"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

"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

Still looking for some pointers and/or advice on current AM
transmitter design.

One place where VHF AM is still alive is radio foxhunting (RDF).
The comments I've seen in the group so far are filament-based,
hardly usable for a portable transmitter to be hidden under the trees.

A lot of ex-PMR equipment has a closed control loop to reduce the
RF output to what the communication plan allows for (so PMR folk
can use the same kit for customers who need 0.3 watts or customers
who need 25 watts; all that's needed is a twist of the pot inside).

This circuit makes AM easy: just inject your audio in this control
circuit and with some luck you get reasonably-quality AM.
It won't be hi-fi but superreg receivers never notice;
similary there may be some FM but again superregs never notice
(and it's a nice way to be able to verify a transmitter w/o
the need to carry AM kit!)

Oh, since AM PEP power is 4 times the AM idle power, pls reduce
the idle output power to at most 25% of the max, or things will clip.

Ex-PMR equipment in many places is dime-a-dozen and this gives one
an easy way to build a transmitter for some RDF bootstrap activity,
or during a JOTA or thereabouts.

[I have one transmitter where I can fluently adjust the power from
50mW to 10W PEP, to adjust for the distance and the difficulty
of the RDF. This means that one can make a small hunt for the
young scouts and a larger hunt, with longer distances,
for the older folk, all with the same equipment.
Just a point for inspiration].

Hope this helps,

Geert Jan PE1HZG

"Geert Jan de Groot" wrote in message

One place where VHF AM is still alive is radio foxhunting (RDF).
The comments I've seen in the group so far are filament-based,
hardly usable for a portable transmitter to be hidden under the trees.

A lot of ex-PMR equipment has a closed control loop to reduce the
RF output to what the communication plan allows for (so PMR folk
can use the same kit for customers who need 0.3 watts or customers
who need 25 watts; all that's needed is a twist of the pot inside).

snip

Hope this helps,
Geert Jan PE1HZG

Thanks Geert - Yes, it does help because it's another strategy to
consider. Your input is appreciated!

Regards,
Bill