In electronic signaling, phase is a definition of the position of a point in time (instant) on a waveform cycle. A complete cycle is defined as 360 degrees of phase as shown in Illustration A below. Phase can also be an expression of relative displacement between or among waves having the same frequency .
Phase difference , also called phase angle , in degrees is conventionally defined as a number greater than -180, and less than or equal to +180. Leading phase refers to a wave that occurs "ahead" of another wave of the same frequency. Lagging phase refers to a wave that occurs "behind" another wave of the same frequency. When two signals differ in phase by -90 or +90 degrees, they are said to be in phase quadrature . When two waves differ in phase by 180 degrees (-180 is technically the same as +180), the waves are said to be in phase opposition . Illustration B shows two waves that are in phase quadrature. The wave depicted by the dashed line leads the wave represented by the solid line by 90 degrees.
Phase is sometimes expressed in radians rather than in degrees. One radian of phase corresponds to approximately 57.3 degrees. Engineers and technicians generally use degrees; physicists more often use radians.
The time interval for one degree of phase is inversely proportional to the frequency. If the frequency of a signal (in hertz ) is given by f , then the time t deg (in seconds) corresponding to one degree of phase is:
t deg = 1 / (360 f )
The time t rad (in seconds) corresponding to one radian of phase is approximately:
t rad = 1 / (6.28 f )