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Using the Instrument
Setting Up Measurements
Triggering a Measurement
Triggering a measurement synchronizes the start of the sweep to an internally
generated trigger signal. Internal triggering ensures continuously triggered
sweeps with the shortest delay between sweeps. Refer to “Trigger Mode,
Internal” on page 3-96.
In some measurements, the spectrum at a particular time within the modula-
tion period is more important than the average spectrum. Gated triggering can
be used to synchronize the data acquisition portion of the OSA to a gating trig-
ger connected to the rear-panel EXT TRIG IN connector. Gated triggering
requires a TTL-compatible signal with a minimum of 0 Vdc and a maximum of
+5 V.
Gated triggering ignores the spectrum when the trigger input is low. It usually
is used in conjunction with the Max Hold function during several sweeps.
Gated triggering is used to select data samples containing valid information.
When the gating signal is high, the data sample is accepted. When the gating
signal is low, the data sample is replaced by a data point with a value of
–200 dBm. Processing continues according to the functions selected, such as,
video bandwidth, max hold, and so forth.
If the low level exists for longer that the time needed for the grating to move
from one trace point to the next, then the trace will have “gaps”. There are
two ways to eliminate the gaps. You can increase the sweep time to at least:
(1.2–2 times the product trace length)
× (the longest “low level” period)