The radar proximity fuse actually sets the round off when the signal from the radar begins to down-doppler as the shell passes the target. The radar used continuous-wave emission, basically as simple as a microwave oven combined with a radar detector. The British invention of the cavity magnetron tube (which has no glass in it, btw) made the whole thing possible, as earlier radar tubes couldn't stand the acceleration of the shell being fired, and were way too big.