Diode and very small amplitude high frequencies signals

[johna@m]

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.

 

[Bill Sloman]

Hello All,

I am trying to simulate a simple 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 sine 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 schematic for AM simple receiver, there is no amplification
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,

The answer to your question is that real, simple AM receivers - crystal
sets - only work where the received signal is quite high. They tend to use
fast, low capacitance diodes. The term "rectifier" tends to be used for
bigger, slower diodes basically intended to handle around an ampere of
current from a 50/60Hz source, that look like capacitors in RF circuits.

Practical AM receivers always amplify the signal before they detect it, and
usually "mix" the amplified signal from the antenna with the output from a
local oscillator at different, if similar, frequency chosen to be 455kHz
away for the transmitted signal. The nominally 455kHz component coming out
of the mixer is then filtered by an elaborate bandpass filter to reject all
the other components, further amplified, and only then detected.

Search on "superheterodyne".

 

[Rene Tschaggelar]

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,

What makes you sure the simulation and the reality agree ?
There is capacive coupling over the PN structure of the
diode to start with. And then at one point a diode has
an exponential characterization instead of binary on/off.

Rene

 

[default]

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

 

[Tim Wescott]

The answer to your question is that real, simple AM receivers - crystal
sets - only work where the received signal is quite high. They tend to use
fast, low capacitance diodes. The term "rectifier" tends to be used for
bigger, slower diodes basically intended to handle around an ampere of
current from a 50/60Hz source, that look like capacitors in RF circuits.

Practical AM receivers always amplify the signal before they detect it,

You're going to get some flak from the crystal radio crowd on that one.

and
usually "mix" the amplified signal from the antenna with the output from a
local oscillator at different, if similar, frequency chosen to be 455kHz
away for the transmitted signal. The nominally 455kHz component coming out
of the mixer is then filtered by an elaborate bandpass filter to reject all
the other components, further amplified, and only then detected.

Crystal radios generally step up the voltage from the antenna, and use a
sensitive, high-impedance earphone -- and still require relatively
strong signals to receive well. For packaged diodes consider a
point-contact germanium or a zero-bias schottkey (although the real
crystal fans will want you to stick with germanium or other material,
per the other poster). Clever tricks to play include audio
amplification after the diode and a very slight forward bias applied to
the diode.

 

[Joe McElvenney]

Hi,

In days of yore a crystal set would have had a longish wire
aerial (several millivolts output), a high-Q tuned circuit
(further magnification) and a high-Z load on the detector all, of
which lessened the need for a perfect characteristic.

And that's before you placed the headphones in a pudding bowl.


Cheers - Joe

 

[johna@m]

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.

 

[Keith Williams]

You're going to get some flak from the crystal radio crowd on that one.


Crystal radios generally step up the voltage from the antenna, and use a
sensitive, high-impedance earphone -- and still require relatively
strong signals to receive well. For packaged diodes consider a
point-contact germanium or a zero-bias schottkey (although the real
crystal fans will want you to stick with germanium or other material,
per the other poster). Clever tricks to play include audio
amplification after the diode and a very slight forward bias applied to
the diode.

One of the Profs in college built a "crystal" radio that was powerful
enough to drive a loudspeaker. It played day and night in the lab.
He had it set up so he could change the detector, but left the LED in
there because it worked as an "on" indicator too. ;-)

 

[Tim Wescott]

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.

 

[Roger Lascelles]

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

 

[Gert Baars]

Diodes have a tresholdvoltage of 0.3-0.7 V.
The amplitude must be greater. That is why this
type of receiver requires a large antenna or a
transmitter nearby.


Regards,


Website: http://members.chello.nl/g.baars13

 

[DaveM]

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!!

 

[Winfield Hill]

DaveM wrote...

...

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/comp/diode/diode-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.

 

[William E. Sabin]

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

 

[lemonjuice]

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.

Yes its what you'd expect. As I explained on the thread "Junction
capacitance of diodes and zeners" the Capacitance of a forward biased
diode increases with the applied bias or if you do the math you'll
see it increases exponentially with it. They are actually 2
capacitances in consideration but thats another question. Its really
hard modelling a non linear component with linear components but the
following can be said to be true for small variations of voltage. You
get your desired precision by adding up together pieces of this model.
If you look at a piece wise linear approximation model of the diode
you'll see it has a conductance or a linear dependent current source in
parallel with 2 condensers and the current through the conductance is
Is*exp(Vapp/Vt). Vapp is the voltage across the condensers. At high
Vapp and low frequencies from above C is high and has a lower
impedance so Idiode is high. At higher frquencies Vapp on the
condensers is pretty low so Idiode is also low. If Vapp is lower you
get an even lower value.

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.

Well if you want to cheat you can have more turns on the primary then
the secondary of the input transformer and you get a higher voltage
(grin). I'd have to see the exact circuit you are talking about to be
of more help.
Good luck.

 

[johna@m]

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.

 

[john jardine]

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.

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

 

[Mark]

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

No it's true. Look at the curves and notice that the current scale on
the x axis is a log scale. When the RF input is very small, the DC out
is proportional to the log of the RF level i.e. the RF in dB. This is
how the normal power meter works. It also provides a true RMS value
for modulated RF signal. Once the signal gets too big and the diode
begins to work as a converntoin rectifier, this relationship no longer
holds true. Notice the curves break upeards. When the RF volatge to
log I curves are straight line, this is rhe square law region where the
diode current gives you true RMS readings of the RF voltage.
Think of it as a voltage in dB to current converter.

Mark

 

[K7ITM]

Indeed, as Win says, you can get some signal out of a diode detector
even for very low input levels. With fairly simple home-brew
techniques but a lot of attention to the details of leakage currents
and op amp offset voltages, I'm able to detect RF signals down to a
very few tens of microvolts. That's using either a zero-bias Schottky
detector diode such as the Agilent HSMS-2860, or an old germanium point
contact diode. At very low signal levels, the optimum load resistance
is quite high. (See Agilent detector diode ap notes for details.)
Things are actually easier if you're only interested in the modulation
component of an AM signal, and not in trying to detect the carrier
level, since the offsets aren't particularly important for AC signals.
A JFET audio amplifier, or even a carefully-designed bipolar amplifier,
can give you a very low noise figure for the high source resistance
that the diode detector running at low input levels gives you.

There are tricks you can play to make a receiver that works from the
power received by the antenna. If you live near a transmitter that's
putting out significant power in your direction, you may be able to set
up a rectifier for that received power and use it to run a micro-power
amplifier following the detector for the station you wish to receive.
If you want to hunt for weak stations, you'll need a carefully designed
and built RF input tank/filter circuit. At night, especially, it's
possible to listen to stations quite a ways away using no active
components in the RF path before the detector.

Cheers,
Tom

 

[lemonjuice]

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.

You'll find that in reality something your simulator won't take into
account that its preferable to transmit at higher frequencies because
reception is better as the 1/f flicker noise is reduced to lower
values. Thats just 1 of the several reasons why frequency multipliers
are used in transmitters and downconverters in receivers.