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From: firstname.lastname@example.org (Steven Swift)
Subject: Re: Can clock jitter in a A/D D/A system cause aliasing?
Date: 12 Sep 2002 03:09:02 GMT
Organization: Eskimo North www.eskimo.com (800) 246-6874
NNTP-Posting-Date: 12 Sep 2002 03:09:02 GMT
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email@example.com (Tom) writes:
>Lately I have been reading literature on nonlinear sampling, with the idea
>that by jittering the sample clock on the A/D one can eliminate aliasing
>which can occur if there are frequency components of the input signal above
>the Nyquist frequency. This relaxes the input anti-aliasing requirements.
>On the other hand, I have heard it said that clock jitter in the D/A clock
>might cause aliasing. I can kind of see this but not quite. I imagine the
>output of the D/A before reconsrtuction filtering, where the aliases of the
>output spectrum are replicated at multiples of the sample rate. I would
>think that clock jitter on the D/A would cause some kind of distortion in
>the output spectrum which I would ecpect would be harmonically related to
>the sample frequency. I can see that some of this distortion might modulate
>down to the baseband part of the spectrum. However, I can't see how this
>distortion, even if it were in the baseband spectrum (below the corner of
>the output reconstruction filter) , could be considered aliasing.
It is really simple to visualize. Think of jitter as FM. Thus the clock
acts as a carrier with the jitter showing up as sidebands. Depending upon
the rate and amplitude this carrier can be quite "smeared." So what you
have is really a bunch of sampling rates. All these can mix with your
input. If they are wide enough to encroach on your anti-aliasing filter,
you'll get aliasing. Really noisy clocks with wide-band jitter just raise
the whole noise floor.
You can use carefully constructed clock jitter for "dithering" to actually
improve your A/D. The concept is to have a known distribution of frequencies.
One simple trick is to run an MLS into a DAC to modulate a VCO. You can
set the MLS to be exactly what you need.
>Reference materials are welcome.
Here's an old (classic) paper:
VANDERKOOY, g., AND LIPSHITZ, S. P. 1984. Resolution below the least
significant bit in digital systems with dither. J. Audio Eng. Soc. 32,
There are many new ones.
Steven D. Swift, firstname.lastname@example.org, http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA
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