I took a 100MHz DSO with 100MHz probes in 10:1 mode and connected it to a 400MHz LVTTL clock. It is barely able to display anything: the signal is visible, but distorted and heavily attenuated. No wonder.
But then I attached the same to the output of my GaN pulser intended for nanosecond-scale ferrite testing. Behold:
This is the voltage at the drain (which means that the current through the load resistor ramps about that fast from 0 to 3.3A). I have all reasons to believe that this curve is accurate -- the fall time is 1.5ns or better. So what makes this scope so incredibly good this time?
BTW, here is a tiny 1 turn:1 turn transformer saturating:
Best regards, Piotr