Chapter 2 133
Making Measurements
Example 11: Time-Gated Measurement
Because the pulse trains of signal number 1 and signal number 2 have
almost the same carrier frequency, their frequency-domain spectra
overlap. Further, the spectrum of pulse train number 2 dominates
because signal number 2 has greater amplitude. Without gating, you
won't see the spectrum of pulse train number 1; it is masked by pulse
train number 2.
To measure pulse train number 1, the gate must be on only during
those pulses. The gate will be off at all other times, thus excluding all
other signals. To position the gate, set the gate delay and gate length,
as shown in Figure 2-63, so that the gate is on only during some central
part of the pulse. Carefully avoid positioning the gate over the rising or
falling pulse edges. When gating is activated, the gate output signal
will indicate actual gate position in time, as shown in the line labeled
"Gate."
Figure 2-63 Timing Relationship of Signals During Gating
Now that you've set up the spectrum analyzer to perform the gate
measurement, you'll see, as shown if Figure 2-65, the spectrum within
the pulses of pulse train number 1 only; the spectrum of pulse train
number 2 is excluded. In addition, when viewing pulse train number 1,
you also will have eliminated the pulse spectrum generated from the
pulse edges. Gating has allowed you to view spectral components that
otherwise would be hidden. (See the following section, “Stepping
through a Time-Gated Measurement” for the detailed measurement
procedure.)
Figure 2-64 Spectrum within pulse #1
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