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I am trying to make a FM transmitter. For the oscillator part I picked a colpitts oscillator as it seemed relatively simple and I could get the pretty high frequency needed for FM with it. I built the Oscillator on a breadboard but unfortunately nothing I tried got it working. I was thinking would it be possible that the inductance of the wires was effecting it? I simulated the circuit on "LT spice" and it worked. I also made a lower frequency version of it on my breadboard and it worked also, so I'm not too sure whats going on

This is an imagine of the circuit diagram that I drew up. Thank you

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  • \$\begingroup\$ Yes, you need to be really, really good about doing a very tight and clean wiring job after doing a careful layout of your parts to minimize lead length. Here is a youtube placed right at the point where you can see what something workable looks like. Here's what not to do. \$\endgroup\$
    – jonk
    Commented Aug 5, 2020 at 1:58
  • \$\begingroup\$ Can you put 0.5 pF between C-E? \$\endgroup\$
    – D.A.S.
    Commented Aug 5, 2020 at 2:29
  • \$\begingroup\$ You need power supply decoupling, not shown on that circuit. \$\endgroup\$
    – user16324
    Commented Aug 5, 2020 at 10:07
  • \$\begingroup\$ Hi, Brian would a decoupling capacitor do the job? \$\endgroup\$
    – MattODwyer
    Commented Aug 5, 2020 at 13:18

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  • Breadboard is likely to have higher parasitic capacitance between tracks than either of your filter capacitors (10 pF and 5 pF) - this means trouble
  • The 100 uH inductor is quite possibly self-resonating below the frequency you are aiming for and this means you won't get sustained oscillations.

For both of the above try lowering the inductor value (L1) by ten (or more) and increasing C2 and C6 by ten (or more).

  • L3 at 1 uH should be significantly higher than L1 for proper operation.

Choose inductors that have a self resonant frequency (SRF) above the desired oscillation frequency. If the data sheet for an inductor doesn't mention SRF, don't use that inductor.

One last point - the values you have in your schematic are for running at around 8.7 MHz - is this what you intended? I point this out because an FM receiver runs at 88 to 108 MHz.

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    \$\begingroup\$ Is it possible that L1 and L3 values are swapped on the schematic? That would also bring the frequency to about the Band II FM range. \$\endgroup\$
    – user16324
    Commented Aug 5, 2020 at 10:09
  • \$\begingroup\$ Hey Andy, thank you very much for your answer. About your last point, I did intend to have it running at 88-108Mhz, I must have made a mistake (I'm a new electronic hobbyist) what values would you suggest to make it run at 88-108Mhz? \$\endgroup\$
    – MattODwyer
    Commented Aug 5, 2020 at 13:03
  • \$\begingroup\$ L1 needs to be closer to 1 uH as mentioned by Brian - that would produce a resonant frequency of around 87.6 MHz. But try C6 a tad lower to centralize this more in the broadcast FM band. \$\endgroup\$
    – Andy aka
    Commented Aug 5, 2020 at 13:06
  • \$\begingroup\$ perfect, thank you Brian and Andy :) \$\endgroup\$
    – MattODwyer
    Commented Aug 5, 2020 at 13:29

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