Specifications

136 Chapter 15
Concepts
Time Gating Concepts
Concepts
Gated LO Concepts (PSA Spectrum Analyzers)
Gated LO is a very sophisticated type of time gating that sweeps the LO
only while the gate is “on” and the gate is passing a signal. See Figure
15-7 for a simplified block diagram of gated LO operation. Notice that
the gate control signal controls when the scan generator is sweeping
and when the gate passes or blocks a signal. This allows the analyzer to
sweep only during the periods when the gate passes a signal. Since
gated LO only sweeps during periods when the gate is “on” and the gate
is passing a signal, gated LO results in faster measurements than gated
video.
Figure 15-7 Gated LO PSA Spectrum Analyzer Block Diagram
NOTE Gated LO is available on all PSA models with firmware release A.04.12
or greater and the appropriate hardware. See service notes E4440A-09,
E4443A-08, E4445A-08, E4446A-06 or E4448A-07 for more information
on the hardware requirements.
Gated Video Concepts (ESA Spectrum Analyzers)
Gated video may be thought of as a simple gate switch, which connects
the signal to the input of the spectrum analyzer. When the gate is “on”
(under the Gate menu) the gate is passing a signal. When the gate is
“off,” the gate is blocking the signal. Whenever the gate is passing a
signal, the analyzer sees the signal. In Figure 15-8 notice that the gate
is placed after the envelope detector and before the video bandwidth
filter in the IF path (hence “gated video”).
The RF section of the spectrum analyzer responds to the signal. The
selective gating occurs before the video processing. This means that
there are some limitations on the gate settings because of signal
response times in the RF signal path.