FSK Demodulation (Matched Filter) output

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	 

_____________________________		
Posted through www.DSPRelated.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

>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