Jesus. I swear, next time I need an RF oscillator - forget Mr. Colpitts, the LM7171 is
much more reliable.
Getting it to work was reasonably simple. I am happy to say that I'm so far doing a good job of proving Danny wrong:
sqrwave risetime: no longer than 10ns, probably around 5-6ns
Let me be the very to say that if you could indeed design and implement something like that fully in the analog domain, you would have no problem work at any test equipment company as a senior person.
To be
very sure I hit the 10ns limit, and the 5ns goal, I've been designing for 3ns. As usual, real life isn't quite as good as simulation, so I'm getting 4.5ns.
That's at low amplitude, though. The only source I have for the
real signal (high rise time
and high amplitude) is the comparator module itself, which is presently tied up with the VCO doing some other tests, so I didn't have an appropriate signal source. That will come a bit later.
The ringing and overshoot are a bit worse than I'd like, but that can easily be attributed to layout. In the "final" PCB, I'll leave a few empty footprints for compensation networks of various types, so I can design those around the final layout without changing the board. (Though the likelihood of a 100% successful first prototype run of the main board is not very high, I admit.)
Some of the overshoot will be from the signal generator itself, as well as the fact that the signal came down a long cable and was not terminated 100% perfectly.
A discrete transistor-based solution was difficult because of the low overhead (output needs to span -10V to 10V before the termination, and the rails are -15V and 15V) - it had to be biased very carefully, and I'm really not a fan of "precision biasing". Recipe for failure IMHO. But taking the output buffer out of the feedback loop did the trick. 7171 stays.
I'll say this: you know you have a high-speed op amp when it doesn't work properly until you trim the leads on the feedback resistors!
The rise time limiter works perfectly as well, though I didn't doubt that (I more or less wanted it present in the prototype to ensure that it did not limit anything when switched
off.)