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From: N1BUG <paul@n1bug.com>
Message-ID: <d97d3a8b-ff32-9be3-e7f7-bad083344d5b@n1bug.com>
Date: Fri, 29 Dec 2017 08:35:59 -0500
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Subject: Re: R: Re: LF: TXing 2200m WSPR
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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