DIGITAL STORAGE OSCILLOSCOPES: CHOOSING THE RIGHT BANDWIDTH

Because there are hundreds of models available with different specifications at a wide variety of prices, choosing the right oscilloscope can be intimidating and confusing for many engineers and technicians. In this article we’ll look at a few aspects of digital storage oscilloscopes that are of particular importance in order to help you avoid a costly mistake.

Some things to consider before we get into the specifics of bandwidth are: where you’ll be using the oscilloscope, how many signals you’ll need to measure at one time, the minimum and maximum signal amplitudes you’ll be measuring, the highest frequency signal you’ll potentially measure, whether your signals are repetitive or single-shot, and whether you need to view signals in the frequency domain (i.e. spectrum analysis) and the time domain simultaneously.

Bandwidth

Bandwidth is the maximum signal frequency that can pass through the front-end amplifiers and the oscilloscope's bandwidth must be higher than the maximum frequency you’d like to measure. However, sufficient bandwidth isn’t the only consideration when making sure that a digital storage oscilloscope can capture a high-frequency signal accurately.

Oscilloscope manufacturers seek a certain type of frequency response when designing their instruments and this response is called the maximally flat envelope delay, or MFED. This type of frequency response provides superb pulse fidelity with very little undershoot, overshoot, and ringing. Because a digital storage oscilloscope is made up of amplifiers, attenuators, analog-to-digital converters, interconnects, and relays, maximally flat envelope delay response cannot be entirely realized — think of it more as an ideal.

Please note that the majority of oscilloscope manufacturers define bandwidth as the frequency at which a particular sine wave signal will be attenuated to seventy-one percent of its actual amplitude, i.e. the trace will be twenty-nine percent in error of the input signal.

Your signal will contain high-frequency harmonics if your input signal isn’t a pure sine wave; e.g. a twenty megahertz signal viewed on a twenty megahertz bandwidth oscilloscope will show up as a distorted and attenuated waveform. A good rule of thumb here is choosing an oscilloscope with a bandwidth five times higher than the maximum frequency signal you’d like to measure. The caveat here is that oscilloscopes with high bandwidths are more expensive, meaning you may have to compromise with regard to your oscilloscope’s bandwidth. Some oscilloscopes have bandwidths that aren’t available for all voltage ranges which is why should carefully read through the oscilloscope’s data sheet.