FM Receiver - single Transistor

Discussion in 'General Electronics Chat' started by Kkein, Jun 4, 2014.

  1. Kkein

    Thread Starter Member

    Jun 10, 2011
    46
    0
    Gents,
    Why isn't my single Transistor FM Radio work?
    My simulation with proteus shows signal at resonance. But I build the circuit and hook the output to a high impedance headphone, no radio reception whatsoever. However, the circuit works well as a transmitter when I connect an audio jack to the antenna.Attached is my circuit. Please note that my inductor coil is 4 turns but I have use 100uH inductor in this case for the simulation. Appreciate your help please in advance.
    Thanks.
     
    • FM.png
      FM.png
      File size:
      256.6 KB
      Views:
      70
    • FM 1.jpg
      FM 1.jpg
      File size:
      69.4 KB
      Views:
      44
    Last edited: Jun 4, 2014
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    A regenerative reciever is not one of the best designs.
    When the adjustment is not correct, it will either oscillate and function as a transmitter or it will be very insensitive.
    For some ideas, you might want to look at the following page:
    http://www.mikroe.com/old/books/rrbook/rrbook.htm

    Bertus
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,658
    632
    Try making C2 .01 uf instead of 100 pf with 10 uf to 22 uf in parallel, then you might be able to get the circuit to oscillate with much less than 1 uH for L1.

    As Bertus points out, the circuit is hard to adjust, so put a parallel combination of 10k and .005 uf in series with R2; this will allow quenching, which is the periodic stopping of the oscillator which makes the circuit much easier to adjust. You probably will find that you don't need the 30k trimmer.

    Change the BC547 to something faster like a 2N3904 or even a 2N2222; the Ft of a BC547 is only 100 MHz.

    Maybe that will get it to work.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    a regenerative reciever of this type cannot recieve fm. they tend to lock onto the carrier and cannot be used to slope detect fm.
     
  5. Kkein

    Thread Starter Member

    Jun 10, 2011
    46
    0
    Thanks friends for your support comments. I now know what the problem is. It's the BE capacitor and CE capacitor that were causing headache. I replaced the BE capacitor with 1nF and CE with 3p3 capacitor and wallah it works. I can tune it to any local channel now. The only caviet though is it drifts away from tuned channels frequently.

    Any better circuit?

    Thanks;.
     
Loading...