Advantages of Time Gating

In some cases, time-gating capability enables you to perform measurements
that would otherwise be very difficult, if not impossible. For example,
consider Figure 2-33a, which shows a simplified digital mobile-radio
signal in which two radios, # 1 and # 2, are time-sharing a single frequency
channel. Each radio transmits a single 1 ms burst, and then shuts off while
the other radio transmits for 1 ms. The challenge is to measure the unique
frequency spectrum of each transmitter.

Unfortunately, a traditional spectrum analyzer cannot do that. It simply
shows the combined spectrum, as seen in Figure 2-33b. Using the time-gate
capability and an external trigger signal, you can see the spectrum of just
radio # 1 (or radio # 2 if you wished) and identify it as the source of the
spurious signal shown, as in Figure 2-33c.

Figure 2-33a. Simplified digital mobile-radio signal in time
domain

Figure 2-33b. Frequency spectrum of combined signals. Which
radio produces the spurious emissions?

Figure 2-33c. Time-gated spectrum of signal # 1 identifies
it as the source of spurious emission

Figure 2-33d. Time-gated spectrum of signal # 2 shows it is
free of spurious emissions