Group Delay Aperture (Trace Format)
The value of (under ) is used when the trace format is set to Group Delay. The aperture is set as a percentage of the current frequency span for frequency-domain data and is set as a percentage of the time-record length for time-domain data.
When group delay is calculated for a given point (which can be a time- or frequency-domain point), the aperture is centered at that point. Larger apertures decrease resolution but they increase the smoothing of the group-delay trace.
The point plotted for group delay is located between the data points used to calculate it. For example, in the frequency domain, the group delay for 100 Hz may be calculated by measuring the change in phase between 90 and 110 Hz. If a start frequency of 90 Hz had been specified, 100 Hz would be the first point with group delay data. This results in a trace that does not extend to the edges of the screen (more noticeable as the delay aperture increases).
The smallest aperture that can be selected depends on the number of frequency points. If an invalid aperture is selected, the VSA automatically selects the smallest valid aperture.
The following table shows the size of the group-delay aperture in number of bins. For example, if the number of frequency points is 401 and the aperture is 16% of span, the size of the aperture is 64 bins.
|
Number of Frequency Points |
|||||||
|---|---|---|---|---|---|---|---|
|
Aperture |
51 |
101 |
201 |
401 |
801 |
1601 |
3201 |
|
0.125 % |
2 |
2 |
2 |
2 |
2 |
2 |
4 |
|
0.25 % |
2 |
2 |
2 |
2 |
2 |
4 |
8 |
|
0.5 % |
2 |
2 |
2 |
2 |
4 |
8 |
16 |
|
1.0 % |
2 |
2 |
2 |
4 |
8 |
16 |
32 |
|
2.0 % |
2 |
2 |
4 |
8 |
16 |
32 |
64 |
|
4.0 % |
2 |
4 |
8 |
16 |
32 |
64 |
128 |
|
8.0 % |
4 |
8 |
16 |
32 |
64 |
128 |
256 |
|
16.0 % |
8 |
16 |
32 |
64 |
128 |
256 |
512 |
|
Aperture |
6401 |
12801 |
25601 |
51201 |
102401 |
|---|---|---|---|---|---|
|
0.125 % |
8 |
16 |
32 |
64 |
128 |
|
0.25 % |
16 |
32 |
64 |
128 |
256 |
|
0.5 % |
32 |
64 |
128 |
256 |
512 |
|
1.0 % |
64 |
128 |
256 |
512 |
1024 |
|
2.0 % |
128 |
256 |
512 |
1024 |
2048 |
|
4.0 % |
256 |
512 |
1024 |
2048 |
4096 |
|
8.0 % |
512 |
1024 |
2048 |
4096 |
8192 |
|
16.0 % |
1024 |
2048 |
4096 |
8192 |
16384 |
Group Delay Aperture and Trace Blanking
When the trace format is set to , the VSA blanks a portion of the data at the beginning of the trace and at the end of the trace. The width of the blanked area is one-half the value of the setting. For example, if is set to 10%, the first 5% and the last 5% of the trace will be blanked.
Group Delay and Segmented Time Data
When analyzing segmented time data,note that the Group Delay calculation is reset at the beginning of each segment.
See Also
