On 2 Feb 2005 05:07:50 -0800, "johna@m" wrote:

Hello All,

I am trying to simulate a simpe AM receiver circuit with diode
detector. I am assuming that the signal received from the antenna
(simulated with a voltage source) has a weak amplitude (around 100 uV)
and a high frequency (around 600 Khz). The issue is that the current
after the diode does not get rectified. The output current is very weak
(less than 250pA) and still contains the full sin signal (both halves
of signals).

When I try the simulation with smaller frequencies (around 5kHz) and
higher amplitude (around 0.2 v), the signal gets correctly
half-rectified, but not anymore when I work with higher frequencies and
smaller signals.

In real shematic for AM simple receiver, there is no ampification
bewteen the antenna (and the tuning LC circuit) and the diode. So how
the diode manage to half-rectifies correctly in real operating mode
when the signal is weak and high frequencies, which is the case of real
radio signals.

I use Ansoft Simplorer mainly. Any other simulators recommended ?
Thanks in advance and best regards,

John.
http://uweb.superlink.net/~bhtongue/.../10npddec.html
http://uweb.superlink.net/~bhtongue/.../16MeaDio.html

Information on diodes for small signal detector use

Many crystal set devotees prefer iron pyrite with a cats whisker over
today's diodes for sensitivity to small signals.

Should not we expect that the current, even at very small level, to be
half rectified by a diode, since the reverse resistance of the diode is
supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

Regards,

John.

johna@m wrote:
Should not we expect that the current, even at very small level, to be
half rectified by a diode, since the reverse resistance of the diode is
supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

Regards,

John.

The diode behavior is a continuous curve, so for a small AC voltage you
won't see much change in the diode's resistance even at zero bias.
Unless you're modeling a really leaky diode, however, you are probably
seeing a situation where the diode's resistance is effectively shunted
by it's capacitance and you are seeing capacitive coupling rather than
conduction.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
johna@m wrote:
Should not we expect that the current, even at very small level, to be
half rectified by a diode, since the reverse resistance of the diode is
supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

Regards,

John.

The diode behavior is a continuous curve, so for a small AC voltage you
won't see much change in the diode's resistance even at zero bias.
Unless you're modeling a really leaky diode, however, you are probably
seeing a situation where the diode's resistance is effectively shunted
by it's capacitance and you are seeing capacitive coupling rather than
conduction.

The point about continuous curve is well made.

The diode doesn't have to hard rectify. As long as it has a non-linear V-I
graph it will produce some audio. The more sharply curved the
characteristic, the more audio is produced.

In the valve days, the anode bend detector worked that way, using a valve
biased to operate on the curved part of the characteristic.

Roger

Roger Lascelles wrote:

"Tim Wescott" wrote in message
...
johna@m wrote:
Should not we expect that the current, even at very small level, to be
half rectified by a diode, since the reverse resistance of the diode is
supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

Regards,

John.

The diode behavior is a continuous curve, so for a small AC voltage you
won't see much change in the diode's resistance even at zero bias.
Unless you're modeling a really leaky diode, however, you are probably
seeing a situation where the diode's resistance is effectively shunted
by it's capacitance and you are seeing capacitive coupling rather than
conduction.

The point about continuous curve is well made.

The diode doesn't have to hard rectify. As long as it has a non-linear V-I
graph it will produce some audio. The more sharply curved the
characteristic, the more audio is produced.

In the valve days, the anode bend detector worked that way, using a valve
biased to operate on the curved part of the characteristic.

Roger

Right. Take a look at a diode curve across a 100 uV region. Most diodes
will look pretty flat, even without considering the effects of
capacitance and other parasitics.

--
Paul Hovnanian
------------------------------------------------------------------
My inner child can beat up your inner child.
-- Alex Greenbank

"johna@m" wrote in message
oups.com...
Should not we expect that the current, even at very small level, to be
half rectified by a diode, since the reverse resistance of the diode is
supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

Regards,

John.


There is nothing wrong with the simulator... the problem is with your idea
of a diode. The general definition of a diode is a component that conducts
normally in one direction, but does not conduct in the other. That
definition only applies to a "perfect" diode. The reality of semiconductor
diodes is that a 'barrier potential" exists across the junction. In
germanium diodes, this is around 0.3 volts; in silicon diodes, it's around
0.6 volts.
In order for the diode to conduct, this barrier potential must be exceeded
by an externally applied voltage. Until that potential is reached, the
diode is said to be reverse biased, and only a very small leakage current
flows. When the barrier potential is reached, the junction becomes forward
biased and conducts heavily.

The small signal voltage that you are trying to simulate may not be enough
to reach the barrier potential of the diode junction, thus, no conduction
(rectification) in either direction. The simulator is aware of the barrier
potential of the diode. If the peak value of your signal voltage is less
than the barrier potential, no rectification occurs. If you increase the
amplitude of the signal applied to the defined barrier potential of the
particular diode in your model, you will see rectification begin. The
higher the signal amplitude, the more rectified signal appears on the
output.


You can make a diode rectify a signal amplitude lower than the barrier
potential by applying a forward voltage that is just under the barrier
potential, so that the signal doesn't have to overcome the full barrier
potential. Fer instance, if you apply a 0.5 volt DC voltage to a silicon
diode, it will start to rectify signal levels as low as 0.1 volts.

