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Two FM circuits

Discussion in 'General Electronics Discussion' started by Integrator741, Nov 12, 2013.

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  1. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    Hello everybody,

    I am trying to do small home FM transmitter project. I found two circuits in the internet and I am struggling to find the difference between them (besides transistors).

    Circuit number 1

    [​IMG]

    Circuit number 2

    [​IMG]

    And also which one would be more efficient?

    Thank you for your help,
    Integrator741
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The circuits are similar.
    Circuit 1 has AF and RF power supply bypass, this should be be included in Cir2
    Circuit 2 has a capacitor in series with the aerial, this is good.

    The 2N3904 may be a better switch so the efficiency of Cir1 could be higher.
     
  3. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    So how would you calculate the efficiency of the circuits?? Is there any specific formula to do it?
     
  4. davenn

    davenn Moderator

    13,765
    1,920
    Sep 5, 2009
    not really, you would have to look at each section in turn

    both these circuits are designed for low power output maybe ~ 5 - 10 mW
    for use over short distances , 50 - 100 metres max, depending on terrain, buildings, trees etc

    over that range you are probably unlikely to find much difference between them

    Experiment!! build them both and see how you go, do some actual real tests

    Dave
     
    Last edited: Nov 25, 2013
  5. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    thanks
     
    Last edited: Dec 3, 2013
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    Not surprised when simulating circuit 1 it failed to maintain oscillation. It kicks on but decays exponentially. It doesn't mean that this circuit is faulty. It just means that sinusoidal oscillators are always a royal PITA to sim.

    Chris
     
  7. philgritsr2

    philgritsr2

    20
    0
    Oct 21, 2013
    I built this one and the range was about 350 feet very easy to put together and because of voltage regulator fairly stable after about five minutes of use guess it had to warm up a bit then do drifting at all good luck

    http://www.techlib.com/files/fmxmit.pdf


    thanks Phil
     
  8. philgritsr2

    philgritsr2

    20
    0
    Oct 21, 2013
    also built this transmitter, these are the videos that helped me put it together. the man in video is very nice and even responded to emails I sent him he has lots of neat projects






    thanks Phil
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    "The Shadow"! That brings back great memories of listening to those broadcasts on my crystal radios in the 50's.

    Chris
     
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