From: Andreas Robinson (sleeper75se_at_yahoo.se)
Date: 2001-12-25 11:39:20
Hello all, and merry christmas!
I did some calculating on AM transmission about a
month ago...
The conclusion is, you can do it, but it is not easy
and probably not cheap either.
The idea is like this, moving from the frontend to the
sound card:
1. First you have a regular input stage, amplifying
the signal about 150 times.
2. The signal is filtered so that you get 96 dB
attenuation at 9kHz. A third order Chebyshev I filter
is enough.
3. The signal is passed through an analog optocoupler
(HCNR200 or IL300)
4. The signal is amplitude modulated to 10kHz. This is
the tricky part. If it is done badly, a lot of
resolution is lost.
There are two ways to do this:
* Modulate using a switch, flipping it back and forth
between the signal and a constant voltage at a rate of
10kHz. Then filter the signal using a 6th order
elliptic filter. If you don't do that, you'll get
aliasing in the sound card.
or
* Modulate with a sine wave, the old fashioned way.
Requires an experienced analog designer to get it
right.
5. A buffer amplifier sends the signal along to the
sound card.
6. The sound card samples the signal in 40kHz or
higher.
Strengths: 16-bit resolution possible, at least in
theory.
Weaknesses: More complex analog section. It requires
either a very high filter order (total of 9) or a low
distortion sine-wave oscillator and modulator. It also
needs more power because it uses one analog
opto-coupler per channel. You have to use two low
noise power supplies (such as batteries) as well.
/Andreas
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