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

DaveM wrote...

"johna@m" wrote ...
Should not we expect that the current, even at very small level, to
be half rectified by a diode, since the reverse resistance of the
diode is supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

There is nothing wrong with the simulator... the problem is with your
idea of a diode. The general definition of a diode is a component that
conducts normally in one direction, but does not conduct in the other.
That definition only applies to a "perfect" diode. The reality of
semiconductor diodes is that a 'barrier potential" exists across the
junction. In germanium diodes, this is around 0.3 volts; in silicon
diodes, it's around 0.6 volts. In order for the diode to conduct, this
barrier potential must be exceeded by an externally applied voltage.
Until that potential is reached, the diode is said to be reverse biased,
and only a very small leakage current flows. When the barrier potential
is reached, the junction becomes forward biased and conducts heavily.

The small signal voltage that you are trying to simulate may not be enough
to reach the barrier potential of the diode junction, thus, no conduction
(rectification) in either direction. The simulator is aware of the barrier
potential of the diode. If the peak value of your signal voltage is less
than the barrier potential, no rectification occurs. If you increase the
amplitude of the signal applied to the defined barrier potential of the
particular diode in your model, you will see rectification begin. The
higher the signal amplitude, the more rectified signal appears on the
output.

You can make a diode rectify a signal amplitude lower than the barrier
potential by applying a forward voltage that is just under the barrier
potential, so that the signal doesn't have to overcome the full barrier
potential. Fer instance, if you apply a 0.5 volt DC voltage to a silicon
diode, it will start to rectify signal levels as low as 0.1 volts.

Dave, your considerable effort to explain the nuances of diodes to John
is commendable, but your explanation is rather misleading. It's not true
that for a diode to conduct, the "barrier potential must be exceeded,"
and "the junction becomes forward biased and conducts heavily." Instead
the diode current has an exponential relationship to the voltage across
it, and gradually turns on over many hundreds of millivolts, not abruptly
at say 600mV. Here, examine some diode measurements I made a long time
ago, http://www.picovolt.com/win/elec/com...de-curves.html

For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win

The difficulty is "what to do with that 1uA current". To put it to practical
use, a signal processor is needed that has a useful output. For example, a
MOSFET amplifier with a 10 megohm input resistance and negligible input
capacitance (for low frequency sigs) could be used.

In this case it would be better to rethink the project.
In addition to the diode, some system design is indicated.

Bill W0IYH

"Winfield Hill" -edu wrote in
message ...
DaveM wrote...

"johna@m" wrote ...
Should not we expect that the current, even at very small level, to
be half rectified by a diode, since the reverse resistance of the
diode is supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

There is nothing wrong with the simulator... the problem is with your
idea of a diode. The general definition of a diode is a component that
conducts normally in one direction, but does not conduct in the other.
That definition only applies to a "perfect" diode. The reality of
semiconductor diodes is that a 'barrier potential" exists across the
junction. In germanium diodes, this is around 0.3 volts; in silicon
diodes, it's around 0.6 volts. In order for the diode to conduct, this
barrier potential must be exceeded by an externally applied voltage.
Until that potential is reached, the diode is said to be reverse biased,
and only a very small leakage current flows. When the barrier potential
is reached, the junction becomes forward biased and conducts heavily.

The small signal voltage that you are trying to simulate may not be
enough
to reach the barrier potential of the diode junction, thus, no conduction
(rectification) in either direction. The simulator is aware of the
barrier
potential of the diode. If the peak value of your signal voltage is less
than the barrier potential, no rectification occurs. If you increase the
amplitude of the signal applied to the defined barrier potential of the
particular diode in your model, you will see rectification begin. The
higher the signal amplitude, the more rectified signal appears on the
output.

You can make a diode rectify a signal amplitude lower than the barrier
potential by applying a forward voltage that is just under the barrier
potential, so that the signal doesn't have to overcome the full barrier
potential. Fer instance, if you apply a 0.5 volt DC voltage to a silicon
diode, it will start to rectify signal levels as low as 0.1 volts.

Dave, your considerable effort to explain the nuances of diodes to John
is commendable, but your explanation is rather misleading. It's not true
that for a diode to conduct, the "barrier potential must be exceeded,"
and "the junction becomes forward biased and conducts heavily." Instead
the diode current has an exponential relationship to the voltage across
it, and gradually turns on over many hundreds of millivolts, not abruptly
at say 600mV. Here, examine some diode measurements I made a long time
ago, http://www.picovolt.com/win/elec/com...de-curves.html

For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win

Hi,

I have re-done the simulation with the diode 1N41481, and it rectified.
Thanks.

For the 600mv, it did give indeed around 0.5mA. I even tried at 100mv
and at 100uv, and it still conducts (respectively 60uA and 25nA).

Regards,

John.

"Winfield Hill" -edu wrote in
message ...
For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win


Must be my lucky week!. I needed to make some similar readings. Thanks!.
I still puzzle over the oft quoted "up to about 30mVrms the output from the
diode offers a square law response and will approximate a true RMS
measurement.
As the OP found, there's very little happening down there. Who makes these
sweeping statements?.
regards
john