Chapter 2 129
Making Measurements
Example 11: Time-Gated Measurement
Example 11: Time-Gated Measurement
What Is Time-Gating?
Traditional frequency-domain spectrum analysis provides only limited
information for certain signals. Examples of these difficult-to-analyze
signal include the following signal types:
• Pulsed RF
• Time multiplexed
• Time domain multiple access (TDMA)
• Interleaved or intermittent
• Burst modulated
In some cases, time-gating capability enables you to perform
measurements that would otherwise be very difficult, if not impossible.
For example, consider Figure 2-58, which shows a simplified digital
mobile-radio signal in which two radios, number 1 and number 2, are
time-sharing a single frequency channel. Each radio transmits for 1 ms.
Figure 2-58 Simplified Digital Mobile-Radio Signal in Time Domain
What if you wanted to measure the unique frequency spectrum of each
transmitter? Unfortunately, a traditional spectrum analyzer cannot
perform that measurement. By the time the spectrum analyzer has
completed its measurement sweep, which lasts about 50 ms, the radio
transmissions will have switched back and forth 25 times. Because the
radios are both transmitting at the same frequency, their frequency
spectra will overlap, as shown in Figure 2-59. The spectrum analyzer
shows the combined spectrum; you will not be able to tell which part of
the spectrum results from which signal.
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