Dave,
If you're talking about risetime, it can be quite a lot smaller than 100 fs.? I would say arbitrarily small, but eventually you reach the limits of double precision math.? Remember not to let LTspice's waveform compression get in the way.? And there are limits of how far you can zoom in, but even that can be overcome if you know how.? And if you don't, there's waveform cursors.
Andy
