The US was working on pulse doppler fighter radars from the late 1950s onwards, for example the Hughes AN/ASG-18 from the F-108 which evolved through the 1960s and ended up being the basis of the AWG-9 radar. Behind the US pulse-doppler radars like AWG-9, APG-63, etc, lay decades of research.
Sorry, don't you mean the US (effectively) stole pulse doppler fighter radars off the Canadians?