Why do I need to determine the test signal bandwidth when selecting an oscilloscope?

　　The traditional view is that analog oscilloscopes have familiar control panels and are inexpensive, so they are always considered "easy to use". However, with the increasing speed and decreasing price of A/D converters, as well as the increasing measurement capabilities and virtually unlimited measurement functions of digital oscilloscopes, digital oscilloscopes have become the leader. However, digital oscilloscopes have the disadvantages of three-dimensional display and slow processing of continuous data, and an oscilloscope with digital-analog hybrid technology is needed, such as the DPO digital phosphor oscilloscope.

　　Determining the Test Signal Bandwidth

　　Bandwidth is generally defined as the frequency at which the amplitude of a sine wave input signal decays to -3dB, or 70.7% of the amplitude. Bandwidth determines the basic measurement capability of an oscilloscope for a signal. Without sufficient bandwidth, the oscilloscope will not be able to measure high-frequency signals, the amplitude will be distorted, the edges will disappear, and the detailed data will be lost; without sufficient bandwidth, all the characteristics of the signal, including ringing and humming, are meaningless.

　　An effective rule of thumb for determining the oscilloscope bandwidth you need is the "5x rule of thumb": multiply the highest frequency component of the signal you want to measure by 5 to achieve a measurement accuracy better than 2%.

　　In some applications, you do not know the bandwidth of your signal of interest, but you do know its fastest rise time. In this case, the frequency response is calculated using the following formula to relate the bandwidth and rise time of the instrument: Bw = 0.35/fastest rise time of the signal.

　　There are two types of digital oscilloscope bandwidth: repetitive (or equivalent time) bandwidth and real-time (or single-shot) bandwidth. Repetitive bandwidth only applies to repetitive signals and displays samples from multiple signal acquisition periods. Real-time bandwidth is the highest frequency that can be captured in a single sampling of the oscilloscope, and is more important when the event being captured is not a frequently occurring or transient signal. Real-time bandwidth is closely related to the sampling rate.

　　The higher the bandwidth, the better, but higher bandwidth often means a higher price, so you should choose the signal frequency components you want to observe according to your budget.