Agilent Technologies 24A Guide de l'utilisateur télécharger pdf (Page 375)

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Operating Concepts

Analyzer Display Formats

Imaginary Format

The softkey displays only the imaginary (reactive) portion of the measured

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.

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