Imaginary Format; Group Delay Principles - Agilent Technologies 8753ET User Manual

Operating Concepts
Analyzer Display Formats

Imaginary Format

The softkey displays only the imaginary (reactive) portion of the measured data on a
IMAGINARY
Cartesian format. This format is similar to the real format except that reactance data is displayed on the
trace instead of resistive data.

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 7-14.
Figure 7-14 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 7-15.
7- 32