Re: KPA100 TR Switch isolation problem

Posted by Don Ehrlich-2 on Dec 06, 2004; 8:54pm
URL: http://elecraft.85.s1.nabble.com/KPA100-TR-Switch-isolation-problem-tp372363p372364.html

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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