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The phase difference between two signals of the same frequency can be thought of as a delay or advance in the zero crossing of one signal with respect to another. Consider a graph of a sinusoidal waveform with amplitude on the y or vertical axis and time on the horizontal or x axis. If signals A and B begin at zero, build to a high positive value, fall through zero, build to a high negative value and return to zero at exactly the same time, the signals are of the same frequency and are said to be in phase, i.e. there is no phase difference between them.
Conversely, if there are two signals of the same frequency (same distance between zero crossings on the x axis) but one signal begins earlier and the other one begins at a later point, they are said to be out of phase by some amount, i.e. there is some time difference between the two signals.
Phase difference is expressed in degrees from 0 to 360, or in radians. If the difference is 180 degrees then the two signals are said to be in antiphase: they are equal but opposite, and if added together will sum to zero. If the phase difference is 90 degrees then the signals are said to be in quadrature.
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