150 Chapter2
Making Measurements
Example 11: Time-Gated Measurement
"Rules" for Making a Time-Gated Spectrum
Measurement
This section summarizes the rules described in the previous sections.
Table 2-6 Determining Spectrum Analyzer Settings for Viewing a Pulsed
RF Signal
Spectrum
Analyzer
Function
Spectrum Analyzer Setting Comments
Sweep
Time
Set the sweep time to be 601
times greater than the pulse
repetition interval (PRI):
Sweep time > 601 × PRI
Because the gate must be on at least
once per trace point, the sweep time has
to be set to the pulse repetition interval
times for every point of the trace. (Each
trace has 601 points.)
Gate Delay The gate delay is equal to the
signal delay plus half of the
pulse width:
Gate Delay = Signal Delay + τ/2
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 is equal to
one-fourth the pulse width:
Gate Length = τ/4
If the gate length is too long, the signal
display can include transients caused by
the spectrum analyzer filters.
Video
Bandwidth
Set the video bandwidth to a
value greater than 1 divided by
the gate length:
Video Bandwidth >1/gate length
The video bandwidth must be wide
enough so that the risetimes of the video
bandwidth do not attenuate the signal.
The maximum video bandwidth for
time-gate measurements is 1 MHz.
Resolution
Bandwidth
Set the resolution bandwidth to
a value greater than 2 divided
by the gate delay minus the
signal delay:
Resolution Bandwidth >
2/(Gate Delay - Signal Delay)
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.
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