Time Gating

Figure 2-32a. Video filtering

Figure 2-32b. Trace averaging

Figure 2-32. Video filtering and trace averaging yield different results on FM broadcast signal

Time gating
Time-gated spectrum analysis allows you to obtain spectral information
about signals occupying the same part of the frequency spectrum that are
separated in the time domain. Using an external trigger signal to coordinate
the separation of these signals, you can perform the following operations:

Measure any one of several signals separated in time; for example, you
can separate the spectra of two radios time-sharing a single frequency
Measure the spectrum of a signal in one time slot of a TDMA system
Exclude the spectrum of interfering signals, such as periodic pulse edge
transients that exist for only a limited time

Why time gating is needed
Traditional frequency-domain spectrum analysis provides only limited
information for certain signals. Examples of these difficult-to-analyze signals
include the following signal types:
Pulsed RF
Time multiplexed
Time domain multiple access (TDMA)
Interleaved or intermittent
Burst modulated