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.

"johna@m" schreef in bericht
oups.com...
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".

-------------
Bill Sloman, Nijmegen

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,

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
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

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.

Bill Sloman wrote:

"johna@m" schreef in bericht
oups.com...

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,

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.

--

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

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

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.

In article ,
says...
Bill Sloman wrote:

"johna@m" schreef in bericht
oups.com...

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,

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.

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. ;-)

--
Keith

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