Decades ago we used a much simpler method. A few 50 or 75 ohm non-inductive resistors and a tunnel diode.

Feed any (slow) pulse generator into the diode and make it switch. Tunnel diodes can have sub-nanosecond switching times.

We also used this technique to check/measure the rise times of our oscilloscopes.

sometimes, I realize how much is taken for granted these days that when I stop to think about is one of those "whoa" (in a Neo voice) moment. being able to time something in the picosecond range just gives me a wry smile.