The classic TEK 'scope you have uses an old definition (not that old means not valid any longer!) of bandwidth, specifically, the frequency at which the display is attenuated by 3db. I don't know what definition these digital 'scopes use, but it may well be the same, as that's the established definition for analog instruments.

However, if it has a 300 MHz bandwidth, a 300 MHz sinusoid will appear 3db down from its actual amplitude. I don't, right offhand, know what a 500 or 800 MHz sinusoid would look like, but I can guarantee you that even a 300 MHz square wave won't look like a square wave, as the harmonics that make it a square wave, will be severely attenuated. Now, a 1 ns pulse would, essentially, be a segment of a 500 MHz wave. If the wave is square, then you probably would be pretty distorted, both in shape and in timing. I don't know how the bandwidth limitation combined with the risetime limit will impact your display. 300 MHz would suggest a 1.4 ns pulse width, wouldn't it? If that's shifted 1.4 ns in time, and attenuated 3db, what do you suppose you should see?

I'd say you need to display slower waveforms and then contemplate how the bandwidth + risetime factors into what you see. Keep in mind that the bandwidth will affect all the signals that you display, and the rise/fall time may well simply be an effect of the bandwidth, but I suspect it will make itself felt on all the waveforms that you display, irrespective of frequency. The bandwidth will do that, too, but will disappear as the frequency is lower.

I doubt this will impact your ability to work with the instrument, but I do believe you need to know in what ways the various limitations of the instrument will impact the way in which you interpret what it displays. Sadly, I can't advise you as to what to expect between the bandwidth, rise/fall time limitation, and sample rate. You may actually see distortions that are really not there.

RE