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Making Measurements
Example 11: Time-Gated Measurement
2. Set analyzer sweep time greater than 601 times PRI (pulse
repetition interval), or longer if MEAS UNCAL appears on the
screen.
To ensure that the gate is on at least once during each of the 601
digital trace points on the spectrum analyzer, you may need to
increase the sweep time of the analyzer. In Figure 2-74, the PRI is 5
ms, so you should set the sweep time to at least 601 times 5 ms, or
3005 ms (3.1 s). Because the next-higher available sweep time
setting is 3.1 s, the sweep time should be set to 3.1 s in this example.
If the sweep time is too fast, some trace points may show values of
zero or other incorrectly low readings. If the signal has an
intermittent or variable repetition interval, use the longest PRI for
calculating sweep time. If the trace seems incomplete or erratic, try
a longer sweep time. You should leave the
MAN, so that the sweep time that you have selected will be retained.
3. Locate the signal under test on the display of the spectrum analyzer.
Set
CENTER FREQ
characteristics that you are interested in measuring. Although the
analyzer is not yet configured for correct gated measurements, you
will want to determine the approximate frequency and span in
which to display the signal of interest. If the signal is erratic or
intermittent, you may want to press
determine the frequency of peak energy.
Set SPAN narrow enough to show the signal characteristics that 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, or if you just wanted to
measure peak amplitude during the gate, you could leave the span
wider.
4. Turn on the gate pressing
Adjust EDGE POL POS NEG, GATE DLY, and GATE LEN as
necessary. Determine the setup time and signal delay for
time-gating.
Generally, you should position the gate 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. Generally, you should maximize the setup time
(SUT). Signal delay (SD) is the delay inherent in the signal; that is,
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 2-75.
144
and
so that you can view the signal
SPAN
TRACE
GATE ON OFF
SWEEP AUTO MAN
,
to
MAX HOLD A
so that ON is underlined.
Chapter 2
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