Xref: utzoo sci.electronics:17799 sci.physics:16818 Path: utzoo!utgpu!cunews!bnrgate!brtph3!batpa1!cadnews From: kahhan@bnr.ca Newsgroups: sci.electronics,sci.physics Subject: Re: A question about the Nyquist theorm Message-ID: <1991Feb17.145953.27252@bnr.ca> Date: 17 Feb 91 14:59:53 GMT References: <1751@manta.NOSC.MIL> Sender: kahhan@batpa3 Organization: none Lines: 30 In article terryb.bbs@shark.cs.fau.edu (terry bohning) writes: >north@manta.NOSC.MIL (Mark H. North) writes: > >> > i.e. If you have a 60HZ sine wave, and you sample at 120HZ, you're >> >only going to get two points per cycle. >> >> And imagine that those two points are phased such that they land at the >> zero crossing of the 60Hz signal. All your samples are zero! This is >> why you must sample at greater than 2nu. >> >The catch is that you *know* you're sampling the highest input freq at >2 points per cycle. That is, the input signal is bandlimited. So if >someone gives you a set of all zero samples and you know the sample >rate is 120 Hz, the only frequency it can be is 60 Hz. Not quite. Another signal that will yield an all zero sample set to the 120 Hz sampling rate is DC. In general, you must sample at greater than twice the maximum frequency, not exactly twice. However, there are techniques that can be used to sample at exactly twice the frequency, under certain conditions (like looking for a single frequency, where you kick the phase of your sampler periodically to avoid sampling the input waveform only at zero crossings). A -- ---------------------------------------------------------------------------------- Larry Kahhan - NRA, NRA-ILA, CSG, GOA, GSSA | The opinions expressed here do | not necessarily represent the | views of the management. ----------------------------------------------------------------------------------