Bob,
Bingo! You nailed it I'm sure. Your explanation accounts for other things
I saw when originally troubleshooting this circuit. I arrived at the 12 pF
'fix' by accident when noticing that a scope probe made the problem go away
but I knew it was only a patch .. and did not address the root problem. It
explains why the dc voltage at Q6/R11 rose to abnormal levels at the low end
of 80M before breaking into a low frequency oscillation (the source of a
buzzing sound on the outgoing signal). Q6 contains a diode (probably
internal ESD protection) that was grounding the negative half of the rf
making it through the resonant circuit and resulting in a rectified dc
voltage at Q6 which I measured as high as 170 volts (normally around 70
volts during transmit).
Thanks a bunch. Now I know what to do to fix it right.
Don K7FJ
I modeled the switch and have identified the source of the problem. There
is a resonance formed between the junction capacity of D12, D14 and the
100uH inductor RFC3. When you add the 12pF cap or a scope probe, all you
do is move it around a little. The easiest way to kill this problem
forever is to replace RFC3 with a 1K resistor. That will introduce an
additional receive path loss of 0.216 dB which is negligable. Try it and
let me know what you see.
Bob
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