How Fast an Analog Coincidence Circuit Can Be Designed

Friends

How accurate we can detect the coincidence events within small error zone Td. I have two signals of almost Gaussian type of different heights (voltage or current gain, random in time) but same shape (exact location for say 50% of the voltage / current.

Signal-A and Signal-B may come within Td limit. The circuit should be able to detect the coincidence within 500ps or at best 1ns value of Td.

This scheme requires Constant Fraction Discriminator (CFD) to get the 50% level walk free point and digital logic to take action if two signals fall within Td time domain for starting point.

If you think you can do it using discrete or integrated circuit then try out your brain here. I will welcome all ideas. I am not interested in any commercial ready made items and it is a problem for sharpening the brain. However for reference you can look at the scheme used by Canberra.

http://www.canberra.com/pdf/Literature/Timing%20Coin%20Counting%20SF.pdf