HP HP 8753E, Руководство пользователя, Страница: 320 / 699

result in the group delay data. These errors can be significant for long delay devices. You can
verify that is by increasing the number of points or narrowing the frequency span (or
both) until the group delay data no longer changes.
a - - -
i
* 1 - - - - -
Figure 6-21. 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 Af is increased (see 6-22). A wider aperture results in loss of the 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.
I I I I I
(a) Overlaid Traces
(b) Split Display
Figure 6-22. Variations in Frequency Aperture
In determining the group delay aperture, there is a tradeoff between resolution of tine 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 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.
Application and Operation Concepts