Oscilloscope Bandwidth Tutorial
B
4000 X-Series Oscilloscopes Advanced Training Guide
163
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 106
100 MHz digital clock signal captured on a 100 MHz bandwidth scope
