current sensing systems have ac outputs in which the phase difference is proportional
to an optical polarization angle change, induced by the magnetic field produced by the
current under measurement. Also, phase information is important in the description of
feedback system performance and filter behavior.
Phase measurements can be classified as being done by analog or digital systems.
To illustrate the concept of phase difference, let us define two periodic functions having
the same frequency, but not coincident in time origin: