7-31
Operating Concepts
Analyzer Display Formats
Figure 7-16 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 7-17. 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 7-17 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.