30m Oscillator (Night 4)

I found some bench time after the family shared a movie Saturday night and went to bed.

I did a bunch of testing and made some changes.

  • I tweaked the gain stages slightly. Q3 and Q4 seem to operate more efficiently with
    a 100 ohm resistor instead of the 10 ohm resistor. I validated both values with the
    spectrum analyzer and I got a bit more gain of the fundamental frequency and less
    impact on the harmonic frequencies.

  • In talking with N0FP earlier in the day I discovered that my output at 50 ohms was
    loading the final transistor. I played with using a matching transformer but was unhappy
    with the result. (I think that I was getting tired and should retest the process with
    the transformer again.)

  • I also tried changing the output from a common-emitter to a common-collector. The
    common-collector design has a lower output impedance and was a better match to the
    50 ohm load. It was a not a perfect match @ 4:1 but it was pretty decent and reduced
    the amount of loading. (In the end I left it as a common-emitter design for now.)

  • I then relocated the low pass filter from after Q2 to it's new home after Q4. I tried
    using a 4:1 transformer from the collector to the input of the filter but was running
    into some flaky issues. I experimented again with common-emitter vs. common-collector
    to try to get a better match. In the end the common-emitter design worked pretty well
    so I left it alone.


  • Power is now running about  3.2mW or ~ +5dBm. (I referenced the spectrum analyzer
    against my signal generator and then compared the final output of the transmitter.)

  • My harmonics are down 55dB against the fundamental frequency. (It would be legal to
    put on the air as is.)

Next steps:

  • Look at some more options to increase the power into the 100-200mW range.

  • Add a key (RCA or 1/8" jack) so that I can try to make a QSO with it.