free_electron: JW avalanche generator is fantastic, and yes it will win the competition if you're talking about slew rate (V/s) .
However I would argue that 50ps rise time @ 0.4Vpp is still better than 400ps @ 12V if we're talking about measuring bandwidth on the scope.
Let me start by noting that there's nowhere in the bandwidth formula Bw = 0.4/Tr , that we find the slew rate or the absolute Vpp value. That's because the bandwidth (frequency) of a pulse signal is determined by how fast it goes from (low) flat to (high) flat, and not really from how low to how high it travels. In other words by increasing the amplitude of your signal to 12V you do not get a faster equivalent bandwidth or rise time ... If you amplify a signal that has a 1Mhz harmonic (equvalent harmonic of 400ps rise time is 0.4/400ps = 1Mhz) to 12v that harmonic will still be 1Mhz ...
The equivalent bandwidth of 50ps is 0.4/50ps = 8Mhz, thus you could evalute scopes up to ~8Mhz/5 = 1.6Mhz with it , or use the more complex formula Bw = 0.4/SQRT(Tr^2 – 50ps^2) and measure even higher bandwidths, that takes into account the 50ps rise time of the test signal.
Let me know what you think guys.