BK Precision 2120C, Instruction Manual, Page: 24 / 30

APPENDIX
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 meas-
ured and the oscilloscope’s own risetime. The two rise times
are combined in square law addition as follows:
T
observed
=
The effect of the oscilloscope’s rise time is almost negli-
gible 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 progres-
sively as the pulse rise time approaches that of the oscillo-
scope. Accurate measurements can still be obtained by
calculation as described below.
Direct Measurements
The Models 2125 C and 2120 C oscilloscopes have a rated
rise time of 12 ns. Thus, pulse rise times of about 36ns or
greater can be measured directly. Most fast rise times are
measured at the fastest sweep speed and using X10 magni-
fication. For the Models 2125
C and 2120 C , this sweep rate
is 10 ns/div. A rise time of less than about four divisions at
this sweep speed should be calculated.
Calculated Measurements
For observed rise times of less than 36 ns, the pulse rise
time should be caluclated to eliminate the error introduced
by the cascaded oscilloscope rise time. Calculate pulse rise
time as follows:
T
pulse
=
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 16 ns minimum for the Models 2125
C and
2120 C .
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.
(T ) + (T )
pulse
2
scope
2
(T ) + (T )
observed
2
scope
2
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