In article ,
"daestrom" wrote:

"Spehro Pefhany" wrote in message
...
On Fri, 23 Dec 2005 18:46:49 GMT, the renowned "daestrom"
wrote:


wrote in message
...
Joel Kolstad wrote:

(I can't tell you how many times I've seen people stating something
like,
'The Nyquist theorem requires sampling at at least twice the highest
frequency present in the signal," when of course it says no such thing.)

What do you think it means?


Nyquist figured out that higher frequency components of the input signal
will 'alias' and you will lose the ability to tell them from lower
frequency
components. In order to avoid 'losing information' and not being able to
tell whether a particular sample stream was from a low or high frequency
component, Nyquist's theorem states you must sample at least twice as fast
as the highest component present.
snip

More than twice the bandwidth.



So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz,
you think I can sample it at just 150 hz and get accurate reproduction?
That's just wrong.

It is the maximum frequency component in the signal that is important. The
bandwidth is not related unless the lower edge of the band is at 0 hz
(whereupon the upper side of the band is equal to the max frequency).

daestrom



You are getting your terms confused here guys. Nyquist requires that
you input both the Center Frequency, and Bandwidth when determining
the Sampling Rate. If the sampling is done at BaseBand then only the
Bandwidth is relevent. If the sampling is not done at baseband, then
the Center Frequency, and Bandwidth are required to determine samling
rate. Example, if the Bandwith of the signal is 3Kc and the sampling is
done at BaseBand then sample rate needed would 6Kc. If the sampling is
done at 100 Mhz with the same 3Kc bandwidth, then a 200.006 Mhz sampling
rate would be required.

It is much easyier to do DSP at baseBand, than at IF Frequencies, and if
you do DSP at IF Frequencies, the lower the IF Frequency, the easyier it
is to do, and the slower the DSP has to run.

Me