[mathcad] Re: DSP method

Author: Eden Mei Posted: Tue, 17 Jan 2006 07:18:00 -0800

Your correlation function will only be sharp if tow things occur:
> High information "distance," e.g., off-peak deltas result in large decorrelations. You could do this by filtering out signals that don't help the correlation
> Sufficient resolution OR phasing of the FFT to decrease the spectral leakage into adjacent bins, i.e., a 50 Hz signal will correlate better if it's not spread out into 40, 45,50, 55, 60 Hz bins. Small changes our sample rate can alter the behavior.

TTFN,

From: Mark Smith
To: Mathcad Discussion List
Cc: ''
Sent: Tuesday, January 17, 2006 2:51 AM
Subject: [mathcad] Re: DSP method

Hi Chris

You wrote... "The Nyquist frequency doesn't limit the accuracy of the phase
measurement.
This is determined more by the total length of the data, and how sharp the
correlation is. You can interpolate to find the peak of the correlation,
but this is only one sample from a distribution. If the same experiment
were repeated multiple times, one would expect to find a scatter on this
position. Depending on the quality of the data this scatter could more or
less than the time step."

Thanks for this it is as I had a feeling that this was the case but is there
a way to determine what the theoretical accuracy is from the data. I want to
only have to sample at a rate sufficient for the accuracy I need given the
level of noise present, slow A2D's are cheeper than fast ones.
I can improve accuracy by averaging many sets of data to reduce scatter but
again I'd like to keep this to a minimum.

cheers
mark

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