"RST Engineering" wrote in message:

The last bastion of VHF-AM is aircraft, both civilian and military. The
civilian aircraft com band goes from 118-137 MHz. and the military from
somewhere around 225 to 400. Those of us who have spent our lives in this
little corner of the design world are few and far between.

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

As to those who say that AM is so simple that anybody could do it, I
respectfully disagree. I even DISrespectfully disagree. Trying to
amplitude modulate a synthesized AM radio without having as much FM in the
output as AM is no mean feat. In the old vacuum tube days with crystals
for
the synthesizer it isn't all that difficult. However, with digital
synthesis and VCOs that will FM at the drop of a hat, it is far from
trivial. Shielding to the center of the earth and power supplies bypassed
from DC to daylight are the order of the day.

Jim


"Netgeek" wrote in message
...
Still looking for some pointers and/or advice on current AM
transmitter design. There doesn't seem to be much out there
apart from the "pirates" who are building SW transmitters
around 5W or so...(e.g. the "Grenade" QRP transmitters and
similar designs)...

Is AM ("Ancient Modulation") considered dead and therefore
there are no links/publications worth reading?

Bill

"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

On Mon, 16 May 2005 19:53:23 -0400, "Netgeek"
wrote:

"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-)

High power VHF/UHF AM transmitters are also used as audio subcarrier
transmitters in System-L television, which is used at least in France.
These require "full" audio bandwidth and good linearity for low audio
distortion in normal TV receivers. The audio exciters used in low
power relays might be in your power range, so try to find some info
about these transmitters, most likely only in French.

Paul