Group Delay - Keysight U8903B User Manual

6 Sweep Function

Group Delay

Group delay is the difference of phase over the difference of the frequency. All signals are
delayed when transiting through a device such as a filter, an amplifier, or a processor. This
small delay is usually not a problem, but if the delay is different for different frequencies,
and the signal is built up by more than one frequency, then the shape of the signal is
distorted. The difference in delay for different frequencies is group delay.
Group delay is a useful measure of phase distortion. The variations in a group delay cause
signal distortion, just as deviations from linear phase cause distortion. Group delay is
calculated by differentiating the insertion phase response of the DUT versus frequency.
Group delay is only meaningful when performing sweep.
The input signal is compared with the output signal to calculate the phase between the
input and output signals. The phase for frequency 1 is then recorded. The output frequency
is then shifted to frequency 2, and the phase is measured again. The result is a group delay
value of (phase 1 – phase 2) / (frequency 1 – frequency 2). This measurement can be done
for a series of frequency points to plot the group delay measurement graph which will show
the linearity of the phase versus frequency plot.
Group Delay measurement function mode is only applicable for digital interface.
Figure 6-2
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Sweep Parameter > App. Type > Group Delay menu page
Keysight U8903B User's Guide