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Re: [Discuss-gnuradio] USB2 <-> fast ADC & DAC

From: Henrique Miranda
Subject: Re: [Discuss-gnuradio] USB2 <-> fast ADC & DAC
Date: Wed, 4 Jun 2003 04:41:42 +0100 (WEST)

Joseph and Matt,

>Basically this sampling technique divides the spectrum up into regions that
>are fs wide.

Not exactly. They are fs/2 wide (the so called Nyquist zones). Each
Nyquist zone contains a copy of the same spectrum (the spectrum in a
Nyquist zone is the mirror image of the spectrum of the neighboring zones).
This applies to real-valued signals only (mirroring does not occur for
complex signals and for these Nyquist zones are fs wide)

For a more in-depth discussion on this and other SDR related topics I
suggest you the following:

Marvin Frerking, "Digital Signal Processing in Communication Systems"

Jeffrey Reed, "Software Radio: a modern approach to radio engineering"


>So basically the difference between optimum and minimum has to do with
>whether your signal lies exactly in the middle of one of the bands you can
>look at. If not, you won't be able to get optimum use of your sampling rate,
>with optimum meaning being able to capture a signal as wide as slightly less
>than half the sampling rate.

Basically, that is what meant with optimum sampling. I would add that
if your IF signal is in the middle of one region you are as far as
possible from the aliasing trouble. This way, you can use IF filters with
less demanding transition bandwidths, and consequently a flatter group

I hope things are now more clear.


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