Agilent Technologies 8719D, User Manual

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Group Delay Principles
For many networks, the amount of insertion phase is not as important as the linearity of the
phase shift over a range of frequencies. The analyzer can measure this linearity and express
it in two different ways: directly, as deviation from linear phase, or as group delay, a derived
value.
Group delay is the measurement of signal transmission time through a test device. It is defined
as the derivative of the phase characteristic with respect to frequency. Since the derivative
is basically the instantaneous slope (or rate of change of phase with respect to frequency), a
perfectly linear phase shift results in a constant slope, and therefore a constant group delay
(see Figure 6-20).
A
Phase
Figure 6-20. Constant Group Delay
Note, however, that the phase characteristic typically consists of both linear and higher
order (deviations from linear) components. The linear component can be attributed to the
electrical length of the test device, and represents the average signal transit time. The higher
order components are interpreted as variations in transit time for different frequencies, and
represent a source of signal distortion (see Figure 6-21).
Figure 6-21. Higher Order Phase Shift