HP 54750A, User Manual, Page: 437 / 478

How the Instrument W orks
System Architecture
System bandwidth
The bandwidth of the combined analyzer and probe system must be
sucient to accurately reproduce the input signal. Otherwise , time-interval
measurements are inaccurate . F or example , if the analyzer and probe have
a combined rise time of 1 ns , and the signal also has a 1 ns rise time , the
measured rise time is:
p
(1 ns)
2
+ (1 ns)
2
= 1: 41 ns
The answer is in error by 41 percent.
If the analyzer and probe have a combined rise time of 330 ps , and the signal
has a 1 ns rise time , the measured rise time is:
p
(1 ns)
2
+ (330 ps)
2
= 1: 05 ns
Now the error is only 5 percent.
There are three rules worth memorizing.
1. The combined system rise time (analyzer and probe) should be less than
1/3 the rise time of the measured signal for an error of less than 5 percent,
or less than 1/7 of the rise time of the measure signal for an error of less
than 1 percent.
2. Rise time and bandwidth are inversely related as shown in the preceding
equations .
3. Rise times add approximately as the square root of the sum of the squares .
The wide bandwidth of the analyzer compared to the bandwidth of a typical
probe means that, for most cases , the combined bandwidth can be assumed to
be the same as that of the probe alone .
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