FSK Demodulation (Matched Filter) output

Started by engrmasood2002 July 4, 2014
Hi, All i have some simple questions for HAM Radio/Caller ID FSK
demodulation.
Studying FSK demodulation using matched filter it is known that the output
will be +ve  or -ve for mark and space frequency, i am unable to figure out
that
1. What will be the output of matched filters when neither 1 (1200 Hz)  or
0 (2200 Hz) is transmitted. ? How to know this condition.

2. Will matched filter(non coherent) demodulation technique also detect
coherent (Syncronous) FSK signals successfully?

Thanks	 

_____________________________		
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On Fri, 04 Jul 2014 13:44:29 -0500, engrmasood2002 wrote:

> Hi, All i have some simple questions for HAM Radio/Caller ID FSK > demodulation. > Studying FSK demodulation using matched filter it is known that the > output will be +ve or -ve for mark and space frequency.
Who by? Y'know, the whole "it is known" thing is really just a way to dodge a forthright statement "so and so said that..." For that matter, what are you defining as +ve and -ve? The outputs of the matched filters themselves will be mostly-sinusoidal signals; you detect a 0 or a 1 by comparing the relative amplitude of these. , i am unable to
> figure out that
> 1. What will be the output of matched filters when > neither 1 (1200 Hz) or 0 (2200 Hz) is transmitted. ?
Noise. Noise that is mostly sinusoidal.
> How to know this condition.
Good question. This isn't easy. You can set some semi-arbitrary threshold on some metric (the ratio between the amplitude of the outputs of the two filters is probably good), and declare anything below that threshold is "noise". This works well when definitive reception is better than, well, noise. Alternately, you can take whatever's coming out of the matched filters and attempt to synchronize your bit clock and decode the data. This works well either when you're desperate, or when the data has some sort of error detection code built in. Then solid reception is determined when good packets start coming in.
> > 2. Will matched filter(non coherent) demodulation technique also detect > coherent (Syncronous) FSK signals successfully?
Yes, but not as well as synchronous detection. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
>On Fri, 04 Jul 2014 13:44:29 -0500, engrmasood2002 wrote: > >> Hi, All i have some simple questions for HAM Radio/Caller ID FSK >> demodulation. >> Studying FSK demodulation using matched filter it is known that the >> output will be +ve or -ve for mark and space frequency. > >Who by? Y'know, the whole "it is known" thing is really just a way to >dodge a forthright statement "so and so said that..." > >For that matter, what are you defining as +ve and -ve? > >The outputs of the matched filters themselves will be mostly-sinusoidal >signals; you detect a 0 or a 1 by comparing the relative amplitude of >these. > >, i am unable to >> figure out that > >> 1. What will be the output of matched filters when >> neither 1 (1200 Hz) or 0 (2200 Hz) is transmitted. ? > >Noise. Noise that is mostly sinusoidal. > >> How to know this condition. > >Good question. This isn't easy. > >You can set some semi-arbitrary threshold on some metric (the ratio >between the amplitude of the outputs of the two filters is probably >good), and declare anything below that threshold is "noise". This works >well when definitive reception is better than, well, noise. > >Alternately, you can take whatever's coming out of the matched filters >and attempt to synchronize your bit clock and decode the data. This >works well either when you're desperate, or when the data has some sort >of error detection code built in. Then solid reception is determined >when good packets start coming in. > >> >> 2. Will matched filter(non coherent) demodulation technique also detect >> coherent (Syncronous) FSK signals successfully? > >Yes, but not as well as synchronous detection. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >
Sorry Tim, you are right i forgot to tell the whole story and i meant what you have understood that after subtracting outputs of mark and space matched filters the output would be +ve or -ve based on what was transmitted. _____________________________ Posted through www.DSPRelated.com