Jeff wrote on 8/2/2017 5:09 AM:

I don't doubt that it works nor do I suggest it isn't a very clever bit of
design. I'm just not sure about the terms used.

Ok, but I don't see what you can be confused about. I believe in
electronics this phase detector is referred to as "bang-bang" where it
outputs a 1 or a 0. So on every measurement the VCO frequency control
signal receives an impulse of one polarity or the other.

I think the confusion occurs because at no time, are the phases of the 2
clocks locked together, even at the point of the impulse. By the very nature
of the design the phase of the 2 pendulums (or should that be pendula to
please Gareth) shift in relation to each other.

In an electronic pll, even one using a bang-bang phase detector, the phases
of the 2 signals are locked together, within the constraints of the loop
filter.

This is another false dichotomy. The aspect of the Shortt clock you are
referring to is that it is *discrete* rather than continuous. So you can
clearly see the fact that the slave oscillator is not in perfect lock step
with the master (reference). The same is true in *all* PLL circuits. The
phase of the oscillator is adjusted by the error signal. There can be no
adjustments without error, so the oscillator will not be in perfect lockstep
with the reference. It will be within some tolerance... same as the Shortt
clock. A PLL can be discrete and the phase will move in patterns with small
offsets in frequency at all times. With a continuous phase comparison the
frequency will vary continuously but still will not be "locked" to the
reference with no error. In fact, PLLs are used to remove short term jitter
from clocks by the use of a slow filter on the control signal.

--

Rick C