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Re: [Discuss-gnuradio] NBFM

From: Al Davis
Subject: Re: [Discuss-gnuradio] NBFM
Date: Fri, 18 Feb 2005 02:06:28 -0500
User-agent: KMail/1.7.2

On Thursday 17 February 2005 07:32 pm, Marcus D. Leech wrote:
> In the analog world, do they implement squelch using
>   detected RF power levels (i.e. prior to FM demod), or
>   something else?

It is usually done by measuring detected noise.  Recall, in FM, 
audio output level is independent of the carrier level, except 
at very low signal levels.  As the signal gets stronger, 
detected noise decreases.

The usual way that squelch works is to use a high pass or band 
pass filter on the output of the detector, to filter out the 
signal.  This leaves only high frequency noise.  Then sense the 
level of that detected noise.  When the noise goes down below 
the set level, open it.

Using carrier level does not work well.  Atmospheric noise would 
be able to open the squelch.  Also, remember that the IF amp is 
a limiter.  The level at the input to the detector does not 
vary at all with signal strength.  There is enough gain so the 
front end noise is brought up to hard limiting.

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