The third step is to determine the oscilloscope bandwidth required to measure this signal, based on your desired degree of accuracy when measuring rise times and fall times. Table 1 shows multiplying factors for various degrees of accuracy for scopes with a Gaussian or a maximally-flat frequency response. (Remember, most scopes with bandwidth specifications of 1GHz and below typically have a Gaussian-type response, and most scopes with bandwidths greater than 1GHz typically have a maximally-flat type response.)
Here is an example: Determine the minimum required bandwidth of an oscilloscope with an approximate Gaussian frequency response to measure a 500-ps rise time (10 to 90%)
If the signal has an approximate rise/fall time of 500 ps (based on a 10- to 90-percent criteria), then the maximum practical frequency component (fknee) in the signal would be approximately 1GHz:
fknee=(0.5/500ps) = 1 GHz
If you are able tolerate up to 20 percent timing errors when making parametric rise time and fall time measurements on your signals, then you could use a 1-GHz bandwidth oscilloscope for your digital measurement applications. But if you need timing accuracy in the range of 3 percent, then a scope with 2-GHz bandwidth would be the better choice. Let's now make some measurements on a digital clock signal with characteristics similar to this example, using scopes with various bandwidths.
Digital Clock Measurement Comparisons
Figure 3 shows the waveform results when measuring a 100MHz digital clock signal with 500-ps edge speeds (10 to 90 percent) using an Agilent MSO6014A 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- to 20-MHz range, but 100-MHz bandwidth is clearly insufficient for this 100-MHz clock signal.
A 500-MHz bandwidth oscilloscope is able to capture up to the fifth harmonic (Figure 4), 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. It 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.
With a 1GHz bandwidth scope, we have a much more accurate picture of this signal, as shown in Figure 5. When we select a rise-time measurement on this scope, we measure approximately 550 ps. This measurement is providing us with approximately 10 percent measurement accuracy and may be a very acceptable measurement solution, especially if capital funding is an issue. However, even this measurement using a 1-GHz bandwidth scope might be considered borderline. If we want to make edge-speed measurements with greater than 3-percent accuracy on this signal with 500-ps edge speeds, we really need to use a scope with 2-GHz bandwidth or higher, as we determined in the walk-through example earlier.
