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simple fm receiver

Discussion in 'General Electronics Discussion' started by scientifico, Jun 5, 2014.

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  1. scientifico

    scientifico

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    Mar 13, 2012
    Hello, do you think this simple fm receiver is working and how does it detect fm signal ?
    It looks more like an am radio....

    [​IMG]

    Thanks
     
  2. duke37

    duke37

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    Jan 9, 2011
    I think this is a superregenerative circuit which oscillates locked to the input signal. I do not understand it!
    The sensitivity is high and was used when a valve (tube) could cost a weeks wages.
     
  3. scientifico

    scientifico

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    Mar 13, 2012
    So that's not a receiver at all?
     
  4. duke37

    duke37

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    Jan 9, 2011
    Oh yes it is! Look up Wikipedia superregenerative receiver.
    The circuit oscillates at two completely different frequencies and detects FM by slope detection.
    The output is a form of pulse width modulation. Like I said, I do not understand it.
    This must be one of the most complcated circuits made from the fewest components.
     
  5. scientifico

    scientifico

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    Mar 13, 2012
    Can I replace the BF199 with a BF255 ?
     
  6. duke37

    duke37

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    BF199 25V 25mA 500Mhz
    BF255 20V 30mA 260MHz

    So the BF255 is similar but somewhat slower and it may not run at high frequencies.
     
  7. scientifico

    scientifico

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    Mar 13, 2012
    I have built that circuit with wires of copper to connect the components using BF255 and DL2032 as power supply, but I can't listen to anything when I connect the headphone output to computer speakers, what could be wrong ?
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    What did you use for C and L?
     
  9. duke37

    duke37

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    Jan 9, 2011
    My eyes are dim and I cannot see if the diagram needs a 5V or 3V supply. I think it is 3V which matches your battery so should be OK.

    I do not have the BF255 connections to hand, look these up and check if you have them correct.

    Too short an antenna will give low sensitivity. Too long an antenna will damp the oscillations.

    What frequency does the L and C tune to? Are there signals in this area?
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  11. scientifico

    scientifico

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    Mar 13, 2012
    Last edited: Jun 8, 2014
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  13. scientifico

    scientifico

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    Mar 13, 2012
    It has to be a Siemens because the central pin is not the base from my test so it was connected correctly
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    In that case, I have nothing more to add. I have no idea how that circuit is supposed to work! I'm not saying that it won't, of course; simply that I personally don't know how it would.
     
  15. duke37

    duke37

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    As I said in #6, the BF255 is not as agile as the BF199 by a factor of 2 0r 3.

    You could try tuning the circuit to a lower frequency to see if it will work or get super fast transistors.

    The antenna loading will be critical. Circuits such as this which need to be set up optimally are not the best for newcomers. Have you read up on superregenerative receivers and understand them?
     
  16. ramussons

    ramussons

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    Jun 10, 2014
    Does this work at all? The DC conditions are such that both T1 and T2 will be saturated mode. Or am I missing something?

    Ramesh
     
  17. duke37

    duke37

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    Jan 9, 2011
    A two transistor bistable circuit looks to have each transistor in saturated mode but they always work. This circuit is similar but I think it should run with two oscillating frequencies. The first controlled by L and C and the second one controlling squegging using the output capacitor as the time setting.

    Thinking about it, the output load may be critical. Apparently simple circuits doing lots of things can be difficult to analyse and optimise. Better to use several stages each doing its own thing.

    Super regen receivers can work and are used in car fobs but will be made to a standard layout.
     
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