Comments have been that the Shortt clock is not
a phase locked loop because of the infrequent checking
but if you were to cast your mind back to the early
days of PLLs in VHF rigs, typically the 145MHz signal was
crystal mixed down to a few hundred kHz before being
phase checked, and you can't get more infrequent
than 1 in 1000, which is really slow compared to the 1 in 30
check rate of the Shortt clock!