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

[Brian Padalino]

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 --

> --
> Nos

Brian