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
IMAGINARY
data on a 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.
7-31