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Old July 8th 03, 04:48 PM
reach me through the sonic server
 
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Default Detector/diode question

Many detectors using diodes have an active bias voltage/current applied.
An Autek WM-1 peak reading watt meter is a great circuit of said.

cheers
skipp
http://sonic.ucdavis.edu

: Clifton T. Sharp Jr. wrote:
: I was going through my bookmarks when I came upon one about diodes
: (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In his
: third paragraph, the author asserts:

: For RF diode detectors to work, one needs a device that has a
: non-linear V/I curve. In other words, the slope of the V/I curve
: must change as a function of applied Voltage. The slope must be
: steeper (or shallower) at higher voltages and shallower (or steeper)
: at lower voltages than at the quiescent operating point.

: If there's any truth to this, someone please enlighten me. I've always
: been of the belief that an envelope detector diode would be most perfect
: if the diode was a perfect switch, i.e. zero attoamps reverse current
: and perfectly linear forward current (as though the diode was a wire
: during the forward conduction period). I don't see how a changing slope
: during forward conduction could do anything other than distort the
: demodulated waveform, especially on tiny signals.

: --
: The function of an asshole is to emit quantities of crap. Spammers do
: a very good job of that. However, I do object to my inbox being a
: spammer's toilet bowl. -- Walter Dnes
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Old July 8th 03, 06:39 PM
Joe McElvenney
 
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Hi,

For RF diode detectors to work, one needs a device that has a
non-linear V/I curve. In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

If there's any truth to this, someone please enlighten me. I've always
been of the belief that an envelope detector diode would be most perfect
if the diode was a perfect switch, i.e. zero attoamps reverse current
and perfectly linear forward current (as though the diode was a wire
during the forward conduction period). I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


You believe correctly! A diode detector is nothing much more than a
rectifier and should be characterised accordingly. Curvature of the
forward characteristic has to be lived with and is not something generally
to be desired. There are exceptions to this where the law of the initial
curvature is employed in certain instruments but not usually in
demodulating an AM signal.

However, with very weak signals a small degree of forward biassing is
sometimes employed to obtain the greatest recovered audio. This comes
about because the positive peaks are driven onto the greatest slope while
the negative ones, which would normally distract from the charge on the
load capacitor, are placed on the flatter part below the bias point and so
not recovered so well.


Cheers - Joe


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Old July 8th 03, 06:39 PM
Joe McElvenney
 
Posts: n/a
Default

Hi,

For RF diode detectors to work, one needs a device that has a
non-linear V/I curve. In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

If there's any truth to this, someone please enlighten me. I've always
been of the belief that an envelope detector diode would be most perfect
if the diode was a perfect switch, i.e. zero attoamps reverse current
and perfectly linear forward current (as though the diode was a wire
during the forward conduction period). I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


You believe correctly! A diode detector is nothing much more than a
rectifier and should be characterised accordingly. Curvature of the
forward characteristic has to be lived with and is not something generally
to be desired. There are exceptions to this where the law of the initial
curvature is employed in certain instruments but not usually in
demodulating an AM signal.

However, with very weak signals a small degree of forward biassing is
sometimes employed to obtain the greatest recovered audio. This comes
about because the positive peaks are driven onto the greatest slope while
the negative ones, which would normally distract from the charge on the
load capacitor, are placed on the flatter part below the bias point and so
not recovered so well.


Cheers - Joe


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Old July 9th 03, 12:13 AM
John R. Strohm
 
Posts: n/a
Default

"Clifton T. Sharp Jr." wrote in message
...
I was going through my bookmarks when I came upon one about diodes
(http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In his
third paragraph, the author asserts:

For RF diode detectors to work, one needs a device that has a
non-linear V/I curve. In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

If there's any truth to this, someone please enlighten me. I've always
been of the belief that an envelope detector diode would be most perfect
if the diode was a perfect switch, i.e. zero attoamps reverse current
and perfectly linear forward current (as though the diode was a wire
during the forward conduction period). I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


Dude, you and they are talking about the exact same thing.

