100nH air core coil. The transmitter as we said transmits at 100 MHz. Should the receiver, have the same LC tank, so as to get 100MHz modulated signal? Or it doesnt work that way? Im a bit confused on how the FM radio works.

    Any explanation is highly appresiated :)
     
  2. SLK001

    Senior Member

    Nov 29, 2011
    1,267
    484
    Some FM demodulators types for you to look up their operations: Foster-Seeley, ratio detector and phase locked loop. A PLL is almost universally used today and is probably the easiest to implement.
     
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,760
    1,293
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,512
    1,817
    The receiver tuned circuit should be resonant at the same frequency as the transmitter so the 33pF/100nH could equally be 66pF/50nH or any other combination of values which give the same frequency.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    5,397
    1,677
    You can use an AM detector tuned off to one side of the FM center frequency. It may not be "Hi-Fi" but it is simple and it works.

    This method is referred to as slope detection
     
    xox likes this.
  6. PaulEngineer

    Thread Starter Member

    Dec 21, 2016
    159
    7
    and soething else. I found somewhere that PLL receivers is more stable receivers. So the question is: should both transmitter and receiver have the PLL IC? For example the LM565 or NE565 is PLL ICs.
     
  7. DickCappels

    Moderator

    Aug 21, 2008
    5,397
    1,677
    If the receiver uses a PLL to demodulate the FM, it would likely be at the intermediate frequency (IF). If your signal is narrow band, a few kHz or so you could get by with an IF of less than 500 kHz which is the maximum frequency for the LM565's oscillator. There are PLL chips that can deal with much higher frequencies if you need to have a higher IF frequency so you can get wider signal bandwidth.

    You can use a PLL in the transmitter, but parts of the PLL have to be able to work at the transmitting frequency unless you want to have an IF in your transmitter, but unless there is an overriding need, an IF in the transmitter should be avoided because it adds complexity and more spurious signals to worry about. Here is an example of an FM transmitter using a PLL on this site:
    https://forum.allaboutcircuits.com/threads/project-pll-fm-transmitter.122209/
     
  8. PaulEngineer

    Thread Starter Member

    Dec 21, 2016
    159
    7
    No, i just need a frequency of 27MHz FM. I just need to make something that looks like FM broadcasting but on 27MHz. The following scheme is what i should to do:

    transmitter
    voice (information) ----> amplification -------> modulation -------> end amplification --------> aerial (ANT)

    receiver
    aerial (ANT) --------> receive signal --------> amplification ----------> demodulation (Filter, or whatever, maybe PLL too) -------> audio amplification --------> information out (speaker).

    I dont need something else yet. I will look any other time, if i will need somthing more complex with PLL :D. But this time i need something simple, like 27MHz radio broadcasting (I know its 87.6-108MHz, but anyways). I want to have stable frequency, so that why i need it for :)

    Thanks for all you did for me. Its highly appreciated sir.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
    5,397
    1,677
    For a simple and stable crystal controlled frequency modulator, scroll down to figure 4 on the web page below. It should be easy to adapt the circuit to 27 MHz.

    http://www.qsl.net/kd2bd/exciter.html

    Make sure that you design your transmitter and antenna such that they do not violate Greek Law concerning unlicensed radio transmitters.
     
  10. PaulEngineer

    Thread Starter Member

    Dec 21, 2016
    159
    7
    ok, thanks sir
     
  11. Motanache

    Member

    Mar 2, 2015
    474
    53

    Yes.
    If you have 100MHz transmission, then at the reception you need LC on 100Mz and an initial amplification of this signal.

    You need a local oscillator. An oscillator like the one in the transmitter at near 100MHz.

    Then these two signals multiply (*).
    In fact, they simply gather (+) at one point. Then the signal is amplified by a transistor that offers non-linear amplification.

    E.g. signal x and signal y
    Add x+y
    Non-linear amp: (x+y)^2=x^2+y^2+2xy(multiplication)
    Then the low frequency (audio) is separated by a LC filter.

