Technical data

5-21
Operating Concepts
Analyzer Display Formats
Figure 5-15. Rate of Phase Change Versus Frequency
When deviations from linear phase are present, changing the frequency step can result in different values
for group delay. Note that in this case the computed slope varies as the aperture
∆f is increased. See
Figure 5-16 on page 5-21. A wider aperture results in loss of the fine grain variations in group delay. This
loss of detail is the reason that in any comparison of group delay data, it is important to know the aperture
that was used to make the measurement.
Figure 5-16. Variations in Frequency Aperture
In determining the group delay aperture, there is a trade-off between resolution of fine detail and the
effects of noise. Noise can be reduced by increasing the aperture, but this will tend to smooth out the fine
detail. More detail will become visible as the aperture is decreased, but the noise will also increase, possibly
to the point of obscuring the detail. A good practice is to use a smaller aperture to assure that small
variations are not missed, then increase the aperture to smooth the trace.
The default group delay aperture is the frequency span divided by the number of points across the display.
To set the aperture to a different value, turn on smoothing in the average menu, and vary the smoothing
aperture. The aperture can be varied up to 20% of the span swept.
Group delay measurements can be made on linear frequency, log frequency, or list frequency sweep types