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From: kkenny@wind.nrtc.northrop.com (Kevin B. Kenny KE9TV)
Newsgroups: sci.electronics
Subject: Re: How to detect SLOOOOW AM?
Keywords: WEFAX WESAT weather satellite facsimile APT
Message-ID: <22623@gremlin.nrtc.northrop.com>
Date: 20 Apr 91 00:12:36 GMT
References: <3632@naucse.cse.nau.edu>
Sender: news@gremlin.nrtc.northrop.com
Reply-To: kkenny@wind.nrtc.northrop.com (Kevin B. Kenny KE9TV)
Organization: Northrop Research & Technology Center, Palos Verdes, CA
Lines: 32

In article <3632@naucse.cse.nau.edu> rrw@naucse.cse.nau.edu (Robert Wier) writes:
> We have GEOS weather satellite receiver.  The converter on the
> dish comes out with an AM signal representing pixel values
> from the images.  It is an a 2KHZ "carrier" (yup, that 
> 2,000 Hz - audio).  There are about 500 pixels per line, so
> I guess that the detected signal I want varies at that rate.
>
> The question is, how does one detect such a low frequency
> AM signal with the information of interest being such a 
> large percantage of the carrier frequency?  

OK, here are a couple of ideas.

If you really want to do this with analog hardware, check out
`synchronous demodulation.'  The idea here is to phaselock a local
oscillator to the 2400 Hz (check your numbers -- the WEFAX carrier is
2400 Hz, not 2000) carrier, and mix it with the incoming signal to get
a level signal that you can then A/D or whatever.

A variant on this technique is to use the local 2400 Hz (or 4800, or
9600; any divisor of 4x the carrier frequency will do) to control a
resettable integrator that is used to demodulate the AM.  I've had
good success with this technique in a homebrew WEFAX demodulator.

The easiest way by far, though, is to use digital techniques.  Just
oversample the signal and demodulate it in software.  

I can probably drag some schematics and code fragments out of my
notebooks if you *really* need them.  Please don't bother me for them
if you don't.... it will be a fair amount of work to find them.

73 es gud luk de ke9tv/6, Kevin