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Re: [Discuss-gnuradio] Two ADCs, one signal

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] Two ADCs, one signal
Date: Wed, 7 Mar 2007 16:43:06 -0500

On 3/7/07, address@hidden <address@hidden> wrote:
Brian Padalino wrote on Mon, 5 Mar 2007 16:05:09 -0500:

>That Analog Devices AD9235-65 looks like it's good if you want to
>sample at something like the USRP is doing right now - 64MHz.  So what
>you'd be looking at is an oscope with a 500MHz bandwidth and a 64MSPS
>sampling rate.  You could possibly double that if you did some
>cascading of the ADCs and used the opposite edge of the clock to also
>clock into a different ADC - giving you (effectively) double the
>samples per second.

If one was to shift the second ADC by 90 degrees to the clock instead
of 180, wouldn't one be gaining I/Q data aswell as increase the
effective sampling rate?

I found a post on dsprelated.com that can help.  I will post relavent
information here.

Taken from http://www.dsprelated.com/showmessage/24329/1.php

-- snip --
I (vaguely) heard that sampling complex-valued data does not abide by the
Nyquist rate criteria, i.e., the sampling rate fs can go lower than Nyquist
rate and it still can avoid aliasing and reconstruct perfectly...

Is that true?

Yes. Real sampling at Fs samples/second provides a "usable" bandwidth
of Fs/2 Hz while complex sampling provides a usable bandwidth of Fs Hz
at the same sample rate.

Any theory behind it?

Yes. Use two pieces of knowledge: a) the Fourier transform property
that H(f) = H*(-f) (this is known as "Hermitian symmetry") for a real
signal h(t), H(f) = F[h(t)], and b) the fact that sampling can be
viewed in the frequency domain as replicating the band from -Fs/2 to
+Fs/2 every Fs Hz.

More simply, a real signal has bandwidth from 0 to Fs/2 available, while
a complex signal has bandwidth from -Fs/2 to +Fs/2 available.

As I recall, Richard Lyon's book "Understanding Digital Signal Processing"
(2nd ed.) discusses this phenomenom at great length.
% Randy Yates
-- snip --



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