Improved diode selection for AM detection
 

Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

I am going to try and drive the detector diode in
my R2000 with a ~20K resistor feeding the junction
of to force the Ge diode slightly into conduction.
(for a diagram of the R2000 please see:
http://www.qsl.net/kc7bum/schematic-1.jpg
for a diagram of the R2000)

But I would prefer to find a diode with the lowest
Vf possible. Back in the mid 1970's GE had a diode
with a very low Vf, but I can't remember the name.

Does anyone know of a source of Shotkey "zero-bias
detector diodes"? And will diodes from a microwave
detector work in at 455KHz? I have several microwave
detector diodes that I would sacrfice in a minute if
there was a good chance they would work.

This weekend I intend to go to a friend to whom I have
loaned my ancient R392 which has a 455KHz IF out port,
and see how it works.


Terry

wrote:

Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

I am going to try and drive the detector diode in
my R2000 with a ~20K resistor feeding the junction
of to force the Ge diode slightly into conduction.
(for a diagram of the R2000 please see:
http://www.qsl.net/kc7bum/schematic-1.jpg
for a diagram of the R2000)

But I would prefer to find a diode with the lowest
Vf possible. Back in the mid 1970's GE had a diode
with a very low Vf, but I can't remember the name.

Does anyone know of a source of Shotkey "zero-bias
detector diodes"? And will diodes from a microwave
detector work in at 455KHz? I have several microwave
detector diodes that I would sacrfice in a minute if
there was a good chance they would work.

This weekend I intend to go to a friend to whom I have
loaned my ancient R392 which has a 455KHz IF out port,
and see how it works.

I'm not an expert on this by any means but as I rcall we went through the
Schottky diode craze about 20 years or so and for the most part it seemed to be
bunk.

Perhaps one of our resident techs might weigh in on this.

dxAce
Michigan
USA

dxAce wrote:

I'm not an expert on this by any means but as I rcall we went
through the Schottky diode craze about 20 years or so and
for the most part it seemed to be bunk.

Perhaps one of our resident techs might weigh in on this.

dxAce
Michigan
USA
---------------------------
I am hoping to improve the AM detecton in the R2000. I have done some
experimenting and decided that changing the caps was worthwhile. I
tapped straight off the detector with a DC coupled hi-Z FET, fed it
to my classs A servo heaphone amplifier and switched back and forth
between the my buffer and the Rec out, and then the input to the power
amp IC. This porved somehting I already knew, but IC audio amps
suck. Lots of crossover distortion. By taking my audio upstream
I was able to understand weak singals. As opposed to just telling
something was there. I using my HiFer beacon feeding a attenuator
and using my MC power divider to feed my antenna and the RF
to the R2000s. This allowed me to nearly duplicate real world
conditions with QRN/M. I used a local 24 hour talk radio station,
for my modulation source. Pulled it from my DX398 tape out.

I am going to be very busy changing caps for the next month.

I have decided that some of the low level "Grunge" that I
wrote off to PLL phase noise is in fact distortion caused by the
caps, and the nasty cross over distorion in the audio power IC.
I am going to replace that stage with a much better design.

While I would love to buy a new, or better used receiver, I have
to stay within a budget, so it is either modify or accept.

Of course some people mihgt nominate me for "sows ear
to silk purse" prize of the year.

Later tonight I am going to try adding some forward bias
to the AM detector diode. Distortion is after all another form
of noise.

Terry
Terry

Terry - let us know how forward biasing the AM detector diode works
out. Hopefully it will reduce distortion.

Frank

wrote in message
oups.com...
Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

I am going to try and drive the detector diode in
my R2000 with a ~20K resistor feeding the junction
of to force the Ge diode slightly into conduction.
(for a diagram of the R2000 please see:
http://www.qsl.net/kc7bum/schematic-1.jpg
for a diagram of the R2000)

But I would prefer to find a diode with the lowest
Vf possible. Back in the mid 1970's GE had a diode
with a very low Vf, but I can't remember the name.

Does anyone know of a source of Shotkey "zero-bias
detector diodes"? And will diodes from a microwave
detector work in at 455KHz? I have several microwave
detector diodes that I would sacrfice in a minute if
there was a good chance they would work.

This weekend I intend to go to a friend to whom I have
loaned my ancient R392 which has a 455KHz IF out port,
and see how it works.

I've considered feeding the tape out of a receiver to an outboard tube audio
amp, a single tube 6V6 or some sort. Have you ever tried this?

