Phase linearity is one way to measure the amount of group delay distortion that a device under test (DUT) introduces. The phase linearity measurement uses a multitone signal to measure the relative delay of each component through the DUT. The multitone test signal must be constructed with zero relative phase between the individual frequency components. In addition, the signal must be constructed with evenly-spaced frequency components given by the following equation:
fi = i * f
This test signal has a high crest factor. You also might use a square wave to measure phase linearity. Square waves are generated easily and have a nominal crest factor of one.
You must construct the multitone signal with frequency components appropriate for the DUT. In general, f must be low enough to measure the group delay that the DUT introduces, as calculated by the following equation:
Aim to use a fundamental frequency, f, such that .
The phase linearity is defined as the maximum deviation from a straight line of the plot of the phase versus frequency. The measurement is performed in steps. First, the multitone signal is generated, passed through the DUT, and acquired on the input channel. The phase of each tone in the multitone signal is measured and plotted versus frequency to yield a graph similar to the one in the following front panel.
The SVT Phase Linearity VI returns the group delay. The group delay is the slope of the best linear fit to the plot of phase versus frequency. Next, the SVT Phase Linearity VI computes the difference between the measured phase and the best linear fit. You can plot this difference against frequency to show the deviation of the measured phase from the best fit line. The following front panel shows this plot.
The maximum absolute value of the phase deviation is the value returned as the phase linearity. In this case the maximum phase deviation value is 0.215 degrees. Phase linearity measurements always are dependent on the bandwidth of the measurement. To completely report the result, specify that the phase linearity is 0.215 degrees with a 20 kHz bandwidth, 1 Vpk test amplitude, and unity gain set for the DUT.