Consider measuring the rise time of a signal that has an actual rise time of
1 ns. If you use an oscilloscope and probe that have a combined rise time of
1 ns, the error will be approximately 40 percent:
Measured
rise
time
=
√
(
1
ns
)
2
+
(
1
ns
)
2
=
1.41
ns
If instead you measure the same signal with an oscilloscope and probe with a
combined rise time of 330 ps, the error will be reduced to approximately 5
percent:
Measured
rise
time
=
√
1
ns
)
2
+
(
330
ps
)
2
=
1.05
ns
Table 13-2 shows the error for other ratios of oscilloscope rise time to signal
rise time. Table 13-3 shows the approximate error in measuring rise times
with various plug-in and probe combinations with the HP 54720.
Table 13-2
Rise-Time Measurement Errors
Signal rise time/
Oscilloscope rise time
Measurement
error percent
140
212
35
43
52
61.4
71
Measurements
Time-interval measurements
13–39
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