    By combining two signals, you get the sum and the difference of those both.
    E.g. 100MHz FM modulated
    At some point of time it can be 100.1MHz or 99.9MHz

    100.1MHz(received and amplified) * 100MHz(from the local oscillator)=
    =200.1MHz + 0.1MHz => low pass filter = 0.1MHz (demodulated signal)

    My suggestion is to ruin cheap radio equipment when work with it.
    You need at least one frequency meter.
    if the radio is very cheap and displays the frequency, it is possible to have a separate frequency meter. You can dismantle it and use it separately.

    If it have digital search of the radio stations is not good for this.
     
  12. Motanache

    Member

    Mar 2, 2015
    474
    53
    The cheapest FM reception is TDA7000 or TDA7088.
    TDA7088 you can find in radio like this at 1-2$:
    [​IMG]
    [​IMG]
    http://electronics-diy.com/fm-radio-receiver-using-tda7088.php
    I have modified such a radio for 27MHz FM.

    C3 and C4 and 70nH antenna filter and initial amplification.
    C16 and 70nH and varicap = LC from local oscillator.

    Very low IF for use of RC filters.

    C1 is charging through resistor slowly. C1=voltage on varicap= function of capacitance = frequency LC local oscillator =frequency received

    Reset discharge C1.
     
  13. Motanache

    Member

    Mar 2, 2015
    474
    53
    You can try to do it only with transistors. Start from an oscillator like this from an transmitter. But it will not be easy to achieve. That's why I suggested integrated circuits.
    you can see my signature to see that I worked with PLL from mobile phone.
    digamyhobbyen.weebly.com
    There's a lot of programming not too didactic for FM receiver.

    You can find plenty of simple transistor FM receivers on the Internet.
    If you want we talk about this schematic you will find it.
    the receiver is much harder to do than the transmitter
     
    Last edited: Oct 20, 2017
  14. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,760
    1,293
    PaulEngineer ......here is something you can breadboard and play around with. You will have to play with the inductor and cap values........to tune in your 27MHz transmitter. This is very simple and can be improved on....but it will get you started.

    http://www.electroschematics.com/5150/tiny-fm-radio/

    And here is a ~27MHz FM transmitter......These coil and cap values are needed in that receiver.

    http://www.buildcircuit.com/simple-steps-for-making-fm-transmitter/

    The problem with 27mhz on low power work is.........the size of the antenna.
     
  15. Motanache

    Member

    Mar 2, 2015
    474
    53
    [​IMG]

    Would you please explain the role of T1 and T2?
     
  16. Motanache

    Member

    Mar 2, 2015
    474
    53
    C2 gets the signal from T1's collector.
    This signal also reaches T2's base.
    T1=ON => T2=OFF
    T1=OFF => T2=ON It seems to be an oscillator on the LC frequency.
    The signal in the antenna is gathered with the LC signal.

    From here I do not understand how it works.................................

    LM368 can only amplify low frequencies, but has no reaction.
     
  17. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,512
    1,817
    A super-regenerative receiver?
     
    Motanache likes this.
  18. DickCappels

    Moderator

    Aug 21, 2008
    5,397
    1,677
    The problem with those (stolen) schematics from some websites is that they come without explanation.

    I mocked up the circuit of post #15 in LTSpice and to my surprise it oscillates! I cast my vote with AlbertHall that it is a regenerative receiver.
     
    Motanache likes this.
  19. Motanache

    Member

    Mar 2, 2015
    474
    53
    The regenerative receiver does not say anything about demodulation.
    I did not understand how it is doing demodulation FM.
    For me, this receiver could receive as well AM.

    It's clear the LM386 is a low pass filter.
    You can in the antenna to inject a few uV simulation.
    Or you can post the LTSpice file, please.
    I think it is possible in LTSpice to perform a frequency analysis and change the frequency of the very weak signal in the antenna.
     
  20. DickCappels

    Moderator

    Aug 21, 2008
    5,397
    1,677
    Motanache likes this.
Loading...