Bandwidth is generally defined as the frequency when the amplitude of sinusoidal input signal attenuates to -3db, that is, 70.7% of the amplitude. The bandwidth determines the basic measuring ability of oscilloscope to signal. If there is not enough bandwidth, the oscilloscope will not be able to measure the high frequency signal, the amplitude will be distorted, the edge will disappear, the detail data will be lost;Without sufficient bandwidth, all the features of the resulting signal, including ringing and ringing, are meaningless.
An effective rule of thumb for determining the bandwidth of an oscilloscope you need -- the "5x rule of thumb" : multiply the highest frequency component of the signal you are measuring by 5, so that the measurement results are more than 2% accurate.
In some applications, you do not know the bandwidth of the signal of interest to you, but you do know its maximum rise time, at which time the frequency response calculates the associated bandwidth and rise time of the instrument using the following formula: Bw=0.35/maximum rise time of signal.
There are two types of digital oscilloscope bandwidth: repeat (or equivalent time) bandwidth and real-time (or single time) bandwidth. Repeat bandwidth only applies to repeated signals, showing sampling from multiple signal acquisition periods. Real-time bandwidth is the highest frequency that can be captured in the single sampling of oscilloscope, and it is more important when the captured events are not frequent or transient signals. Real-time bandwidth is closely related to the sampling rate.
The higher the bandwidth, the better, but higher bandwidth often means higher prices, so choose the frequency component of the signal you're looking at on a budget.