Interesting article on fading distortion
 

In article ,
"Frank Dresser" wrote:

"Telamon" wrote in
message

.com...

[snip]


Some detector designs would use a DC bias on the diode to put it on
the edge of its liner region to improve its small signal
sensitivity. The optimum bias voltage will depend on the diode
characteristics.


There's a linear region in the usual model of a semiconductor diode
(a fixed voltage drop with a series resistance), but that model is
only an approximation. The other model, the square law model, is
also just an approximation, although it's supposed to be close enough
over small parts of the curve.

However, the diode doesn't have to be linear in order to have a
fairly linear diode detector circuit. Imagine we have a diode whose
forward resistance drops in a square law with the voltage. At 0.1V
the forward resistance is 1 meg. At 0.2V the forward resistance is
1K. At .0.3V the forward resistance is 32 ohms. At 0.4V the
resistance is 5.6V, and so on.

Now, let's put this nonlinear diode in series with a linear load
resistance and decide that the circuit is pretty much linear once the
diode resistance drops to 10% of the load resistance. Well, it's
obvious that diode detector circuits which work into higher
resistance loads will linearize themselves at lower voltages than
diode detectors which work into lower resistance loads.

Below a certain voltage, the diode's non linear characteristics will
dominate the detector. Low voltage signals will have much more of
their waveform in this funky reigion than high voltage signals, even
at the same modulation index.

So, as I see it, there's alot more to know about a diode detector's
audio distortion than only the modulation index. There's the actual
characteristics of the diode, the resistance of the load and the
signal voltage the detector is operating at.

There's also the RF filtering, which will tend to "sawtooth" the
audio a bit, much as the rectifier and capacitor do in a power
supply. There's also some resistances/capacitances in the AVC line.

But I could be wrong. If so, let me know!

I don't anything wrong with what you wrote but you seem to think that
the diode used makes no difference because you can make it up its
deficiencies with an amplifier whose input impedance and gain adjusts
for it. Basically that is true that you can use a less efficient diode
but you will have to provide higher signal levels to it and weak
signals will still be distorted due to compression. I suppose you could
use a logarithmic type amplifier following the detector in order to make
up for the compression.

If you look at the diode curves germanium has one of the better forward
current to input voltage ratios of several diode types. Not being a
radio designer my approach would be to use a diodes fairly liner region
with a better forward current to input voltage ratio where the least
distortion and compression would be due to it and therefor the least
needed correction to be made up for by a amplifier with a fixed
correction. Another reason to use a more efficient diode besides the
signal level power needed is the power the diode itself burns when you
bias the diodes with larger forward voltage junctions.

--
Telamon
Ventura, California

Interesting article on fading distortion
 

"Telamon" wrote in message
...

I don't anything wrong with what you wrote but you seem to think that
the diode used makes no difference because you can make it up its
deficiencies with an amplifier whose input impedance and gain adjusts
for it.

I don't think we disagree on anything important, but I wanted to say that,
after a point, it won't make any practical difference to the distortion of
the detector if a diode has a linear region or a very non linear square law
region. The resistance of the load soon dominates the characteristiscs of
the circuit.

The rest of my reply was mostly aimed at the original article's contention
that a diodes distortion level can be derived from only from a diode's
presumed square law characteristics and the modulation index.

Basically that is true that you can use a less efficient diode
but you will have to provide higher signal levels to it and weak
signals will still be distorted due to compression. I suppose you could
use a logarithmic type amplifier following the detector in order to make
up for the compression.

I suppose, but I don't see any need. The distortion of the diode detector
can be quite low if it's driven at a proper level to minimize the the amount
of the waveform in the non linear region of the detector.


If you look at the diode curves germanium has one of the better forward
current to input voltage ratios of several diode types.

Right. A germanium diode would generally give less distortion and better
sensitivity than a silicon diode. More than that, there used to be a bunch
of specialized germanium diodes intended for radio audio detection, video
detection and such. It seems now it's 1N34A types.


Not being a
radio designer my approach would be to use a diodes fairly liner region
with a better forward current to input voltage ratio where the least
distortion and compression would be due to it and therefor the least
needed correction to be made up for by a amplifier with a fixed
correction. Another reason to use a more efficient diode besides the
signal level power needed is the power the diode itself burns when you
bias the diodes with larger forward voltage junctions.


Efficiency is a bigger consideration with crystal sets.

Frank Dresser

Interesting article on fading distortion
 

Interesting article on fading distortion
 

On 30 Jul 2006 09:06:34 -0700, 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.

Terry

I used a similar technique with my R-390A. I'd tune one side of the
carrier and use the BFO to provide a local one. You have to back way
off on the RF gain as the 390A BFO is sort of weak.

Interesting article on fading distortion
 

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.

Almost sounds like the 'syncho-phase' detector on the R7/A. Use of a narrower
filter and tilting the passband control to either USB/LSB.

dxAce
Michigan
USA

multipath distortion
 

Sorry for being such a RX newbie here. I guess I shoulda stayed awake
more during signals class. My expertise is very large switching and
resonant power supplies and transmitters.

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
here. 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.

I Imagine it would be a chore to build a sync detector from the ground
up, but I would also think it must have already been put into an ic
chip, no???

I like the link R2000swl posted to AmWindow for the precision full-wave
rectifier. I think I'll stick it on a pc board and give it a try. If
anybody wants a board let me know. They are very inexpensive.

bm, or anyone else, if you have good link to ELPAF or alternative RX
circuits then maybe I could throw that down on same board if not too
much room.

Details of board size at www.expresspcb.com I do the small one double
side and no silkscreen 3 boiards 60bucks.

You can contact me off board at

73
Bob
N9NEO





bm wrote:
Well, it works.

I have been playing with the ELPAF since last autumn; first on my
R-390A, which, despite having done the AF Deck mod, does have its
quirks with regard to audio quality. The ELPAF cleaned up audio
admirably. Mostly doing MW DX then. Then, this summer together with a
modified IC-703 mostly on SW. It practically eliminates the distortion
caused by fades, as well as high-frequency hiss and noise giving an
audibly better signal to noise ratio. The trade-off is of course a
more limited audio response. Personally I can live with that - I never
use bandwidths wider than 6-7 kHz anyway. My ELPAF has a bypass switch
so it is easy to compare audio quality.

I used to have an SE-3 as well, and enjoyed the excellent audio it
produced. The ELPAF does little less with regard to audio recovery. I
am thinking about doing an A-B comparison between the two later on.

BM

multipath distortion
 

Back to the Future movie is on Radio tv.See if your radio has a Flux
Capacitor.According to the doc,that's what makes time travel possible.
Calling Art Bell,calling George Noory.I found the secret to time travel.
cuhulin

multipath distortion
 

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

Sorry for being such a RX newbie here. I guess I shoulda stayed awake
more during signals class. My expertise is very large switching and
resonant power supplies and transmitters.

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
here. 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.

I Imagine it would be a chore to build a sync detector from the ground
up, but I would also think it must have already been put into an ic
chip, no???

I like the link R2000swl posted to AmWindow for the precision full-wave
rectifier. I think I'll stick it on a pc board and give it a try. If
anybody wants a board let me know. They are very inexpensive.

bm, or anyone else, if you have good link to ELPAF or alternative RX
circuits then maybe I could throw that down on same board if not too
much room.

Details of board size at www.expresspcb.com I do the small one double
side and no silkscreen 3 boiards 60bucks.

You can contact me off board at

73
Bob
N9NEO


http://users.adelphia.net/~alexmm/Prod_det/detector.htm

multipath distortion
 

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

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

multipath distortion
 

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