207
Concepts
Time Gating Concepts
which to display the signal of interest. If the signal is erratic or intermittent, you may
want to hold the maximum value of the signal with Max Hold (located under the
Trace/Detector menu) to determine the frequency of peak energy.
To optimize measurement speed in the Gated LO case, set the span narrow enough so
that the display will still show the signal characteristics you want to measure. For
example, if you wanted to look for spurious signals within a 200 kHz frequency
range, you might set the frequency span to just over 200 kHz.
Step 4. Determine the setup time and signal delay to set up the gate signal. Turn on the gate
and adjust the gate parameters including gate delay and gate length as shown below.
Generally, the gate should be positioned over a part of the signal that is stable, not
over a pulse edge or other transition that might disturb the spectrum. Starting the gate
at the center of the pulse gives a setup time of about half the pulse width. Setup time
describes the length of time during which that signal is present and stable before the
gate comes on. The setup time (SUT) must be long enough for the RBW filters to
settle following the burst-on transients. Signal delay (SD) is the length of time after
the trigger, but before the signal of interest occurs and becomes stable. If the trigger
occurs simultaneously with the signal of interest, SD is equal to zero, and SUT is
equal to the gate delay. Otherwise, SUT is equal to the gate delay minus SD. See
Figure 15-17
Figure 15-17 Positioning the Gate
There is flexibility in positioning the gate, but some positions offer a wider choice of
resolution bandwidths. A good rule of thumb is to position the gate from 20 % to
90 % of the burst width. Doing so provides a reasonable compromise between setup
time and gate length.
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