Combining digital electronics with tube audio would be great. I own a couple
of Hammerlunds and love that sound.

wrote in message
oups.com...
Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

I am going to try and drive the detector diode in
my R2000 with a ~20K resistor feeding the junction
of to force the Ge diode slightly into conduction.
(for a diagram of the R2000 please see:
http://www.qsl.net/kc7bum/schematic-1.jpg
for a diagram of the R2000)

But I would prefer to find a diode with the lowest
Vf possible. Back in the mid 1970's GE had a diode
with a very low Vf, but I can't remember the name.

I think I would do away with the simple diode envelope detector and
substitute something more sophisticated. In my homebrew RX I am using an
NE602 as a multiplier- this from Wes Hayward's latest text- Experimental
Methods in RF Design.
Another approach I have used with good fidelity is an MC1330 (?) video
detector. Finally, see:
http://www.amwindow.org/tech/htm/alowdisdet.htm
for a nice treatise on the shortcomings of classic diode AM detectors.

Dale W4OP

wrote in message
oups.com...
Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

True the Schottkey diodes will have a lower Vf than
your standard garden variety germanium. You will also
have to take into account that the rcvr will be more prone
to hear more man-made interference and also the AGC will
still be active and possibly negate any gains in sensitivity.
Not sure if they will work at the lower freq's well as they are
designed for microwave uses although some do run in
the RF-ID range of 36 MhZ.
A good experiment project but methinks you would be
better off with an external FET amplifier or high gain
directional loop antenna for BCB Dx.

see these for datasheets and app notes:

http://www.microwaves101.com/encyclopedia/detectors.cfm

http://www.spelektroniikka.fi/kuvat/schot8.pdf

http://www.findarticles.com/p/articl...46/ai_17990746

"DesignGuy" wrote:

I've considered feeding the tape out of a receiver to an outboard tube
audio
amp, a single tube 6V6 or some sort. Have you ever tried this?

Combining digital electronics with tube audio would be great. I own a
couple
of Hammerlunds and love that sound.
--------------------------------------------
I used a stereo class A headphone amplifier buitl around MOS-FETs,
with a JFET input. I don't have the link here, but I will post it when
I get
home.
I'm using a 30+ year old set of Senheiser HD424 "open air" stereo
earphones. They Hi-z, 2K units and I have found them to be very
easy to where for hours on end. I have a set of Koss Pro4A, God
I am telling my age here, that I use when listening in noisy locatons,
but
they have always given me a head ach and sweaty ears if I where them
longer then 30 minutes or so.

I built a stereo headphone amp using 2 12AU7's to compare the
sound quality of the MOS-FET to holllow state. No difference that
I could hear.

My wife finds trying to reduce distortion on shortwave as "silly".
From her viewpoint "if you want clarity you wouldn't be trying to
listen to 20W transmitters on the far side of the world".

Terry

wrote:

Almost all AM detectors use Germanium diodes.
This link shows an interesting alternative:
http://www.wsdmag.com/Articles/Print.cfm?ArticleID=8628
Ge diodes have a typical Vf (forward conduction volage)
of .4V.
"Silicon Schottky diodes are commercially available
in four different versions. They offer forward
voltages of approximately 600 mV for high barrier,
330 mV for medium barrier, 280 mV for low barrier,
and 180 mV for zero-bias detectors.".

I am going to try and drive the detector diode in
my R2000 with a ~20K resistor feeding the junction
of to force the Ge diode slightly into conduction.
(for a diagram of the R2000 please see:
http://www.qsl.net/kc7bum/schematic-1.jpg
for a diagram of the R2000)

But I would prefer to find a diode with the lowest
Vf possible. Back in the mid 1970's GE had a diode
with a very low Vf, but I can't remember the name.

Does anyone know of a source of Shotkey "zero-bias
detector diodes"? And will diodes from a microwave
detector work in at 455KHz? I have several microwave
detector diodes that I would sacrfice in a minute if
there was a good chance they would work.

This weekend I intend to go to a friend to whom I have
loaned my ancient R392 which has a 455KHz IF out port,
and see how it works.

Terry

The early "Microwave" diodes like the 1N21 are extremely sensitive to
static damage, and had a low breakdown voltage. They were shipped with
a thin piece of lead sheet wrapped around them to protect them. Some
Ohm meters will destroy the junction, so be careful.
--
Former professional electron wrangler.

Michael A. Terrell
Central Florida