Most Digital Storage Oscilloscopes (DSOs), including Pico PC oscilloscopes, feature equivalent–time sampling (ETS). ETS allows a digital oscilloscope to capture high–frequency signals with a much higher effective sampling rate.
Equivalent–time sampling works by constructing a picture of the input signal by accumulating the samples over many wave cycles. Because ETS samples the waveform over a number of cycles it can only be used to measure signals that are repetitive. ETS cannot be used for single–shot or non–repetitive signals.
Figure 1 below shows a 25 MHz sine wave that was captured using a Pico handheld oscilloscope, using a real–time sampling rate of 100 MS/s. Figure 2 shows the same 25 MHz sine wave sampled using digital ETS. From these waveforms you can see that the real–time sampling gives a wave shape so distorted as to be nearly useless, while ETS gives a much more accurate representation of the input signal.
On repetitive waveforms, a Pico oscilloscope with equivalent–time sampling can provide the same accuracy as higher–cost oscilloscopes that have higher real–time sampling rates.
Figure 1: 25 MHz sine wave sampled using real–time sampling
Figure 2: 25 MHz sine wave sampled using equivalent–time sampling