User`s guide

7-31

Operating Concepts

Analyzer Display Formats

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, ∆ Φ over a speciﬁed frequency aperture, ∆ f, to obtain an approximation for

the rate of change of phase with frequency. Refer to Figure 7-16. This value, (τ)

represents the group delay in seconds assuming linear phase change over ∆f. It is

important that ∆Φbe ≤ 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.

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 Figure 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.