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From: "Peter Martinez" <peter.martinez@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Fri, 16 Jun 2006 06:41:01 -0000
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Subject: LF: Re: PAs
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>From G3PLX:

Uwe:

I am not an expert on LF power-amplifiers (I worked on 10kW HF 
power-amplifiers 40 years ago but not recently), but it looks to me that the 
voltage and current waveforms of your home-brew PA have some harmonics. I 
took the image of the homebrew waveform and did a mirror-image (flip 
vertically) then superimposed it on the original. This shows clearly that 
the voltage waveform has different rising and falling slopes, which means 
there is some even-order harmonic present. Maybe this is due to some 
unbalance between the two halves of a push-pull circuit. The current 
waveform is more confused but that is probably because the harmonic currents 
are higher. This is some effect of the low-pass filtering and the antenna 
tuning.

There is also some broadening of the scope trace at some parts of the 
waveform, which shows there is some signal there which is not 
harmonically-related. It could be HF/VHF parasitic oscillation, but it's not 
possible to tell from the scope trace for sure. It could be just some 
power-line-frequency effect. Only a spectrum analyser could show this 
properly, but you may get a clue if you can hook the scope ahead of the PA 
low-pass filter. If this broadening of the trace is worse here, it may mean 
that there is some HF parasitic oscillation.

73
Peter