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Re: [Discuss-gnuradio] Multiple applications using same A/D converter?

From: Dave Emery
Subject: Re: [Discuss-gnuradio] Multiple applications using same A/D converter?
Date: Sat, 22 Mar 2003 18:11:23 -0500
User-agent: Mutt/1.4i

On Sat, Mar 22, 2003 at 01:39:05PM -0800, Eric Blossom wrote:
> rate to run the A/D.  After all, at any given instant the RF front end is
> tuned to a single center frequency.  If the signals of interest are
> within the pass band of the front end, and if you're sampling rate is
> sufficient to capture them all, and if the A/D converter's got enough
> usable dynamic range to handle the span between the weakest and
> strongest signals of interest, this will work.

        A minor quibble - but not unimportant either.  One needs sufficient
dynamic range to handle the strongest signal within the passband of the
the tuner, not the strongest signal of interest.   This may seem a nice
distinction, but in fact most real communications bands (or for that
matter even the FM broadcast band) have one or more very powerful local
signals and the IF and RF gain of the system WILL have to be set so these
signals do not saturate the ADC.

        There are, of course, good ole fashioned analog techniques such
as narrow notch filters that might help eliminate or more likely just
reduce the amplitude of strong undesired signals inside the tuner's
passband - but these add complexity and cost and have limitations of
various kinds and require that the notch filters be tuned to the
particular signals in a particular area which of course would require
instrumentation and knowlage and tools.   Having sufficient SFDR makes
this problem much easier of course.

        Of course if one wants to look at a lot of signals, it may
get more interesting to use hardware digital downconverters and
decimators rather than performing this cpu intensive function
in software.  But this point has been made already.

        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
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