"Bob Lombardi" wrote in message

You won't find much of this information without buying the tech manuals on
these aviation band radios and reading about them.

I was hoping to avoid the "overt plagiarism" approach - but I guess I
could reserve the option of "covert plagiarism"??? 8-)

You'll find that the better sounding VHF-AM transceivers do not use simple
open loop modulation as has been described ("just hook up a modulation
transformer"). That's 1970s technology. We use control loops to
linearize
the transmitters and reduce distortion. That's one part of the "how do
you
AM modulate a synthesizer without FM'ing even more" question - the other
part is good layout/bypassing practices as someone pointed out.

I suppose I've been pursuing the path of open loop because that's what
most of the literature describes - mostly for reasons of efficiency I
suppose (as in the case of continuous broadcast/commercial AM) but -
now that you mention it - much of that doesn't apply here. The duty
cycle for transmissions is *really* low and short duration, and the output
levels are fairly modest (a few watts or so) - so the difference between
using some high-level modulation (e.g. transformer) on a Class C
final or doing series modulation through linear stages isn't really all
that significant. Guess I'll go re-think it... It makes sense to, as you
suggest, close the loop and govern the modulation where the "rubber
hits the road" (or airwaves).

The high-level mod scheme counts on doing a good job of
compression and limiting at the source - and then setting the final PA
to fall within a certain range. I assume that the closed-loop scheme
you suggest involves some "tastefully designed" VCAs somewhere
in that loop???

Thanks for the input! Lots to think about........

We modulate at low level (pre-driver - milliwatt levels) with either
simple
mixers or complex I/Q modulators. The feedback is either envelope ALC,
polar or Cartesian.

"Simple stuff", right??????????? 8-) Just kidding -

Thanks again,

Bill