[Elecraft] KPA100 TR Switch isolation problem

[Don]

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
>