N9NEO wrote:
So the multipath distortion causes fading of the carrier only?? This
makes some sense to me. A small set of the lower sideband frequencies
would also cause phase cancellation, but since the audio spectrum is
moving around so fast no one notices. I think I'm on the right track

No, a simple 2-path is effectively a comb filter with the separation
between
frequencies of constructive and destructive interference determined by
the
difference in propagation delay. With enough delay, this separation can
be
much less than the bandwidth of the desired signal, causing multiple
cancellations within the passband. The delay difference is not a
constant
due to the roiling ionosphere so the frequencies at which destructive
interference occurs and their separations are constantly changing. The
severest form of distortion is when a cancellation occurs at the
carrier
frequency but if you have ever heard of 'flanging' in the recording
industry, you'll know what the interference effect can be when a
cancellation occurs in the sidebands. It is is very noticeable.

So use another carrier slaved to the received carrier and you
get better reception during fade. Even if it wanders a few cycles
during fade you probably don't hear anyway. I guess that is how a sync
detector works.

That's basically it.

Tom

Thanks Terry, I just signed up.

regards,
Bob


wrote:
David wrote:
On 30 Jul 2006 09:28:29 -0700, "N9NEO"
wrote:

http://home.att.net/~wa1sov/technical/sync_det.html

While this synchronous detector works quite well, there is a link
http://home.worldnet.att.net/~wa1sov/technical/allpass/allpass.html
to a filter that allows improved reception by "rejecting" signals other
then the desired signal. While this does improve reception, it isn't
quite as effective as some literature would suggest.

If you really interested in synch detectors, Tom Holden has an
excellent
page, http://groups.yahoo.com/group/Synch_AM/messages that has
some links to very good resources.

Terry

In article .com,
wrote:

wrote: snip
Except we are not talking about IF filters .The "fading" filter is
at the end of the chain, i.e. past the demod.

snip

Except this is at audio frequencies, where the component sizes are
much larger. Again, this is not at IF frequencies.

Mr. Lankford's main concept is that by using a narrow enough IF
filter, a narrow filter with with a step attenuation skirt, and by
offset tunning to only get the carrier and the desired sideband, and
with a following suitable AF LP fitler can do wonders. It is not
magic, and doesn't work with every receiver and under every
condition. In the ret of this, and all future posts, I will simply
call it "ELPAF".

I can say is that it is a usefull technique, and will even help when
used premium receiver like an AOR7030 or R390, the filter can really
reduce the effects of "fading". With a modest receiver like the R2000
that has

been upgraded with a suitablely narrow IF filter, the results are
impressive. With a "marginal" receiver like the ATS909/DX398 the
results are nothing short of amazing.

Any time you narrow the frequency range with a filter you lower the
noise floor possibly making the faded carrier large enough so the
detector does not greatly distort the audio.

--
Telamon
Ventura, California

wrote:
wrote:
snip
Except we are not talking about IF filters .The "fading" filter is at
the end of the chain, i.e. past the demod.
snip
Except this is at audio frequencies, where the component sizes are much
larger. Again, this is not at IF frequencies.

Mr. Lankford's main concept is that by using a narrow enough IF filter,
a narrow filter with with a step attenuation skirt, and by offset
tunning to
only get the carrier and the desired sideband, and with a following
suitable AF LP fitler can do wonders. It is not magic, and doesn't work

with every receiver and under every condition. In the ret of this, and
all
future posts, I will simply call it "ELPAF".

My point still holds in that nobody serious builds LCR filters for
auido (speaker crossovers exempted). His LCR filter IS in the audio
chain, not the IF.




I can say is that it is a usefull technique, and will even help when
used
premium receiver like an AOR7030 or R390, the filter can really reduce
the effects of "fading". With a modest receiver like the R2000 that has

been upgraded with a suitablely narrow IF filter, the results are
impressive.
With a "marginal" receiver like the ATS909/DX398 the results are
nothing
short of amazing.

Terry

wrote:
wrote:
snip

My point still holds in that nobody serious builds LCR filters for
auido (speaker crossovers exempted). His LCR filter IS in the audio
chain, not the IF.


Your comment that "nobody serious" is so off base as to be asiniine.
Dallas Lankford is clearly one of most serious DXers alive today.
Why not take a look at all the technical information at
http://www.kongsfjord.no/dl/dl.htm
before jumping to silly conclussions.

Terry

wrote:
wrote:
wrote:
snip

My point still holds in that nobody serious builds LCR filters for
auido (speaker crossovers exempted). His LCR filter IS in the audio
chain, not the IF.


Your comment that "nobody serious" is so off base as to be asiniine.
Dallas Lankford is clearly one of most serious DXers alive today.
Why not take a look at all the technical information at
http://www.kongsfjord.no/dl/dl.htm
before jumping to silly conclussions.

Terry

I've done (as in been paid for) filter design in telecom/datacom
applications, including elliptic filters, delay equalizers, etc. I know
of what I speak.

There is a leapfrog design on this page if you want to educate
yourself:
http://www.filter-solutions.com/active.html

wrote:


I've done (as in been paid for) filter design in telecom/datacom
applications, including elliptic filters, delay equalizers, etc. I know
of what I speak.

There is a leapfrog design on this page if you want to educate
yourself:
http://www.filter-solutions.com/active.html


I give up, you win.
Only idoits and fools would bother to build any passive
AF fitlers. Please build all the active ELPAF filters your
heart desires. I made the mistake of assuming you where
a serious SWL.

I bet you would find the thought of building a clipper to limit
the AF level to a set of earphones with something as simple
as a couple of parallel 1N4004 diodes and a series resistor
so repulsive as to nearly make yo puke.

I can hear it now, "Oh My God, how simple."

I bet you could whip up a active limiter that would achieve
+/-0.0001dB clipping accuracy.

Too bad you make Cuhulin seem reasonable.

I can't PLONK you with Google beta, but I damn sure will ignore
your posts.

Terry

Hey, is idoit something like an idiot?

Your reaction is bizzare to say the least, but plonk away.

wrote:
wrote:


I've done (as in been paid for) filter design in telecom/datacom
applications, including elliptic filters, delay equalizers, etc. I know
of what I speak.

There is a leapfrog design on this page if you want to educate
yourself:
http://www.filter-solutions.com/active.html


I give up, you win.
Only idoits and fools would bother to build any passive
AF fitlers. Please build all the active ELPAF filters your
heart desires. I made the mistake of assuming you where
a serious SWL.

I bet you would find the thought of building a clipper to limit
the AF level to a set of earphones with something as simple
as a couple of parallel 1N4004 diodes and a series resistor
so repulsive as to nearly make yo puke.

I can hear it now, "Oh My God, how simple."

I bet you could whip up a active limiter that would achieve
+/-0.0001dB clipping accuracy.

Too bad you make Cuhulin seem reasonable.

I can't PLONK you with Google beta, but I damn sure will ignore
your posts.

Terry