I figured for a first try, I'd use a $6 ADCMP606 CML-output comparator. It's not the fastest available, with leisurely 160 ps typical Tr/Tf, and 1.2 ns minimum pulse width. There are faster ones available, like the $70 HMC874, with 24 ps rise and 15 ps fall times and 60 ps minimum pulse width. I wanted to get some experience with these things before plunking down that much cash.
I designed a quick circuit using a 74LVC1G14 Schmitt-trigger inverter as an oscillator at about 500 kHz. The oscillator drives a 74LVC1G04 as a buffer than then drives the + input of the ADCMP606 comparator. The - input of the comparator is held at mid-supply with a bypassed voltage divider. The output is AC-coupled to an SMA connector. I made a mistake on the PCB and used the wrong footprint - it was supposed to be a PCB-mount male BNC so the board could be connected right to a 50-ohm terminated oscilloscope with no cable. More on this below.
There's a jumper to select free-running or triggered pulses. With the jumper removed, two of the pins are the trigger input and ground. There's a USB connector for power, but I didn't populate it.
There's no added hysteresis in the circuit - If I make another one, I'll probably add some, since there is a little oscillation on the falling edge - I suspect ground bounce.
I tried to keep all the high-speed stuff small and near the connector in the PCB layout, using 0402 parts where it counts around the SC-70 comparator. I didn't have any large value 0402 capacitors, so the output pulses don't look square - the 10 nF cap and the 50-ohm impedance make a nice high-pass filter. When I get some, I'll fix it.
So, how does it work? I measured 400 ps rise time with a 1 GHz scope. I did have a section of coax in between the pulser and the scope, which could be increasing transition times through dispersion - I have some adapters on order so I can connect it without any cable and see if the measurement changes.
This measurement is also problematic because it's (probably) very close to the rise time of the scope itself. So, I may be measuring the scope more than the pulse.
The bandwidth of the scope and its rise time are inversely related. Tektronix recommends using the equation:
for modern real-time digital oscilloscopes (older analog scopes are better approximated with a constant of 0.35). Even though the scope (a modded TDS754D) is running in equivalent-time sampling mode at these speeds, it's an interesting data point. This equation would imply a 1.125 GHz bandwidth, which seems reasonable for a 1 GHz scope. I had previously estimated the scope's 3 dB bandwidth at 1.05 GHz. So, the actual pulse edge could be considerably faster, maybe even approaching the typical 160 ps from the comparator's datasheet.
I would be satisfied with this result except for one thing. The ebay seller from whom I bought the oscilloscope had posted an image showing a 284 ps rise time when measured with a 40 ps edge!
You can estimate the rise time elongation of the scope by applying the formula:
Using this, and assuming the 284 ps rise time is correct, the pulser would be creating 281 ps edges (sqrt(400^2 - 284^2)). That sounds plausible, too, but I have no way of proving it one way or the other at the moment.
It's also possible I may have exhausted my measurement capability on the first shot. If so, it's time to bootstrap some way to measure even faster pulses.