Specifications

146 Chapter 15
Concepts
Time Gating Concepts
Concepts
Reducing the video-bandwidth filter too fast causes the signal to appear
to drop in amplitude on the screen.
If you are in doubt about the proper video bandwidth to choose, set it to
its maximum and reduce it gradually until the detected signal level
drops slightly. Then reset it to the value it was at just before the signal
dropped.
Leave both RBW and VBW in the manual mode, not Auto. This is
important so that they will not change if the span is changed. The
setting readout on the bottom line of the analyzer screen should show a
"#" sign next to the function names (for example, #Res BW, #VBW, and
#Sweep), indicating that they have been set manually.
Setting the ESA VBW:
To ensure that a true peak value is obtained before the gate goes off, the
video filter must have a charge time of less than the gate length. For
this purpose, you can approximate the charge time of the video filter as
1/VBW, where VBW is the
−3 dB bandwidth of the video filter.
Therefore, you will want to be sure that:
For example using ESA, if you use a 1 kHz video bandwidth for noise
smoothing, you need a gate length greater than 1 ms. Alternatively, if
you use a gate as narrow as 1
µs, you should use a video filter of 1 MHz.
Setting the PSA VBW:
For gated LO measurements the VBW filter acts as a track-and-hold
between sweep times. With this behavior, the VBW does not need to
resettle on each restart of the sweep.
Step 6. Adjust span as necessary, and perform your measurement.
The analyzer is set up to perform accurate measurements. Freeze the
trace data by activating single sweep, or by placing your active trace in
view mode. Use the markers to measure the signal parameters you
chose in step 1. If necessary, adjust span, but do not decrease resolution
bandwidth, video bandwidth, or sweep time.
gate length
1
ESA
()
VBW
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