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
