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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 ultra-wideband ADCs in the Agilent Acqiris
digitizers:
• A much higher sampling rate
• The lack of a hardware decimator, which means the decimation must be
performed by the software from same-length records taken at much higher
sampling rates
• Limited memory capacity
5.1 The 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 1-3-10. The Time tab can then be used to set
Main Time Length up to this maximum limit. This setup keeps Main Time Length
maximized and within the DSP requirements.
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.
5 Time and
Frequency Span
Operating Region