http://www.kongsfjord.no/dl/dl.htm
Dallas Lankford has done some serious research on the cause and cure
of/for
the distortion caussed by ionospheric "hops".
All of his PDFs are informative, but his "Elliptic Low Pass Audio
Filters" series
are must reads. His conclussions in his "Elliptic Low Pass Audio
Filters (Amplified) -
Simplified and improved", 29-MAY-2006, are downright fascinating. I
have been
playing with his design and a couple of steep cut off 3600Hz filters. I
don't
have the tools to duplicate his research. I knew from the "get go" that
my
remote/weak signal source with which I use to test detector, and to a
lesser
degree, antenna/feedline combination suffered from the major weakness
that
there was no multipath effects. All of my experiments were local ground
wave
and I couldn't, and still can't, "messure" the effects of such fading.

I would love to have a Drake R8B, but I was forced to deal with the
receivers I do
have. I used R390, R392, R2000(modified),
R2000(stock),(borrowed)AOR7030,
and a DX398 for some simple tests. I am just out of the ground wave
for several
MW stations and around dawn and dusk I get serious and nasty fading. So
for
the last few weeks I have been comparing stock, ie non-synchronous,
detectors
with synchronous detectors, and the addition of a brick wall 3.5KHz low
pass fitler.

As Mr. Lankford concludes a synch detector is only (and that might
ought to be
"may") be slightly better then a AF LP good filter. It is rather
frustrating to have
spent the last 18 months building an outboard synchronous detector to
find that
a simple LP filter offered so much improvement. Don't get me wrong, a
synch
detector is a usefull addition, but not the end all I had hoped.

I will disagree with his use of a simple bipolar 2W AF amp. The one
thing I have
descerned is that after AF detection, any additional distortion rapidly
degrades
intelligibility. I found MOSFETs, and vacuum tubes, amps allowed me to
understand
signals better then 6dB down from a "good" bipolar AF amp.

"My" third R2000 was siezed by my wife. I had added a MOSFET amp,
redesigned
the treble cut to a tone-tilt control and just completed adding
switchable 3KHz/4KHz
Filters as designed by Mr. Lankford. My wife has been doing some casual
SWL for the
last few days and agrees the filters are very good for nasty band
conditions.

Email is abandoned and dead.
Terry