Re: High speed digitiser
you can use (push pull or) amplifier (rf amplifier / uhf amplifier ) for that use rf range transistor . followed by schmitt trigger inverter from high speed logic such as MECL , 74S , 95H etc which has less gate delay even HCL will not work ( ~ 7 ns ) i think!.
no no ,
first tell about signal whether it has fix frequency or not , fix voltage or voltage range ?
this is later part
if your amplitude is below mV then you have to amplify it.