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Fm receiver circuit .

Discussion in 'General Electronics Discussion' started by Yap, Mar 22, 2017.

  1. Yap

    Yap

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    0
    Feb 13, 2017
    Why the capacitor C3 be connected in parallel to the transistor ? Can anyone explain the function of it ?
    upload_2017-3-22_12-41-21.png
     

    Attached Files:

  2. Yap

    Yap

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    Feb 13, 2017
    Can anyone explain to me where does the oscillation take place ?
     
  3. duke37

    duke37

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    Jan 9, 2011
    The oscillator tank is L1, C2, C5
    Energy is taken out of the circuit via C3 and is amplified and fed in by T1 collector.

    Is this a superregenerative receiver?
     
    Yap likes this.
  4. Yap

    Yap

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    0
    Feb 13, 2017
    yaya it is a superregenerative receiver. How does this circuit select the desired frequency? via the tunable capacitor? then after the signal getting amplified, it will flow through the L2 and C4 and go the amplifier stage ?
     
  5. Yap

    Yap

    14
    0
    Feb 13, 2017
    yaya it is a superregenerative receiver. How does this circuit select the desired frequency? via the tunable capacitor? then after the signal getting amplified, it will flow through the L2 and C4 and go the amplifier stage ?
     
  6. davenn

    davenn Moderator

    13,168
    1,713
    Sep 5, 2009
    yes
     
  7. Audioguru

    Audioguru

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    Sep 24, 2016
    An amplifier oscillates when it has positive feedback. Transistor T1 is an RF amplifier and C3 applies positive feedback to it.

    A simple regen receiver is overloaded by strong local stations and since it has poor selectivity then it will probably pickup a few strong stations at the same time. A strong station will be over top a weak station.

    There is no way a BC559 transistor can drive a speaker. if your schematic showed a power supply voltage then we can calculate how hot the BC559 and the speaker get. Both might instantly burn out.

    EDIT: A superregen is really an AM radio that picks up all the Amplitude caused noise heard on an AM radio. It can pickup an FM station if it is tuned to a side of the signal then it "slope-detects" the amplitude changes as the signal frequency swings towards and away from resonance of the tuned LC. It is a horrible "radio".
     
    Last edited: Mar 23, 2017
    bushtech likes this.
  8. duke37

    duke37

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    Jan 9, 2011
    A superregenerative receiver is quite complex. It has an oscillator which has so much feedback that it runs out of puff and stops, then when the bias subsides, it has another go. The rate of build up of oscillation depends on the input from the aerial.
    The bursts of oscillation must be well above the top audio frequency to be received. The gain can be above a million.

    The oscillation will be transmitted and will cause interference to others using the same frequency band.

    "Electronic Designers Handbook". Landee, Davis and Albrecht 1957
    "Radio Designers Handbook" F. Langford-Smith 1934 - 1963
     
  9. Teh

    Teh

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    Mar 25, 2017
    Hello everybody, I am working with Yap on this circuit. Can anybody explain how this circuit rectifies the slope detected AM signal? And am I right to say that slope detection is achieved through tuned circuits L1, C2 and L2, C4?
     
    Last edited: Mar 25, 2017
  10. Teh

    Teh

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    Mar 25, 2017
    We used a 9V battery for this circuit. We can hear an output at the headphones when injecting a signal and so far the components haven't burned out.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,158
    2,676
    Jan 21, 2010
    You really want to avoid this.

    Finding good explanations of how these receivers work is not trivial. They have complex modes of operation and you often find either extremely complex descriptions by people who *might* understand, overly simplistic descriptions by people who might *think* they understand, but very few that leave you feeling you really know more than you did before you started.

    I think this page does a reasonably good job. I don't claim to have a great understanding, but this page at least tries to hammer things through my skull.

    Oh, how are you with valves?
     
    Arouse1973 and Teh like this.
  12. Teh

    Teh

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    Mar 25, 2017
    I'm sorry, i'm really not that familiar with valves. Thanks for your reply.
     
  13. duke37

    duke37

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    Jan 9, 2011
    I am in (*steve*)'s second category and know enough to realise that I am ignorant. So take what I say with a pinch of salt.

    L2, R2 are there to isolate the emitter from the rest of the circuit at RF. They should not be resonant?

    It does not slope detect an AM signal. The AM signal is detected from the change in pulse rate due to the amplitude of the input signal.
    FM is detected as AM from the slope.
     
    Arouse1973 likes this.
  14. Teh

    Teh

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    Mar 25, 2017
    Is T1 what configuration is T1 connected, common base or common emitter? Does the signal go down or up through T1, referring to the circuit? If T1 is off, does that mean oscillation stops? Does this circuit have an envelope detector?
     
    Last edited: Mar 25, 2017
  15. Audioguru

    Audioguru

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    Sep 24, 2016
    The schematic is missing a solder dot at the base of T1. It should be obvious that T1 operates as a common base amplifier since C1 grounds its base. T1 is a variation of a Colpits oscillator.

    Signals do not go up and down. T1 is never turned off, it oscillates all the time. As the FM signal frequency increases and decreases then the tuned LC in the oscillator becomes closer to its frequency and becomes farther from its frequency which causes T1 to conduct more and conduct less similar to an envelope detector. It is called "slope detection" because this awful radio is tuned to one side of the frequency of an FM radio station.
     
  16. duke37

    duke37

    5,191
    704
    Jan 9, 2011
    Modern schematics do not need a dot at a T junction for connection but do at a cross

    It is both common base and common emitter, depending on the frequency being processed.
    A difference between a regenerative detector and a superregenerator detector is that the oscillator works intermittently in the superregenerator.The regenerative detector oscillates close to the received frequency so the morse tone is the difference between the two. The superregenerative detector oscillates on the received frequency and also oscillates above the audio frequency.
    T1 runs as a common base oscillator with suficient feedback to promote 'squegging'. C1 is biassed negatively due to the base current and oscillation stops, After a little while, current through R1 charges C1 sufficiently to start oscillation. The time to restart the oscillation depends on the input from the aerial. This is common base.
    T2 runs in common emitter.

    The average current through the transistor forms the output with T1 running in common emitter. C5 voltage is the output. I do not think that this complex circuit could be called an envelope detector, it all depends what you mean by envelope.

    Here is a case where bypass capacitors are critical. Simpler circuits have bypass capacitors which as long as they are big enough, they will do.
     
  17. GhostLoveScore

    GhostLoveScore

    71
    5
    Nov 27, 2016
    I also want to recommend you to avoid superregenerative radios. I tried to make one work, it took many attempts. And even after that the frequency was not what I expected to be, you couldn't really tune it - you could change the frequency, but not by much. And you need to get it to work just below the oscillation threshold (if I remember correctly). I wouldn't make one again.
     
    davenn likes this.
  18. Teh

    Teh

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    Mar 25, 2017
    Can anybody explain the function of R5 and C6?
     
  19. Audioguru

    Audioguru

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    Sep 24, 2016
    R5 adds DC negative feedback to stabilize the operation of T2 since its DC gain could be high or low.
    C6 bypasses R5 so that T2 has a very high AC gain and lots or distortion.
     
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