Think about the V-I curve on an ideal diode. Observe that, if the diode is
reverse-biased, no conduction takes place. In principle, you can crank the
reverse voltage up as high as you like, and STILL no conduction takes place.
(In practice, you run into avalanche and zener breakdown.) If the diode is
forward-biased, the diode conducts like crazy. There is a CORNER in the
curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V.
(Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs,
about 2.2V for yellow, about 2.4V for green, and so on and so forth...)


  #5   Report Post  
Old July 9th 03, 12:13 AM
John R. Strohm
 
Posts: n/a
Default

"Clifton T. Sharp Jr." wrote in message
...
I was going through my bookmarks when I came upon one about diodes
(http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In his
third paragraph, the author asserts:

For RF diode detectors to work, one needs a device that has a
non-linear V/I curve. In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

If there's any truth to this, someone please enlighten me. I've always
been of the belief that an envelope detector diode would be most perfect
if the diode was a perfect switch, i.e. zero attoamps reverse current
and perfectly linear forward current (as though the diode was a wire
during the forward conduction period). I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


Dude, you and they are talking about the exact same thing.

Think about the V-I curve on an ideal diode. Observe that, if the diode is
reverse-biased, no conduction takes place. In principle, you can crank the
reverse voltage up as high as you like, and STILL no conduction takes place.
(In practice, you run into avalanche and zener breakdown.) If the diode is
forward-biased, the diode conducts like crazy. There is a CORNER in the
curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V.
(Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs,
about 2.2V for yellow, about 2.4V for green, and so on and so forth...)




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Old July 9th 03, 05:40 PM
Clifton T. Sharp Jr.
 
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Default

John R. Strohm wrote:
"Clifton T. Sharp Jr." wrote...
(http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html).

In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


Think about the V-I curve on an ideal diode. Observe that, if the diode is
reverse-biased, no conduction takes place. In principle, you can crank the
reverse voltage up as high as you like, and STILL no conduction takes place.
(In practice, you run into avalanche and zener breakdown.) If the diode is
forward-biased, the diode conducts like crazy. There is a CORNER in the
curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V.
(Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs,
about 2.2V for yellow, about 2.4V for green, and so on and so forth...)


I'm not sure I was clear on my question. The only nonlinearity I want in
my detector diode is the noncontinuous function

| Vin when Vin = 0.000000 micronanofemtoattovolts
Vout = |
| 0 when Vin 0.000000 micronanofemtoattovolts

....while the author seems to be saying that the nonlinearity which exists
just above the barrier voltage is essential to detection. He does say,

"The slope must be steeper (or shallower) at higher voltages and
shallower (or steeper) at lower voltages than at the quiescent
operating point."

Given my perfect diode and (let's say) a 0.25V peak signal, I contend
that my output envelope will be an exact reproduction of the input
envelope; but (see his graph #2) his required nonlinearity will seriously
distort the output envelope.

He seems to be saying that without that, you can't demodulate the signal.
I assert that without that, I demodulated it better than he did.

--
The function of an asshole is to emit quantities of crap. Spammers do
a very good job of that. However, I do object to my inbox being a
spammer's toilet bowl. -- Walter Dnes
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Old July 9th 03, 05:40 PM
Clifton T. Sharp Jr.
 
Posts: n/a
Default

John R. Strohm wrote:
"Clifton T. Sharp Jr." wrote...
(http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html).

In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.


Think about the V-I curve on an ideal diode. Observe that, if the diode is
reverse-biased, no conduction takes place. In principle, you can crank the
reverse voltage up as high as you like, and STILL no conduction takes place.
(In practice, you run into avalanche and zener breakdown.) If the diode is
forward-biased, the diode conducts like crazy. There is a CORNER in the
curve, i.e. a change in the slope of the V/I curve, at NOMINALLY V=0V.
(Actually, V=0.7V for silicon, V=0.3V for germanium, V=1.7V for red LEDs,
about 2.2V for yellow, about 2.4V for green, and so on and so forth...)


I'm not sure I was clear on my question. The only nonlinearity I want in
my detector diode is the noncontinuous function

| Vin when Vin = 0.000000 micronanofemtoattovolts
Vout = |
| 0 when Vin 0.000000 micronanofemtoattovolts

....while the author seems to be saying that the nonlinearity which exists
just above the barrier voltage is essential to detection. He does say,

"The slope must be steeper (or shallower) at higher voltages and
shallower (or steeper) at lower voltages than at the quiescent
operating point."

