China Electronics Technology Instruments AV3672 Series, User Manual

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8 Basis of Network Measurement
8.5 Group Delay
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Relationship of the aperture and group delay:
If the aperture is too wide and the phase shift of adjacent frequency points is more than 180°, errors will occur in group
delay calculation.
2) Apply the smoothing function of the analyzer.
Perform sweep once, enable the smoothing function and change the value percentage in smoothing settings. This is
similar to change of the frequency interval between sweep points. This method allows a wider group delay aperture,
and the phase shift is allowed to exceed 180° within the smoothing aperture.
Group delay measurement can be done in the following sweep types:
a) Linear frequency;
b) Logarithmic frequency;
c) Segment sweep.
The group delay aperture depends on the frequency interval and the sweep point density, therefore, it changes in the
logarithmic frequency and segment sweep mode.
8.5.4 Improvement of Measurement Accuracy of Group Delay
It is important to keep the phase difference of adjacent measurement points less than180° during group delay
measurement; otherwise, the phase and group delay information will be incorrect, as shown in Fig. 8.15.
Insufficient sampling may occur in measurement of the device of long electrical delay. Adjust the following
settings until the measurement trace does not change so as to ensure that the phase difference of two adjacent points
is less than 180°.
1) Increase the number of points.
2) Reduce the frequency span.
Fig. 8.15 Influence of Insufficient Sampling on Group Delay Measurement
The frequency response is a main error of group delay measurement. This error can be significantly reduced by
means of through type response calibration. Perform the full double-port calibration to realize higher accuracy. For
the amplifier, the response may vary with the temperature. Therefore, the test should be done at the working
temperature of the amplifier.
8.5.5 Steps of Group Delay Measurement
1) Reset the analyzer. Adjust the source power of the analyzer to test the amplifier.
a) Set the source power of the analyzer within the linear zone of the analyzer response (generally at least 10dB less
than the 1dB input compression point).
b) If required, connect one attenuator to the output port of the amplifier to fully attenuate the output power of the
amplifier and prevent the receiver at the reception end of the analyzer from compression or burning.
2) Connect the DUT, as shown in Fig. 8.16.