Electronic – DIY colpitts oscillator not oscillating

accircuit-designoscillatortransmitter

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

Best Answer

  • 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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