Monophasic (Rtrn) input signal from the Simulator at point (A) on the Signal Gate circuit diagram and the gate opening pulse, (gop), that opens the analog switch, U3, and causes the peak signal voltage to remain on the 0.001 mF capacitor at pin #3 of follower, U4.  This (gop) pulse will serve a similar function for magnetic- and voltage-field signals processed by the Track/Hold circuit board.