Decades ago we used a much simpler method. A few 50 or 75 ohm non-inductive resistors and a tunnel diode.<p>Feed any (slow) pulse generator into the diode and make it switch. Tunnel diodes can have sub-nanosecond switching times.<p>We also used this technique to check/measure the rise times of our oscilloscopes.
This is a great post about the basics of what happens in transmission lines.<p>If you need really fast rise times, there are cheap pulse generators that are a couple orders of magnitude faster: <a href="https://leobodnar.com/shop/index.php?main_page=product_info&products_id=295" rel="nofollow">https://leobodnar.com/shop/index.php?main_page=product_info&...</a> At this level everything has to be optimized including physical geometry.
I am using Leo Bodnar’s fast pulse generator (SMA) in my lectures to teach transmission lines. With sufficient length (I use ~1m) it works quite well to demo with a low cost scope. I originally bought it for TDR with 40GSPS/15GHz scope, which works very well with few orders of magnitude smaller lengths. Old on has upper length limit with 10MHz fixed frequency. There is a new one available with external sync and variable frequency, but I have not bought/tested it.
The real gem is the answer at the bottom about doing the same thing with a bit of transmission line you treat as a capacitor.
Yeah, that's a pretty awesome idea. The only critical spec for the transistor is the time to get into saturation, the actual pulse length coming from the propagation of the discharge of the coax segment from the collector end to the far end. When the capacitance of that coax is discharged, the pulse stops, even with the transistor still on.
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: <a href="https://github.com/podonoghue/Jim_Williams_Pulse_Generator?tab=readme-ov-file" rel="nofollow">https://github.com/podonoghue/Jim_Williams_Pulse_Generator?t...</a>
Avalanche transistors, like the tunnel diodes mentioned by another poster, had been widely used in the past for generating fast pulses.<p>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.<p>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.
Great article!
aside: I've never seen Stack Exchange used as a blogpost medium (which normally this kind of write-up would be) and I like it! It's still formatted as Q&A so people with the same question can find it, and what's more, suggest edits or write alternative solutions (as OP explicitly invites here) on equal footing themselves. A collaborative quest for the answer, but not anonymized like a wiki.
Any recommended resources for learning the first principles required to understand this and all the components involved?
For transmission lines: _Similarities of Wave Behavior_, presented by Dr. J. N. Shier (of Bell Labs fame, and whose team invented the phototransistor):<p><a href="https://www.youtube.com/watch?v=DovunOxlY1k" rel="nofollow">https://www.youtube.com/watch?v=DovunOxlY1k</a><p>It's an easy thing to watch at any level, with both brilliant practical demonstrations and supporting math provided.
W2AEW on YouTube was one of my introductions to high-speed analog electronics (and I had the pleasure of working with him later on).
An Electrical Engineering course, or a book on practical circuit design.
That's pretty cool :)