Some years ago, I've been into building a sampling scope using the comparator-as-sampler approach. Unfortunately, I never finished it
Sampler comparator was an ADCMP582.
For trigger generation, I used a startable ring oscillator (basically an and gate and transmission lines on the PCB) and an MC100EP196 delay line with analog fine tuning. The ring oscillator's output is divided by 2 to to reduce the length of the delay line. This signal is fed an ECL counter, that generates a pulse every 256(?) clock cycles. By presetting this counter, I can control the sampling instant with a granularity of approx. 10ns. To get down to ~1ps, the delay line is used.
Since both of these delay elements are rather inaccurate, the MCU measures the frequency of the ring oscillator. To calibrate the MC100EP196, it can be configured as ring oscillator as well, so the MCU can measure its frequency as well.
Unfortunately, I haven't been to characterize the start-up behaviour of the ring oscillator yet.
In general the approach of using a startable ring oscillator has the disadvantage, that for fast start up, the oscillator should have low Q, but for low jitter it should have high Q.
Anyone got any expericene with going the analog way? I.e. steering currents in and out of a capacitor for ramp generation