Specifications

Chapter 15 139
Concepts
Time Gating Concepts
Concepts
• The gate trigger signal (a periodic TTL level signal).
• The gate signal. This TTL signal is low when the gate is "off"
(blocking) and high when the gate is "on" (passing).
The timing interactions between the three signals are best understood
if you observe them in the time domain (see Figure 15-10).
The main goal is to measure the spectrum of signal 1 and determine if it
has any low-level modulation or spurious signals.
Because the pulse trains of signal 1 and signal 2 have almost the same
carrier frequency, their spectra overlap. Signal 2 will dominate in the
frequency domain due to its greater amplitude. Without gating, you
won't see the spectrum of signal 1; it is masked by signal 2.
To measure signal 1, the gate must be on only during the pulses from
signal 1. 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 15-10, 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 15-10 Timing Relationship of Signals During Gating
Once the spectrum analyzer is set up to perform the gate measurement,
the spectrum of signal 1 is visible and the spectrum of signal 2 is
excluded, as shown if Figure 15-12. In addition, when viewing signal 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.