Agilent Technologies 8719D User Manual page 312

The analyzer computes group delay from the phase slope. Phase data is used to lind the phase
change, A4, over a specified frequency aperture, Af, to obtain an approximation for the rate of
change of phase with frequency (see Figure 6-22). This value, us, represents the group delay in
seconds assuming linear phase change over Af. It is important that Aq5 be <l&W', or errors will
result in the group delay data. These errors can be significant for long delay devices. You can
verify that A$ is s180° by increasing the number of points or narrowing the frequency span (or
both) until the group delay data no longer changes.
Figure 6-22. Rate of Phase Change Versus Frequency
When deviations from linear phase are present, changing the frequency step can result in
different values for group delay. Note that in this case the computed slope varies as the
aperture Af is increased (see Figure 6-23). A wider aperture results in loss of the fine grain
variations in group delay. This loss of detail is the reason that in any comparison of group delay
data, it is important to know the aperture that was used to make the measurement.
Application and Operation Concepts
Figure 6-23. Variations in Frequency Aperture