FPGA Central

I read somewhere that if sampling two signals with frequency f and
sampling rate of N. Then the uncertainty in phase shift between the tw
signals will be 360*f/N. Someone know under which conditions this is tru
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 b
360*n*f/N?

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

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