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Concepts
Time Gating Concepts
Figure 15-18 Best Position for Gate
As a general rule, you will obtain the best measurement results if you position the
gate relatively late within the signal of interest, but without extending the gate over
the trailing pulse edge or signal transition. Doing so maximizes setup time and
provides the resolution bandwidth filters of the signal analyzer the most time to settle
before a gated measurement is made. “Relatively late,” in this case, means allowing
a setup time of at least 3.84/resolution bandwidth (see step 5 for RBW calculations).
As an example, if you want to use a 1 kHz resolution bandwidth for measurements,
you will need to allow a setup time of at least 3.84 ms.
Note that the signal need not be an RF pulse. It could be simply a particular period of
modulation in a signal that is continuously operating at full power, or it could even
be during the off time between pulses. Depending on your specific application,
adjust the gate position to allow for progressively longer setup times (ensuring that
the gate is not left on over another signal change such as a pulse edge or transient),
and select the gate delay and length that offer the best representation of the signal
characteristics of interest on the display.
If you were measuring the spectrum occurring between pulses, you should use the
same (or longer) setup time after the pulse goes away, but before the gate goes on.
This lets the resolution bandwidth filters fully discharge the large pulse before the
measurement is made on the low-level interpulse signal.
Figure 15-19 Setup Time for Interpulse Measurement
Step 5. The resolution bandwidth will need to be adjusted for gated LO and gated video. The
video bandwidth will only need to be adjusted for gated video.
Resolution Bandwidth:
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