5 Time and
Frequency Span
Operating Region
The operating region curves for the broadband VSAs are different from those of standard VSAs because the Main Time Length may be limited for some sample rate, center frequency, and span combinations. The differences are due to three hardware characteristics of the
•A much higher sampling rate
•The lack of a hardware decimator, which means the decimation must be performed by the software from
•Limited memory capacity
5.1The relationship of main time length, span, and sample rate
The record length or Main Time Length of a measurement is shown in the lower right corner of a spectrum or time display window. This is the length of time the digitizer sampled the input waveform for the measurement. These samples are transferred to the DSP and measurement algorithms running on the software.
Maximum Main Time Length is normally adjusted by setting Number of Frequency Points in the MeasSetup/ResBW menu ResBW Coupling is set to Auto, and ResBW mode is set to Arbitrary or
Although Main Time Length can be adjusted directly by the MeasSetup/Time menu, it cannot be greater than the limit imposed by the Number of Frequency Points line on the operating region chart. See the online Help tool for a complete discussion.
Note: The maximum Main Time Length can be found as follows: The relationship between the parameters is shown below:
Max Main Time Length = Number of frequency points
Span
where Span = Sample rate 1.28
Max Main Time Length = 1.28 x Number frequency points Sample rate
where sample rate = current sample rate computed by the software for alias- free measurements
Maximum Main Time Length may be further limited by digitizer memory depth.
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