"Netgeek" wrote in message
...
This is what I'm discovering. Not only are the people involved few and
far between but so is much of the documented background material
anyone new to the scene might rely on for help....8-( I hope some of
the old-timers will scribble some of this stuff down while they're still
around!

As for the "simplicity" theory - you've hit it spot on with your following
comments. For example, roll-your-own VCOs are only a few bucks
and pretty easy. Off-the-shelf Minicircuits types (e.g. POS-200) are
only a few bucks more. But it sure is interesting to note that
Minicircuits
has a new line of VCOs *specifically* for "aircraft communications"
with an asking price of nearly $50 (yipes!)... The difference - VERY
low phase noise.

Apparently there are quite a few variables in implementation of these
"simple" designs - and what you can hear out there on the band is all
over the map. The difference in perceived quality between the "good"
radios and the really "rat-s**t" radios is amazing - and that's listening
to
aircraft under identical conditions (for example over the same VOR
checkpoint). Sure leads me to believe that there isn't a set "formula" for
this stuff.......8-)

Bill


Bill,


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

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.

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.




73,
Bob
W4ATM