If you had to pick just one load, it would be 50 ohms. Notice I didn't
write "50 ohms and 30 pf" or anything like it. To get the best sims we
need the high-frequency content of output waveforms, and artificially
filtering it out by adding a lowpass is not exactly helpful.
Of course, we need both the turnon and turnoff responses for both
the pullup and pulldown paths, so that's four tables (rising into
50 ohm pullup, falling into 50 ohm pulldown, rising into 50 ohm pulldown,
and falling into 50-ohm pullup.
Output timing should be to the 10% (initial) point of the edge. That's
easy to plug in as Vmeas so that the timing numbers add up, and also
easy enough to measure on a tester so that you're actually testing to
the critical design parameter.
No lumped caps anywhere (was this a surprise?)
-- D. C. Sessions firstname.lastname@example.org
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