Specifications

Chapter 15 145
Concepts
Time Gating Concepts
Concepts
Step 5. The resolution bandwidth will need to be adjusted for gated LO and
gated video. The video bandwidth will only need to be adjusted for gated
video.
Resolution Bandwidth:
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 15-19 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: and
where SUT is the same as the gate delay in this example. In this
example with SUT equal to 1.5 ms, for ESA, RBW is greater than 2/1.5
ms; that is, RBW is greater than 1333 Hz. The resolution bandwidth
should be set to the next larger value, 3 kHz.
Video Bandwidth:
Just as the resolution bandwidth filter needs a finite amount of time to
charge and discharge, so does the video filter, which is a post-detection
filter used mainly to smooth the measurement trace. Regardless of the
length of the real RF pulse, the video filter sees a pulse no longer than
the gate length, and the filter will spend part of that time charging up.
ESA()SUT
2
RBW
-------------
> PSA()SUT
2.16
RBW
------------- 3.3µs+>
PSA()Gate Length
3
RBW
------------- 1.5µs+>