|Figure 1: Here is an example of aliasing that results from|
sampling a signal at less than the Nyquist rate of 2fmax
According to Nyquist's Theorem, the sampling rate of a digital oscilloscope must be at least twice the speed of the signal's highest-frequency content. Any signal energy at frequencies higher than Nyquist will be undersampled and aliased (Figure 1).
In general, an oscilloscope's sampling rate may change with the number of channels in use and the time/div setting. At long signal capture times, the sampling rate is reduced so that the acquisition memory can cover the elapsed signal time. At lower sampling rates, a high-bandwidth front-end amplifier turns out to be disadvantageous, because it will send lots of high-frequency content in the signal to the ADC, where those frequencies will suffer aliasing.
Say you have three oscilloscopes in your lab. All three may have the same specified bandwidth and sampling rate. But when capturing longer signals, the sampling rates could be very different depending on how much acquisition memory each instrument contains. When capturing signals of long duration, such as those from power devices or serial data streams, be sure that you're aware of your actual sampling rate (which may not be the instrument's maximum specified rate). Also, make sure that you enter the oscilloscope's menu of horizontal settings and select the maximum amount of acquisition memory. Some oscilloscopes also allow you to specify whether all channels are in use, or only one or two. If only two are in use, some instruments can add the memory from unused channels to the active channel(s), thereby increasing the amount of time at which a signal can be captured at that maximum sampling rate.