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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Undersampling and its complement?
Date: Sat, 02 Nov 2002 02:06:18 GMT
NNTP-Posting-Date: Fri, 01 Nov 2002 18:06:18 PST
James Meyer wrote:
> I have to extract information from an 8 MHz signal. Nyquist says that
> 16 MHz should be my minimum sampling frequency. However... if the bandwidth of
> the signal I'm interested in is something like 10 KHz centered around 8 MHz,
> then I really only need to sample at 2 times 10 KHz, let the signal be "folded"
> or aliased down to 0 to 10 KHz and I wil not have lost any information. That's
> My question is.... is there anything similar regarding generating an 8
> MHz signal? Do I really need a 16 MHz or better DAC in order to generate 8 MHz
> signals if the signal will not have a very wide bandwidth?
Neither way works very well. Sampling an 8 MHz signal at 20kHz to
extract 10kHz information works but you also get aliases all over the
map. To get rid of the aliases, you have to filter before sampling.
Also, the Nyquist criterion is a sort of limit given you have infinite
time to extract the information. As a practical matter, sampling at 10
times the highest frequency of interest will give useful information in
a reasonably short time. Turning things around is really almost a mirror
image. You end up generating "imposters" ad nauseum. Imposters are a
sort of dual of aliases. To get rid of imposters, you need a filter
again not to mention the fact that the imposters grow weaker at higher
There are some other problems having to do with whether the 8MHz is an
exact integer multiple of the sample rate and things like that. That is,
you could be folding on the error between 8MHz and the nearest multiple
of the sample rate.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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