Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!crdgw1!ge-dab!sunny!harrison
From: harrison@sunny.DAB.GE.COM (Gregory Harrison)
Newsgroups: comp.dsp
Subject: Re: FFTs of Low Frequency Signals
Message-ID: <2639@ge-dab.GE.COM>
Date: 14 Nov 89 01:25:03 GMT
References: <10208@cadnetix.COM>
Sender: news@ge-dab.GE.COM
Reply-To: harrison@sunny.UUCP (Gregory Harrison)
Distribution: usa
Organization: GE Simulation & Control Systems Dept., Daytona Beach, FL
Lines: 18

In article <10208@cadnetix.COM> jensen@cadnetix.COM () writes:
>
>	The signal of interest is 0.5Hz.  For various reasons, I'm sampling at
>	a 128Hz rate.  If I collect samples for 1 sec. (128 samples), then I
>	have only captured 1/2 of a cycle of the 0.5Hz signal.
>
>	If I do an FFT on the 128 samples, will the 0.5Hz signal show up in the
>	FFT output? I'm aware that it would be spread across the discrete 

Sampling only half a sinewave will give you a DC level in the FFT.  You must 
capture more than exactly half(or less than exactly half) to get non-DC 
energy.   If you want to get energy that is due to the frequency of the sine
wave, and not just attributable to the discontinuities at the edges of
the input time series, I believe that you must capture at least a full
sine wave.

Greg Harrison
My opinions are not intended to express those of GE.