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3000 X-Series Oscilloscopes Advanced Training Guide
B
Oscilloscope Bandwidth Tutorial
Digital Clock Measurement Comparisons
shows the waveform results when measuring a 100 MHz digital
clock signal with fast edge speeds using a 100 MHz bandwidth
oscilloscope. As you can see, this scope primarily just passes through the
100 MHz fundamental of this clock signal, thus representing our clock
signal as an approximate sine wave. A 100 MHz scope may be a good
solution for many 8- bit, MCU- based designs with clock rates in the
10 MHz to 20 MHz range, but 100 MHz bandwidth is clearly insufficient
for this 100 MHz digital clock signal.
Using a 500 MHz bandwidth oscilloscope,
shows that this scope
is able to capture up to the fifth harmonic, which was our first rule of
thumb recommendation. But when we measure the rise time, we see that
the scope measures approximately 750 ps. In this case, the scope is not
making a very accurate measurement on the rise time of this signal. The
scope is actually measuring something closer to its own rise time (700 ps),
not the input signal’s rise time, which is closer to 500 ps. We need a
higher- bandwidth scope for this digital measurement application if timing
measurements are important.
Figure 87 100 MHz digital clock signal captured on a 100 MHz bandwidth scope