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.


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.