BK Precision 2160A User Manual

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 Model 2160A oscilloscope has a rated risetime of 5.8
ns (8.8 ns for Model 1541D). Thus, pulse rise times of about
17 ns or greater can be measured directly (26 ns for Model
1541D). Most fast rise times are measured at the fastest
sweep speed and using X10 magnification. For the Models
2160A and 1541D, this sweep rate is 10 ns/div. A rise time
of less than about two divisions at this sweep speed should
be calculated for Model 2160A (three divisions for Model
1541D).
Calculated Measurements
For observedrise times of lessthan 17 ns (26 ns for Model
1541D), 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 7.5 ns minimum for the Model 2160A and 12 ns
for Model 1541D.
Probe Considerations
Forfast risetime measurementswhich approachthe 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 alsocascaded 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
22

BK Precision 2160A Specifications

General

Brand	BK Precision
Model	2160A
Category	Test Equipment
Language	English