Agilent Technologies 8510C Operating And Programming Manual page 265

Comparing aperture, resolution, and noise.
aperture is increased. A wider aperture results in loss of the ne grain variations in group
delay. This loss of detail is the reason that in any comparison of group delay data you must
know the aperture used to make the measurement.
In selecting the aperture, there is a tradeo between resolution of ne detail and the e ects of
noise. The e ects of noise can be reduced by increasing the aperture, however, this will tend
to smooth out the ne detail. In decreasing the aperture, more ne detail will become visible
but the noise will also increase, possibly to the point of obscuring the detail.
Measuring aperture and phase slope.
or phase slope characteristic of the test device must be considered. To maintain group delay
resolution uncertainty below 1 percent, use an aperture which results in a phase change of at
least 1 degree.
Using aperture and smoothing.
measurement. For example, with smoothing o , group delay is computed using the phase
change between each frequency step. With smoothing on, the phase change over the selected
percent of sweep is used to compute group delay.
The two LCD/CRT display plots in Figure 9-13 show the e ect of increasing the aperture.
Suggestion
Measuring Deviation from Linear Phase Measurement
For coaxial devices, Insertion Phase consists of two components, linear and non-linear. The
linear component can be attributed to the electrical length of the test device and represents
the average transit time. As for group delay, the non-linear phase components, or the
variations from constant group delay, are interpreted as variations in the electrical length,
or transit time, at di erent frequencies and represent a source of signal distortion. This is
also true for dispersive media like waveguide, except that the phase shift attributed to the
electrical length of the device is not linear.
9-12 Transmission Measurements
www.valuetronics.com
For a speci c measurement, the average electrical length
Smoothing is used to change the aperture during the
Use a smaller aperture to assure that ne grain variations are not missed, then
increase the aperture to smooth the trace.
Figure 9-13. Group Delay Plots with Different Aperture Selections
Note that the slope (group delay) varies as the