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.
Results:
Next steps:
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.
Results:
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.