HP 54750A, User Manual, Page: 250 / 478

Increasing Measurement Accurac y
and Time-Interval Measurement
Time-Interval Measurements
Y ou can extend this concept to channel-to-channel time-interval
measurements between edges of opposite polarity . Measuring the time
interval between two closely spaced edges of opposite polarity on two
channels is equivalent to measuring the width of a narrow pulse as shown in
Figure 9-8 .
Figure 9-8.
Error in pulse width at 10% level is approximately equal to the error in 10%{90% rise time.
Probe capacitive
loading
The capacitance of the probe tip to ground forms an RC circuit with the
output resistance of the circuit under test. The time constant of this RC
circuit will slow the rise time of any transitions , increase the slew rate , and
introduce delay in the actual time of transitions . The approximate rise time of
a simple RC circuit is 2.2RC. Thus , for an output resistance of 100 W and a
probe tip capacitance of 8 pF , the real rise time at the node under test cannot
be faster than approximately 1.8 ns . Without the probe , the rise time might
be faster .
9-18