FM Transmitter:Crystal Problem

Discussion in 'Wireless & RF Design' started by Bill B, Jul 10, 2012.

  1. Bill B

    Thread Starter Active Member

    Nov 29, 2009
    Hey all, I found this transmitter circuit on the internet and decided to try building it. I can't get the crystal to start. The crystal in the schematic is a 3rd overtone crystal, I have a 16 MHz fundamental crystal in my circuit. My crystal has a 20pF load capacitance and a max ESR of 40Ω. The series combination of the 47pF and 33pF caps is about 15pF. I know this setup is a Colpitts ocsillator, but the research I have done has not led me to a solution for my problem. Any suggestions would be greatly appreciated. I plan to change the 47pF cap to 56pF to achieve a series combination capacitance of about 20pF to match the load capacitance. Other than that I'm at a loss.
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    First, be sure the crystal itself is good, The best way to do this is with a known good circuit. I have an ancient SENCORE frequency counter that has a crystal checker....I'm sure there are other similar instruments out there.

    Secondly...try a different type of oscillator.....the PIERCE oscillator is the most reliable for getting balky crystals to start up.

  3. Bill B

    Thread Starter Active Member

    Nov 29, 2009
    Thanks for the advice Eric. I have tried different crystals in the circuit and can't get them to start either. I even put them into a working oscillator circuit and got nothing. Back to the drawing board I guess. Electronics wouldn't be any fun if everything worked right off the bat anyway.:)
  4. bertus


    Apr 5, 2008
  5. DickCappels


    Aug 21, 2008
    Not to overlook the obvious, have you double-checked the basing of the transistor? In most circuits, the only work if you connect them correctly.

    Also consider using something with a higher cutoff frequency. The 2N3904, 2N2222, 2N918, 2N3363, 2N3866, 2N4401, 2N5108, 2N5179, 2SC1923, 2SC2570A, 2SC3355, 2SC2668, BF198, BF421, BFQ11, BFQ161, BFQ131, BFR93, KSP10, KTC3197, KTC3198, and MPSH34, would be much better choices.
    Willen likes this.
  6. radiohead

    Senior Member

    May 28, 2009
    Try putting a 33pF or a 47pF capacitor in parallel with the crystal.

    What is your construction technique? Is it on a breadboard, perf-board, or an air circuit on a clad board?

    breadboards can have a lot of stray capacitance that will affect how the circuit operates. Air circuits typically have components with long leads that can affect how the circuit will oscillate.