Here just before the last paragraph we read :

C27 is placed at the output of the ADC Buffer in the rightmost of this schematic.This picks up the trigger signal and it will be fed to an 74AHC86 ,... .
Here is what i exported :

Could i use a simple amp to detect this trigger (After the HF filter) ?Could you give me some ideas what this chip is doing here ,I can't think of anything other than a Schmit Trigger ?

And Look at the third circuit here :
http://www.bitscope.com/design/hardware/?p=4
I remmember that placing an inductor was prohibited in the Ground return ?

Regards,
Farshid