Agilent Technologies 8753ET User Manual page 401

Figure 7-15 Higher Order Phase Shift
The analyzer computes group delay from the phase slope. Phase data is used to find the phase change,
over a specified frequency aperture, f, to obtain an approximation for the rate of change of phase with
frequency. Refer to Figure
phase change over f. It is important that  be 180, or errors will result in the group delay data. These
errors can be significant for long delay devices. You can verify that  is 180 by increasing the number of
points or narrowing the frequency span (or both) until the group delay data no longer changes.
Figure 7-16 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 f is increased. See
7-17. 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.
7-16. This value, () , represents the group delay in seconds assuming linear
g
Operating Concepts
Analyzer Display Formats
Figure
7- 33