Bandwidth is the specification that most engineers consider first when they select an oscilloscope. In this application note, we will provide you with some helpful hints on how to select an oscilloscope with the appropriate bandwidth for both your digital and analog applications. But first, let’s define oscilloscope bandwidth.
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Defining Oscilloscope Bandwidth
All oscilloscopes exhibit a low-pass frequency response that rolls off at higher frequencies, as shown in Figure 1. Most scopes with bandwidth specifications of 1 GHz and below typically have what is called a Gaussian response, which exhibits a slow roll-off characteristic beginning at approximately one-third of the –3 dB frequency.
Oscilloscopes with bandwidth specifications greater than 1 GHz typically have a maximally–flat frequency response, as shown in Figure 2. This type of response usually exhibits a flatter in-band response with a sharper roll-off characteristic near the –3 dB frequency. There are advantages and disadvantages to each of these types of oscilloscope frequency responses. Oscilloscopes with a maximally-flat response attenuate in-band signals less than scopes with a Gaussian response, meaning that scopes with maximally-flat responses are able to make more accurate measurements on in-band signals. But a scope with a Gaussian response attenuates out-of-band signals less than a scope with a maximally-flat response, meaning that scopes with Gaussian responses typically have a faster rise time than scopes with a maximally-flat response, given the same bandwidth specification. But sometimes it is advantageous to attenuate out-of-band signals to a higher degree in order to help eliminate higher-frequency components that can contribute to aliasing in order to satisfy Nyquist criteria (fS > 2 x fMAX). For a deeper understanding of Nyquist’s sampling theory, refer to the Keysight Technologies, Inc application note, “Evaluating Oscilloscope Sample Rates vs. Sampling Fidelity” listed at the end of this document.
Whether your scope has a Gaussian response, maximally flat response, or somewhere in between, the lowest frequency at which the input signal is attenuated by 3 dB is considered the scope’s bandwidth. Oscilloscope bandwidth and frequency response can be tested with a swept frequency using a sine wave signal generator. Signal attenuation at the –3 db frequency translates into approximately –30% amplitude error. So, you can’t expect to make accurate measurements on signals that have significant frequencies near your scope’s bandwidth.
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