3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
point was gathered, relative to the start point. Each trace is time stamped as it
starts, and this time is remembered for each trace. As successive traces appear their
start times get successively larger, relative to the start time of the oldest trace. If a
marker is placed on the live trace and its readout is set to Time, the time of this
marker will increment by about the sweep time for every new sweep. See the
diagram below for a graphical representation of how this appears:
Each trace point has a time value; the value of the start time of the trace is
accurately time stamped, but each point within the trace is the start value plus the
proportion of sweep time represented by that position in the trace. This means the
time value of the points within a trace is not as accurate as the start point, which is
actually the case even in the Normal view, when you use a Time readout for markers
in the frequency domain. This problem is particularly acute with "Sweep Type" on
page 491 set to FFT, since the calculated nominal FFT sweep time estimate can be
off by a large percentage. Therefore, in FFT sweeps, to prevent overlaps of time on
traces, and to make Sweep Type FFT consistent with Swept, the end time for each
trace is calculated to yield a continuous functional Z axis time value for each position
on the trace. Since any inaccuracies within each trace are therefore reconciled with
the start of the next sweep, you can consider the time values along a trace to be
accurate enough for the purpose of making delta time measurements between
traces.
198 Spectrum Analyzer Mode User's &Programmer's Reference
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