Dear Tom, LF Group,
It should be pointed out that the series tank circuit in the Decca PA
amongst other things isolates the PA devices from changes in the
load impedance. It removes a lot of spikyness from the current
waveform in the PA. Another effect is to reduce switching losses,
and therefore heating of the MOSFETs. It definitely is an asset .
I tried simulating Alan's circuit using PSpice. It certainly evens out
impedance variations at high frequencies, however, when
combined with a resonant load at 136k (ie. the LF antenna), rather
sharp parallel resonances lead to rapid swings of impedance
around the operating frequency. I have not investigated exactly
what effect this would have, but I guess it would make the tuning
rather unpredictable. It is a bit more civilised if the cross-over
frequency is increased by a factor of 1.5 to 240kHz (ie, divide all L
and C by 1.5).
I have no access to the university e-mail system over the Easter
break, but hope to be QRV for some of the time at least. Hope
everybody enjoys the holiday :-)
Cheers, Jim Moritz
73 de M0BMU
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