"square wave" infinite freq is theoritical term, in reality, our square waves have limited/finite freq harmonics, RLC will dampen the theoritical nature of it, hence that depends on risetime capability of a generator. so as someone puts, you need to observe the "risetime", 10X sampling and BW during the "risetime period only" is much preferable but that can be a premium. thats why they called 1054z series a "hobbiest scope", 1GSps only able to recontruct at decent 100MHz (with its ringing and overshoot) or 10ns of risetime give or take. at 450MHz @ ~2ns of risetime it can only capture 2 points, and not much you can see SI wise with that even if the scope BW permits. i fully agree with tggzz, without proper dso with eye pattern display, you need to take measure in every possible aspects to solve the SI problem, down to the bare metal and pure imagination.