# Oscilloscope design question

To begin with,I am not designig an oscilloscope but am curious about how certain parameters are determined. My questions might sound very trivial but I would be grateful if someone could answer them convincingly.

1. I have seen oscilloscopes with say a 50 MHz bandwidth but 1 GS/s sampling. My question is, why do I need such a high sampling rate when the signal is restricted to 50 MHz? I read on an oscilloscope manufacturers website that a good rule of thumb is that to visualize a signal of say X MHz, you should have an oscilloscope between 4X to 6X MHz bandwidth. What is the justification for such high sampling rates then (When compared to the bandwidth)?

2. I have also seen probes with a bandwidth limitation. Is it a must to use these if the scope has a filter as well? On the other hand, can there be an oscilloscope which leaves the filtering entirely to the probe and has no filtering of its own?

• 1GS/s is only 20 samples for a 50MHz signal. An analog scope gives you much better fidelity. Mar 3, 2021 at 11:39
• @Kartman nonsense; Shannon-Nyquist literally says you can reproduce the exact 50 MHz bandwidth with only >100 MHz sampling rate. What is much better might be the fidelity of operation if bandlimiting to 50 MHz isn't exactly achieved. Also, some people design their oscilloscopes (or configure them) to connect sample values with straight lines, which is mathematically incorrect; but that's nothing to do with analog vs digital, but with "appropriate" and "incorrectly designed". Mar 3, 2021 at 11:58
• @MarcusMüller "We mustn't give in to the naysayers who claim analog scopes are any better." - Oh no, I agree. And in my experience 10 samples/cycle is plenty to match or beat analog scopes. My point was more that "you only need 2 samples/cycle because Shannon-Nyquist" is not true in practice. Mar 3, 2021 at 13:18
• @JohnB But what if the signal isn't a perfect sine wave? Or you don't even know the shape of the signal? What if your sine wave has noise? Or amplitude or frequency modulation? Yes, if the signal is a perfect sine wave and you know this, 2 samples/cycle is all you need. But in that case, why use an oscilloscope? A frequency counter would suffice. Mar 3, 2021 at 13:21
• @marcelm if the signal isn't a perfect sine wave, then it's not the highest frequency component in the signal, and therefore your sampling rate doesn't meet the Nyquist criteria for that signal. Mar 3, 2021 at 13:23