DI-194 (was Re: Data Acquisition newbie question)

[Robert W. Hughes]

> Nyquist's theorem says you'll need at least 2 samples
> for each cycle of the waveform you are trying to
> measure, so that means w/ only 1 channel enabled to
> get the max of 240S/s, the max frequency you'll be
> able to measure is 120Hz, which is 7200 RPM

A little warning here - if you sample a 120Hz signal at 240S/s the only
thing you will recover is the frequency, the amplitude will be constant
and somewhere between the positive and negative peak. Draw a sine wave
and plot sample points on it if you don't believe this. 

In the real world if you want to sample a complex (non-sinusoidal)
signal with some hope of recovering a reasonable representation of it
use a sample rate of 5 to 10 times the highest frequency component.
Aliasing is very real also and an alias filter set at 1/5 to 1/10 of the
sample rate will be worthwhile.

-- 
Robert W. Hughes (Bob)
BackYard Engineering
29:40.237N, 95:28.726W or perhaps 30:55.265N, 95:20.590W
Houston, Texas "The city with too much Oxygen"
rwhughe at ev1.net
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