RadioBanter - View Single Post - Diode and very small amplitude high frequencies signals

February 2nd 05, 07:03 PM

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