Agilent Technologies 8719D User Manual page 313

In determining the group delay aperture, there is a tradeoff 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. 'Ib 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 (not in CW or power sweep). Group delay aperture varies depending on the
frequency spacing and point density, therefore the aperture is not constant in log and list
frequency sweep modes. In list frequency mode, extra frequency points can be defined to
ensure the desired aperture.
To obtain a readout of aperture values at different points on the trace, turn on a marker. Then
aperture is varied its value in Hz is displayed below the active entry area.