3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
value of (span/sweep points) then the Measure at Marker window will not
display, but instead an advisory message, “Span per point too large, narrow span
or increase RBW or number of points”.This means you have chosen a
combination of RBW, span and sweep points that makes each trace point much
wider than the RBW, so that the trace point in which the signal appears is an
inadequately precise measure of its frequency—for example, with a 30 MHz to
1000 MHz span, 601 trace points and 120 kHz RBW, each trace point is 13 times
as wide as the RBW. In this case, a SCPI query of the results will yield –999 dBm
for each detector
–
If the zoom is successful, the instrument goes to zero span at this frequency.
–
Each detector is then read in successive single-point zero span sweeps, using a
sweep time equal to the specified dwell time. The value displayed by Measure at
Marker represents the maximum value output by the detector during the dwell
timeAutocoupled bandwidth and average type settings are used for each
detector unless the BW & Avg Type control is set to As Set, in which case the
current bandwidth and average type settings are used.
–
Each result is then displayed in the measure at marker window as it becomes
available
–
The instrument returns to its pre-Measure at Marker span and settings after
executing a Measure at Marker function, including Bandwidth, Avg Type, and
EMC Std - regardless of the setting of BW & Avg Type
–
Finally, if the sweep had to be aborted, the aborted sweep is restarted
Zooming
We want to make the "measure at marker" measurements with zero span data
acquisition that is sometimes "zoomed." In this case, the definition of "zoomed" is "at
a frequency within +/–0.5 bucket of the marker frequency, selected to optimize
accuracy, based on the trace on which the marker is placed."
This definition of zoom is different from that used in counter zooming and
preselector centering zooming and tune-and-listen zooming, all of which assume a
reasonably stationary signal. With a stationary signal, zooming via repeatedly
narrowing data acquisitions to find the signal with subbucket resolution is feasible;
with EMC measurements, many signals (for example pulsed RF) are not stationary or
not constant. Furthermore, the reason for zooming in EMC is because the user’s
setup often represents a combination of RBW, span and sweep points that makes
the bucket location of a signal an inadequately precise measure of its frequency (for
example, if the user is looking at one or more full CISPR bands). A commonly used
workaround for not zooming is to use more display points. However, with measure at
marker, the peak detector is not going to do a good job in zero span unless the
tuning is within about 5% of an RBW.
364 Spectrum Analyzer Mode User's &Programmer's Reference
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