Given my perfect diode and (let's say) a 0.25V peak signal, I contend
that my output envelope will be an exact reproduction of the input
envelope; but (see his graph #2) his required nonlinearity will seriously
distort the output envelope.

He seems to be saying that without that, you can't demodulate the signal.
I assert that without that, I demodulated it better than he did.

--
The function of an asshole is to emit quantities of crap. Spammers do
a very good job of that. However, I do object to my inbox being a
spammer's toilet bowl. -- Walter Dnes
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Old July 9th 03, 05:43 PM
Clifton T. Sharp Jr.
 
Posts: n/a
Default

reach me through the sonic server wrote:
: Clifton T. Sharp Jr. wrote:
: (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html).
:
: For RF diode detectors to work, one needs a device that has a
: non-linear V/I curve. In other words, the slope of the V/I curve
: must change as a function of applied Voltage.

: If there's any truth to this, someone please enlighten me. I've always
: been of the belief that an envelope detector diode would be most perfect
: if the diode was a perfect switch, i.e. zero attoamps reverse current
: and perfectly linear forward current (as though the diode was a wire
: during the forward conduction period). I don't see how a changing slope
: during forward conduction could do anything other than distort the
: demodulated waveform, especially on tiny signals.

Many detectors using diodes have an active bias voltage/current applied.
An Autek WM-1 peak reading watt meter is a great circuit of said.


And that removes a lot of the nonlinearity (especially by no longer
chopping the bottom 0.2V off the input signal). But this author seems to
be saying that detection can't work without that nonlinearity.

--
The function of an asshole is to emit quantities of crap. Spammers do
a very good job of that. However, I do object to my inbox being a
spammer's toilet bowl. -- Walter Dnes
  #9   Report Post  
Old July 9th 03, 05:43 PM
Clifton T. Sharp Jr.
 
Posts: n/a
Default

reach me through the sonic server wrote:
: Clifton T. Sharp Jr. wrote:
: (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html).
:
: For RF diode detectors to work, one needs a device that has a
: non-linear V/I curve. In other words, the slope of the V/I curve
: must change as a function of applied Voltage.

: If there's any truth to this, someone please enlighten me. I've always
: been of the belief that an envelope detector diode would be most perfect
: if the diode was a perfect switch, i.e. zero attoamps reverse current
: and perfectly linear forward current (as though the diode was a wire
: during the forward conduction period). I don't see how a changing slope
: during forward conduction could do anything other than distort the
: demodulated waveform, especially on tiny signals.

Many detectors using diodes have an active bias voltage/current applied.
An Autek WM-1 peak reading watt meter is a great circuit of said.


And that removes a lot of the nonlinearity (especially by no longer
chopping the bottom 0.2V off the input signal). But this author seems to
be saying that detection can't work without that nonlinearity.

--
The function of an asshole is to emit quantities of crap. Spammers do
a very good job of that. However, I do object to my inbox being a
spammer's toilet bowl. -- Walter Dnes
  #10   Report Post  
Old July 9th 03, 07:35 PM
Roy Lewallen
 
Posts: n/a
Default

You're right. The author is wrong.

Roy Lewallen, W7EL

Clifton T. Sharp Jr. wrote:


I'm not sure I was clear on my question. The only nonlinearity I want in
my detector diode is the noncontinuous function

| Vin when Vin = 0.000000 micronanofemtoattovolts
Vout = |
| 0 when Vin 0.000000 micronanofemtoattovolts

...while the author seems to be saying that the nonlinearity which exists
just above the barrier voltage is essential to detection. He does say,

"The slope must be steeper (or shallower) at higher voltages and
shallower (or steeper) at lower voltages than at the quiescent
operating point."

Given my perfect diode and (let's say) a 0.25V peak signal, I contend
that my output envelope will be an exact reproduction of the input
envelope; but (see his graph #2) his required nonlinearity will seriously
distort the output envelope.

He seems to be saying that without that, you can't demodulate the signal.
I assert that without that, I demodulated it better than he did.


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