Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!caen!b-tech!ais.org!danr
From: danr@ais.org (Daniel Romanchik)
Newsgroups: sci.electronics
Subject: Re: Re: A question about Nyquist theorm
Message-ID: <7WB-9MC@irie.ais.org>
Date: 22 Feb 91 17:43:26 GMT
References: <1991Feb17.115102.15399@Neon.Stanford.EDU> <605.27c28109@zodiac.rutgers.edu> <5782@mit-caf.MIT.EDU>
Sender: danr@ais.org
Organization: UMCC, Ann Arbor, MI
Lines: 41

In article <5782@mit-caf.MIT.EDU> fritz@mit-caf.UUCP (Frederick Herrmann) writes:
>In article <605.27c28109@zodiac.rutgers.edu> bittel@zodiac.rutgers.edu writes:
>>I had a professor that loved to explain the sampling theory this way.
>>It is not correct!!!  What does the bandwidth have to do with it???
>>Say you have a signal with frequency components from 5000 to 5100 Hz.
>>The bandwidth is 100 Hz.. Does that mean you can sample at 200 samp/sec and
>>get the signal??? NO!!
>
>YES!!  Your professor was right.  You have to sample for all time, of course,
>but you can get the signal.
>
>That's why you need anti-alias filters, so you get the band you want.  Without
>a filter, all the 100Hz bands you can think of get aliased to the same place.
>0-100 Hz, 5-5.1 KHz, 1-1.0000001 GHz.
>
>How do you think digital sampling scopes work?  The HP54501A has a 100 MHz
>bandwidth, but samples at only 10 Msamples/sec.  Yes, I know these instruments
>do a lot more than simple sampling, but they do acquire signals faster than
>their sampling rate.  And they do alias, if you're not careful then
>what you see may not be what you got.

The Nyquist criteria really only applies to single-shot sampling.

The scope that you mention above can really only measure signals at
100 MHz if they are repetitive signals.  The scope takes a sample or
two on successive cycles, and then recreates the waveform from samples
from many cycles.  This is the reason the sampling rate may be much
less than the analog bandwidth.

The reason DSOs can acquire signals faster than their sampling rate
is that they do a lot more than simple sampling, *and* that there are
restrictions on the signals they can measure.

Have fun,  Dan
===============================================================================
           Dan Romanchik              |       writer@irie.ais.org
          Technical Editor            |         CI$: 76236,2372
      Test and Measurement World      |         (313) 930-6564
===============================================================================
                      If it ain't broke, don't fix it.
===============================================================================


Brought to you by Super Global Mega Corp .com