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Phase Linearity
 
In an ideal transmission medium of "infinite" bandwidth such as free space or (in most cases) a well-terminated coaxial transmission line, the phase of a signal passing through the transmission path is directly related to the signal frequency and the propagation velocity. A plot of frequency vs. phase would result in a straight line with a slope dependent on the propagation velocity and effective electrical length of the path. In this ideal case, the phase delay term defined by equation 1 on the following page would remain constant regardless of the signal frequency.

When a single-frequency signal is applied to a filter, amplifier or other device with limited bandwidth, the phase delay becomes frequency-sensitive. The plot of phase vs. frequency is no longer a straight line and the phase delay (tf) would vary as the frequency is changed.

Phase linearity can be expressed as the maximum deviation from the ideal straight line phase vs. frequency plot which would be produced by an ideal transmission line of similar electrical length, or simply by reference to a tabular listing of phase deviation at a number of discrete frequencies.

In most cases, the value of knowing the phase deviation for the single-frequency signal passing through a device is limited, since amplifiers are generally called upon to process signals consisting of many frequency components such as modulated or keyed carriers.

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this page last updated: 1 October 1999