Specifications

144 Chapter 15
Concepts
Time Gating Concepts
Concepts
Figure 15-17 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 spectrum analyzer the most time to settle
before a gated measurement is made. "Relatively late," in this case,
means allowing a setup time of approximately 2 divided by the
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 2 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
signal-to-noise ratio 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 15-18 Setup Time for Interpulse Measurement