Thanks Andy. Assuming by driver stages you mean what is external to
the PA, the exciter. This is one reason I tried driving it with the
function generator (HP 3325B) instead of the usual exciter (QRP Labs
Ultimate 3S). There was no change at all in the undesired behavior.
How likely can it be that two very different drivers would be
affected to exactly the same degree?
Of course it can be feedback from output to input within the PA
itself. That's what I was suspecting, and that somehow this feedback
is triggered by a load that is highly reactive off frequency. But
it's just a wild guess, I really don't know what is happening.
Perhaps I can manage to screen the input circuit.
Paul N1BUG
On 12/29/2017 07:06 AM, Andy Talbot wrote:
Sounds more like feedback. RF feeding back into the driver
stages. The LF component you see is, perhaps, a beat between
generated RF and oscillatory RF due to feedback. Or feedback itself
oscillating at LF. Or something like that
Vaguely recall something similar happening here in the dim and
distant past. Perhaps a couple of times even In fact, I think
it explains why my 475kHz 400W Class-E PA which is built open-style
has an aluminium screen above the input circuitry (Of course, I may
have added that due to an earlier poor-screening experience - it's
all too long ago to remember).
'jnt
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