If you consider what's happening on your analog oscilloscope, you'll realize that the refresh is accomplished as a result of a very precise trigger circuit and the many, MANY repetitions of the signal under study. If an instrument samples that same signal digitally, assuming a small phase change with respect to the trigger, but often enough that it repeats its first offset every, say, 1000 cycles, then it will display every bit as good a representation of the signal as the analog 'scope does, particularly if it runs a "moving average" by combining many samples. What you see on the display will be a bunch of sweeps, each containing the product of many, MANY, repetitions. What more could you ask? Moreover, if the sample rate is 100 MHz and the phase shift is 200 ps, you'll get a really good picture of the waveform despite the ADC's limitations.

RE