Chapter 2 147
Making Measurements
Example 11: Time-Gated Measurement
Figure 2-78 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. Although the charge-up is asymptotic,
you can consider it to be complete after the length of time defined by
two divided by resolution bandwidth, where the resolution
bandwidth is the −3 dB resolution bandwidth being used.
Because setup time should be greater than filter charge times, be
sure that:
or
where, in this example, 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.
SUT
2
RBW
-------------
>
RBW
2
SUT
-----------
>
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