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Concepts
Time Gating Concepts
The resolution bandwidth you can choose is determined by the gate position, so you
can trade off longer setup times for narrower resolution bandwidths. This trade-off is
due to the time required for the resolution-bandwidth filters to fully charge before the
gate comes on. Setup time, as mentioned, is the length of time that the signal is
present and stable before the gate comes on.
Figure 15-20 Resolution Bandwidth Filter Charge-Up Effects
Because the resolution-bandwidth filters are band-limited devices, they require a
finite amount of time to react to changing conditions. Specifically, the filters take
time to charge fully after the analyzer is exposed to a pulsed signal.
Because setup time should be greater than filter charge times, be sure that:
where SUT is the same as the gate delay in this example. In this example with SUT
equal to 1.5 ms, RBW is greater than 2.56 kHz; that is, RBW is greater than 1333
Hz. The resolution bandwidth should be set to the next larger value, 2.7 kHz.
Video Bandwidth:
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.
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