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Bandwidth: The most important characteristic of an oscilloscope

Now that we are in the era of the digitizing scope, there’s more to scope bandwidth than just the bandwidth of the analog amplifiers alone. To ensure that your scope has enough bandwidth for your application, you need to take into account the bandwidths of the signals you will be looking at with the scope.

Bandwidth is the most important characteristic of the oscilloscope, as it determines the range of signals to be displayed, and to a large extent, the price you’ll need to pay. When making your bandwidth decision, you need to balance today’s budget limitations with your expected needs over the life of the scope in your lab.

Look to your application to determine the bandwidth you need

In today's digital technologies, the system clock is usually the highest-frequency signal the scope is likely to display. Your scope should have a bandwidth at least three times greater than this frequency in order to obtain a reasonable display of the shape of this signal.

Another characteristic of the signals in your system that determines the bandwidth requirements of your scope is the signals' risetimes. Since it’s likely you will not be looking at just pure sine waves, your signals will contain harmonics at frequencies beyond the fundamental frequency of your signal. For instance, if you are looking at a square wave, the signal actually contains frequencies that are at least 10 times the fundamental frequency of the signal. If you don't ensure proper scope bandwidth when looking at a signal such as a square wave, you will see rounded edges on your scope display instead of the clean, fast edges you were expecting to see. This, in turn, will affect the accuracy of your measurements.

How do I determine how much bandwidth I need?

Fortunately, there are a few very simple equations that will help you to determine proper scope bandwidth, given your signal characteristics:

1. Signal bandwidth = 0.5 / signal risetime
2. Scope bandwidth = 2 x signal bandwidth
3. Scope real-time sample rate = 4 x scope bandwidth

Watch a short video - How much bandwidth do you need?