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Re: Alternating Current

Yes, it is true that to synthesize a .01 sec burst of a 120Hz sinusoid (zero
"elsewhere"), a "large" spread of pure (infinite duration) sine waves would
be needed; this describes its Fourier spectrum. This has little to do with
the measurement under discussion.

Your analysis doesn't even depend on the sampling time of the ADC process,
which is more than likely the limiting factor in the case in question. Your
analysis addresses only the Fourier description of the burst signal, not the
measuring (ADC process).

Your scope views the signal for a very long time: the scanning interval
times the
number of scans that you see.

Even if I captured a single .01 sec burst on a single sweep scope, I could
still beat your 100Hz error! All I need do is read a time on the sweep
axis!

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor

-----Original Message-----
From: JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE,
IL 60439 <JLU@HEP.ANL.GOV>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Friday, January 29, 1999 11:07 PM
Subject: Re: Alternating Current


To Bob Sciamanda-
You misread me. Or, at least, you misquote me. Or, I spoke
imprecisely.
If the length of me sample is .01 s, then my uncertainty in frequence is
100 Hz.
Your scope views the signal for a very long time: the scanning interval
times the
number of scans that you see.
Stated another way, take the Fourier transform of a section of a
120 Hz
sinusoid that is .01 sec long. The spectral content of the FT would be
enough
to choke a horse. Are we now communicating?
Regards,
Jack

"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography