rrw@naucse.cse.nau.edu (Robert Wier) (04/16/91)
I'm wondering if anyone can point me in some direction to
figure out the following problem:
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?
Presumably, one could use a full wave detector, with a
capacitor to "smooth out" the carrier, but it seems like it
would take a large (and lossy) cap to do this (not like
standard AM radio detectors). (EE's - forgive this description,
I'm a digital type :-)
Any suggestions on this?
Please either E-Mail me or post here.
THANKS!
- Bob Wier
-------------- insert favorite standard disclaimers here ----------
College of Engineering
Northern Arizona University / Flagstaff, Arizona
Internet: rrw@naucse.cse.nau.edu | BITNET: WIER@NAUVAX | WB5KXH
or uucp: ...arizona!naucse!rrwpepke@gw.scri.fsu.edu (Eric Pepke) (04/17/91)
In article <3632@naucse.cse.nau.edu> rrw@naucse.cse.nau.edu (Robert Wier) writes: > It is an a 2KHZ "carrier" (yup, that > 2,000 Hz - audio). 2 KHz? I'd run it through an A-D and do the "demodulation" in software. I know it's perverse, but it should work. Eric Pepke INTERNET: pepke@gw.scri.fsu.edu Supercomputer Computations Research Institute MFENET: pepke@fsu Florida State University SPAN: scri::pepke Tallahassee, FL 32306-4052 BITNET: pepke@fsu Disclaimer: My employers seldom even LISTEN to my opinions. Meta-disclaimer: Any society that needs disclaimers has too many lawyers.
tonya@hpldsla.sid.hp.com (Tony Arnerich) (04/18/91)
> 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? What's the percentage? You haven't told us what the pixel transmission rate is. It *sounds* like you might think it's 500 pixels per second. tonya@sid.hp.com
kkenny@wind.nrtc.northrop.com (Kevin B. Kenny KE9TV) (04/20/91)
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