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38
Rise Time
In the digital world, rise time measurements are critical. Rise time may
be a more appropriate performance consideration when you expect to
measure digital signals, such as pulses and steps. Your oscilloscope
must have sufficient rise time to accurately capture the details of
rapid transitions.
Rise time
describes the useful frequency range of an oscilloscope.
To calculate the oscilloscope rise time required for your signal type, use
the following equation:
Note that this basis for oscilloscope rise time selection is similar to that for
bandwidth. As in the case of bandwidth, achieving this rule of thumb may
not always be possible given the extreme speeds of today’s signals.
Always remember that an oscilloscope with faster rise time will more
accurately capture the critical details of fast transitions.
In some applications, you may know only the rise time of a signal.
A constant allows you to relate the bandwidth and rise time of the
oscilloscope, using the equation:
Some logic families produce inherently faster rise times than others, as
illustrated in Figure 49.
XYZs of Oscilloscopes
Primer
Typical Signal
Calculated
Rise Time
Signal
Logic Family
Bandwidth
TTL
2 ns
175 MHz
CMOS
1.5 ns
230 MHz
GTL
1 ns
350 MHz
LVDS
400 ps
875 MHz
ECL
100 ps
3.5 GHz
GaAs
40 ps
8.75 GHz
Figure 49.
Some logic families produce inherently faster rise times
than others.
Figure 48.
Rise time characterization of a high-speed digital signal.
Oscilloscope Rise Time Required =
Fastest Rise Time of Measured Signal ÷ 5
Bandwidth = k
Rise Time
where k is a value between 0.35 and 0.45, depending
on the shape of the oscilloscope’s frequency response
curve and pulse rise time response. Oscilloscopes
with a bandwidth of <1 GHz typically have a 0.35 value,
while oscilloscopes with a bandwidth >1 GHz usually
have a value between 0.40 and 0.45.
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