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APPENDIX I
IMPORTANT CONSIDERATIONS FOR RISE TIME AND FALL TIME MEASUREMENTS
Error in Observed Measurement
The observed rise time (or fall time) as seen on the
CRT is actually the cascaded rise time of the pulse being
measured and the oscilloscope’s own risetime. The two
rise times are combined in square law addition as follows:
2 2
T
observed = ( T pulse) + ( T scope)
The effect of the oscilloscope’s rise time is almost
negligible when its rise time is at least 3 times as fast as
that of the pulse being measured. Thus, slower rise times
may be measured directly from the CRT. However, for
faster rise time pulses, an error is introduced that
increases progressively as the pulse rise time approaches
that of the oscilloscope. Accurate measurements can still
be obtained by calculation as described below.
Direct Measurements
The Model 2522C has a rated rise time of 18 ns. Thus,
pulse rise times of about 54 ns or greater can be
measured directly. Most fast rise times are measured at
the fastest sweep speed and using X10 magnification. This
sweep rate is 10 ns/div. A rise time of less than about five
divisions at this sweep speed should be calculated.
Calculated Measurements
For observed rise times of less than 54 ns, the pulse rise time should
be calculated to eliminate the error introduced by the cascaded
oscilloscope rise time. Calculate pulse rise time as follows:
2 2
T
pulse = ( T observed) + ( T scope)
Limits of Measurement
Measurements of pulse rise times that are faster than the scope’s
rated rise time are not recommended because a very small reading
error introduces significant error into the calculation. This limit is
reached when the “observed” rise time is about 1.3 times greater than
the scope’s rated rise time, about 23 ns minimum for the Model
2522C.
Probe Considerations
For fast rise time measurements which approach the limits of
measurement, direct connection via 50
Ω
coaxial cable and 50
Ω
termination is recommended where possible. When a probe is used,
its rise time is also cascaded in square law addition. Thus the probe
rating should be considerably faster than the oscilloscope if it is to be
disregarded in the measurement.
