My first try at Fm transmitter

Discussion in 'Wireless & RF Design' started by be80be, Apr 25, 2018.

  1. be80be

    Thread Starter AAC Fanatic!

    Jul 5, 2008
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    Been playing with this a bit using my scope i can see the modulation.
    Got find a FM radio lol.
    Screenshot from 2018-04-25 02-16-02.png

    Here what it looks like
    IMG_20180425_011511.jpg IMG_20180425_011511.jpg
     
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  2. ericgibbs

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    Jan 29, 2010
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    hi burt,
    I like that unusual way of mounting components and at the same time having a 0V/Ground plane.:)

    Eric
     
  3. DickCappels

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    How well does it work? In particular, is it stable frequency-wise?

    That technique is an early form of Manhattan style circuit construction.
    Manhattan Style Circuit Construction
     
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  4. be80be

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    Well it looks to work I looked at it with my scope you can see the modulation
    But I think I have to find a better transistor I didn't have a 2SC2001.

    But with the scope I can see the modulation when I talk.

    I was thinking I have a SDR maybe i can figure where it at on the FM band with it I don't have a FM radio no more lol.
     
  5. Dodgydave

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    Jun 22, 2012
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    Have you got an FM radio on your mobile phone?
     
  6. be80be

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    Here a short video of what the scope showing on the output with me tapping on the mic i think i'm nowhere near the FM band tho.

     
  7. Ian Rogers

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    Dec 12, 2012
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    Hey Burt! Haven't I seen the circuit somewhere before!! Ah yes Back to the future ( on Doc's head LOL!! )

    Last one I built was AudioGuru's FM transmitter... ( Shown somewhere on another forum ).... It worked great.. I used it for the kids to talk through the radio!!
     
  8. bertus

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  9. Ian Rogers

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    Cheers for that! If its Audio's, you know its going to work!!
     
  10. DickCappels

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    The bandwidth of your scope is 25 MHz which might mean it is insensitive to signals around 100 MHz. What you see is the baseband microphone signal because the transistor is an emitter follower that passes the microphone signal through at near unity gain.

    It will be interesting to see what you get with your SDR tuner.
     
  11. danadak

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  12. be80be

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    I'm thinking this thing is more like 3 to 5 mhz LOL
     
  13. Audioguru

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    Dec 20, 2007
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    It will not work with a condenser mic that needs a 48V power supply. But it will work with an electret mic that has the needed 48V embedded in its electret material.
    It will have these problems that I fixed when I designed my FM transmitter:
    1) Its radio frequency changes when something moves towards or away from it or its antenna. Therefore I added a buffer transistor at its output to isolate its radio oscillator from capacitance changes at its antenna.
    2) Its audio level and radio frequency change as its battery runs down. Therefore I added a low dropout voltage regulator.
    3) Its audio level is faint because it needs a preamp that mine has.
    It sounds muffled because it is missing the pre-emphasis that mine has and all FM radio stations have.

    You will not be able to see FM modulation on a 'scope because the frequency change is extremely small. This transmitter transmits AM and FM at the same time. You will see loud amplitude changes (AM).
     
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