China Electronics Technology Instruments AV3672 Series, User Manual

Page: 259 / 424

8 Basis of Network Measurement
8.5 Group Delay
255
1) Calculate the phase change (-dΦ) according to the phase data.
2) Use the specified frequency aperture as the frequency change (dω).
3) Using the above two values, calculate the approximate value of the phase change rate over the frequency. The
approximate value is the group delay (assuming that the phase changes linearly within the specified frequency
aperture).
8.5.2 Why to measure the group delay?
The group delay is more accurate than the linear phase deviation to represent the phase distortion, as shown in Fig.
8.13.
Fig. 8.13 Comparison of Group Delay and Linear Phase Deviation
1) The upper half part of the figure shows the measurement results of the linear phase deviation of Device 1 and 2.
Although the trace shape varies, the linear phase deviations of both devices are identical.
2) The lower half part of the figure shows the measurement results of the group delay of Device 1 and 2. The group
delay of Device 1 is different from that of Device 2, as it is calculated by the analyzer according to the phase ripple
slope. The phase ripple slope depends on the ripple quantity within the unit frequency.
8.5.3 What is the aperture of group delay?
The analyzer will measure the phase of two adjacent frequency points and calculate the phase slope in group delay
measurement. The frequency interval (frequency interval) between two phase measurement points is referred to as
the aperture. Different group delay values can be obtained by changing the aperture. This is why the aperture of
measurement must be known before comparison of group delay data.
The default aperture of group delay of the analyzer is the frequency interval of two adjacent sweep points. Apply
the following two methods to change the aperture:
1) Change the number of measurement points or the frequency span.
a) Increase the number of points or reduce the frequency span to narrow the aperture.
b) Decrease the number of points or increase the frequency span to widen the aperture.
Fig. 8.14 Influence of Group Delay Aperture on Measurement Results
Attention