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phase accuracy and sampling rate | PLL and FFT

Started by dralban December 13, 2009
I read somewhere that if sampling two signals with frequency f and a
sampling rate of N. Then the uncertainty in phase shift between the two
signals will be 360*f/N. Someone know under which conditions this is true
and how this uncertainty can be minimized?

Next question is, suppose the signals contains harmonics with order n,
will then the uncertainty in phase angle between the harmonic be
360*n*f/N?

The background is I try evalute high frequency powers from sampled data of
V and I. The problems are negative powers and strange phase shifts etc..

Someone can explain if zero-point detection and PLL can remove the
uncertainties in FFT and sampling?