I've been wondering if it's possible to recreate this with today's technology. The HP implementation uses a custom ASIC with very fast transistors, so it's not obvious.

https://febo.com/pipermail/time-nuts_lists.febo.com/2025-Jul...

If you want to make your own interval counter today, the to-go-to method is to use a TDC7200 chip. If that's not good enough, you can also look at the schematics of an SRS SR620 time interval counter. It has a similar 25ps precision and uses an ADC to interpolate between samples of a 90 MHz clock. These kind of ADCs weren't available in 1978.