Technical data

5-20
Operating Concepts
Analyzer Display Formats
Figure 5-13. 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 5-14 on page 5-20.
Figure 5-14. Higher Order Phase Shift
The analyzer computes group delay from the phase slope. Phase data is used to find the phase change,
∆ Φ
over a specified frequency aperture,
∆ f, to obtain an approximation for the rate of change of phase with
frequency. Refer to Figure 5-15. This value, (
τ)
g
, represents the group delay in seconds assuming linear
phase change over
∆f. It is important that ∆ Φ be ≤ 180°, or errors will result in the group delay data. These
errors can be significant for long delay devices. You can verify that
∆ Φ is ≤ 180° by increasing the number
of points or narrowing the frequency span (or both) until the group delay data no longer changes.