Nice write up and sneaky introduction to time-domain reflectrometry but I'd like to point out the classic answer to this question is the famous Jim Williams pulse generator: https://github.com/podonoghue/Jim_Williams_Pulse_Generator?t...
However, nowadays it is difficult to find any bipolar transistors that are suitable to be operated in the avalanche mode or any tunnel diodes, because these were fabricated using older technologies that are not suitable for the semiconductor devices that are popular today, so most such fabrication lines have been closed, due to insufficient demand.
Only for extremely few bipolar transistors the characteristics of the avalanche mode operation were specified by their manufacturer, so for most devices using avalanche transistors the transistors for each built device had to be cherry picked by testing many transistors of a type known to include suitable transistors.
But, really, I just took the opportunity to talk about Jim Williams and his magnum opus that is AN47!
Sadly, such great technical documentation exists no more. The companies that make such products are no longer your business partners, but they are adversarial entities, whose only goal is how to confuse and to fool their customers into paying as much as possible for products whose quality is as low as possible.
Educating your customers about how to better use your products is no longer a business goal. Another current thread on HN is about the fear that the huge decline in the quality of technical documentation during the last 3 decades will be accelerated by the replacement of professional technical writers with AI.
I've found ADI still has some great educational material, although that's partly because they've been better at maintaining their webpages from the 90's and 00's, not because they're putting out much new material.