Specifications

Chapter 15 147
Concepts
Time Gating Concepts
Concepts
"Quick Rules" for Making Time-Gated Measurements
This section summarizes the rules described in the previous sections.
Table 15-1 Determining Spectrum Analyzer Settings for Viewing a Pulsed
RF Signal
Spectrum
Analyzer
Function
Spectrum Analyzer Setting Comments
Sweep Time
(gated video
only - ESA)
Set the sweep time to be equal
to or greater than
(number of sweep points - 1)
×
pulse repetition interval (PRI):
Because the gate must be on at least
once per trace point, the sweep time
should be set such that the sweep time
for each trace point is greater than or
equal to the pulse repetition interval.
Gate Delay The gate delay is equal to the
signal delay plus one-fourth the
pulse width:
Gate Delay = Signal Delay +
τ/4
The gate delay must be set so that the
gating captures the pulse. If the gate
delay is too short or too long, the gating
can miss the pulse or include resolution
bandwidth transient responses.
Gate Length The gate length minimum is
equal to one-fourth the pulse
width (maximum about
one-half):
Gate Length =
τ/4
(PSA) Gate Length > 3/RBW +
1.5 µs
If the gate length is too long, the signal
display can include transients caused by
the spectrum analyzer filters.
The recommendation for gate placement
can be between 20% to 80% of the pulse
for PSA and 25% to 80% of the pulse for
ESA.
Resolution
Bandwidth
Set the resolution bandwidth:
(ESA) RBW > 2/(Gate Delay
−
Signal Delay)
(PSA) RBW > 2.16/(Gate Delay
− Signal Delay) + 3.3 µs
The resolution bandwidth must be wide
enough so that the charging time for the
resolution bandwidth filters is less than
the pulse width of the signal.
Video
Bandwidth
Set the video bandwidth:
(ESA) VBW > 1/gate length
The video bandwidth must be wide
enough so that the rise times of the video
bandwidth does not attenuate the signal
(in gated video applications). There are
no requirements for PSA VBW settings
using gated LO.