Re: Alternating Current

[Bob Sciamanda <trebor@VELOCITY.NET>]

Jack,
I think you'd better re-think this argument.
You may have both Heisenberg and Nyquist rolling over in their graves!
(Or do I mis-read you?)
Certainly I can measure the frequency of .01sec of a 120Hz signal (say on a
scope) to better than an error of 100Hz!

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 10:08 PM
Subject: Re: Alternating Current


> Hi all-
>        As others have pointed out, David is sampling the power,
> so that his "expected result" is 120 HZ, not 60 Hz.  Now let's understand
> why he got 124 Hz instead of 120, given that the power company makes
> clocks run on time.  Following Brian's comment, let's estimate the
> uncertainty in David's result.
>        David used a sampling interval of 10^-4 s, and took 100 data
points.
> He therefore sampled over a time interval of 10^-2 s.  What was the
precision
> of his result?  Aha, the uncertainty principle rears her pretty head:
>        (delta t)*(delta f) = 1
> But delta t = 10^-2 s (according to David's message) so delta f can be
> no better than 100 Hz.  How can this be if David sees 124 Hz with good
> precision?
>        It can't.  Either David's precision is much worse than he thinks
> or there was a typo in his report of a sampling interval.  If the sampling
> interval was 10^-3 instead of 10^-4, then he sampled over a time interval
> of 0.1 s, and recorded about 8 cycles.  His uncertainty was then ~10 Hz,
> and 124 Hz is a reasonable estimate of the "expected value" of 120 Hz.
>                                Regards,
>                                        Jack
> ********************************************************************