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

Great info Terry! Keep it up and keep us informed.

Frank
K3YAZ

wrote:
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

How can any audio filter make up for severe distortion?

--
Brian Denley
http://home.comcast.net/~b.denley/index.html

wrote:
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

It is an interesting idea, but nobody builds LCR filters. Rather, you
use the LCR filter as a prototype, then build a leapfrog active filter
from signal flow graphs based on the physical LCR filter.

The problem with elliptic filters with sharp cutoffs is they ring. I
think that would be more annoying than the flutter it is trying to
remove.

Brian Denley wrote:
How can any audio filter make up for severe distortion?

--
Brian Denley
http://home.comcast.net/~b.denley/index.html
-----------------------------------
Please read the pdf at:
http://www.kongsfjord.no/dl/Audio/On%20The%20Causes%20And%20Cures%20Of%20Audio%20Dis tortion%20Of%20Received%20AM%20Signals%20Due%20To% 20Fading%20II.pdf
Lots of nifty formulae and even has FFT trasform screen captures to
show his reasoing.

Terry

On 26 Jul 2006 23:00:58 -0700, wrote:



It is an interesting idea, but nobody builds LCR filters. Rather, you
use the LCR filter as a prototype, then build a leapfrog active filter
from signal flow graphs based on the physical LCR filter.


Really? What kind of filters does the Drake R8 series use?

wrote in message
ups.com...

Brian Denley wrote:
How can any audio filter make up for severe distortion?

--
Brian Denley
http://home.comcast.net/~b.denley/index.html
-----------------------------------
Please read the pdf at:

http://www.kongsfjord.no/dl/Audio/On...ures%20Of%20Au
dio%20Distortion%20Of%20Received%20AM%20Signals%20 Due%20To%20Fading%20II.pdf

Lots of nifty formulae and even has FFT trasform screen captures to
show his reasoing.

Terry


Am I reading the nifty formulae wrong? It looks to me like he's deriving
the distortion of a diode detector from the modulation index only. My sense
of these things says that a 50% modulated signal at a tenth of a volt is
going to have much more distortion than a 50% modulated signal at 10 volts.

Frank Dresser

Frank Dresser wrote:

Am I reading the nifty formulae wrong? It looks to me like he's deriving
the distortion of a diode detector from the modulation index only. My sense
of these things says that a 50% modulated signal at a tenth of a volt is
going to have much more distortion than a 50% modulated signal at 10 volts.

Frank Dresser

Very few radios drive the detector with anything near 10V.
The R390 and R392 have the highest diode drive voltages I have
seen and I think they are less then about 3V.

Most modern, IE "solid state", receivers I have measured have less
1V. All that I have seen that use discrete diode detectors as oppossed
to ICs, have farily high AF gain stages.

I didn't post this as an attemp to claim that "Synchronous detectors"
are a hoax,
but to offer another viewpoint that is backed up by what appears to be
valid
engineering to me.

ASCII text is not my choice for this arcane topic because of the great
difficulty
in expressing meaningfull equations.

This is merely another tool to be used in trying to receceive fading
signals.
His filters work much better then I expected. I found that by forward
biasing
the detector in my R2000 I got a much cleaner, ie lower distortion,
signal.
This was difficult to manage over very modest temperature changes.
A full wave "improved AM detector" gave even better results.
http://www.amwindow.org/tech/htm/alowdisdet.htm
A synch detector in an outboard detector gave even better results.

But the simple improved AM detector with a 4000Hz LP filter is a pretty
close
match to the synch detector at 1/100 the effort.

The above link goes into the math, this link starts with simpler math
and may
help the none engineers enter the fray.
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page2.html
Another unusual but good detector can be seen at:
http://www.pan-tex.net/usr/r/receivers/elrpicamdetect.htm
Tom Holden's Synch detector group has a link to a very detailed math
examination of "detection". I lost the link to that paper so you will
have to ask
Tom or join his group.

And please note Mr. Lankford is not merely slapping a 4000Hz LP AF
filter
in the audio chain, he is offset tunning, with good narrow IF fitlers,
to eliminate
one sideband.

Terry

On 27 Jul 2006 17:09:16 -0700, "N9NEO"
wrote:

Terry, Frank, et al.

And David, what was that comment about the Drake receivers? Do you
know what type of filters they use?


''The R8 IF filters
hark back to the models 1, 2 and early R4,
is they are LC tuned circuits, not crystal
filters. Crystal filters have gotten more
expensive over the years (is the world
running out of quartz, too?), and apparently
LC filters are now more cost-effective.
Some receiver users claim that LC filters
provide better audio response for listening
to broadcast stations. From a performance
standpoint, I wouldn’t have known the R8
had I.C. filters if I hadn’t read about them
in the Owne r 's Manual.''

http://www.dproducts.be/drake_museum/qst-r8.pdf

N9NEO wrote:
Terry, Frank, et al.

Yes, an interesting article. Thanks for bringing it up to the front
burner again.

I don't understand his representation of the fading term as it has no
link to the actual fading frequency. I would think the distortion
would have to be created by the fade. The only frequencies discussed
are w_carrier, and w_audio.

Also his depiction of instantaneous fourier spectrum at the point of
deepest fade leaves me guessing. I would have liked to see him strobe
the fade in and out at a fixed frequency and see the spectrum output of
that. All in all a good article. I read it a few weeks ago briefly,
but haven't had the time to get into it. I'll for sure make the time
knowing there might be some meaningfull discussion here. Usually the
threads degenerate at warp speed.

My boss just bought a smokin nice spectrum analyzer that I might be
able to share with him.

And David, what was that comment about the Drake receivers? Do you
know what type of filters they use?


regards,
Bob
N9NEO


One big problem with recreating real world HF propagation is the random
nature of multipath. Lankford's sweeping RF notch recreates a simple
single fade, but can't
produce the multiple "comb filter" like effect I have noticed. I built
a simple HiFer
"beacon" that allows me to check a "real world" RF signal for just how
much
SN effected intelligibility. I found that many non obvious things had a
big effect.
One of the simplest source of nastiness is the post detection AF chain.
I had
doubted that minor things like the type of capacitor could "really make
any diference".
My whole goal was to improve my best radio as much as I could for as
little money
as possible. A synch detector is one improvement. But even though a
"good" synch
detector with a phasing fitler is supposed to reject non signal out of
band signals.
see :http://home.worldnet.att.net/~wa1sov/technical/sync_det.html
It does help, in some conditions.
I found the biggest jump in intelligibility for the least money came
from an "Improved
AM detector". See: http://www.amwindow.org/tech/htm/alowdisdet.htm
To qoute "There are no intermodulation terms to contend with. In fact,
functionaly,
this is identical to synchronous detection. The negative half wave
signal is derived
in a similar fashion."
For clear signals, with no or minimal fading this detector is cleaner
then any other
detector I have played with. With this detector, a decent 4KHz IF
filter, and using
offset tuning on AM with a 4KHZ AF LP fitler, the result is very close
in performance
a Synch detector. If you wish to follow my "saga", please look up the
various threads from last summer.


IF I were doing this project over I would likely go with the better
Kiwa fitler module and the improved AM detector. Along with an improved
AF chain. My wife's R2000 has a
4HKZ mechanical fiter in the AM-N/SSB slot, the improved AM detector
and a home
built 2W MOSFET audio chain. And she loves it.

Terry