Jeff wrote on 8/5/2017 5:45 AM:

What we are seeing is that even after the 30 second 'kick' the 2 pendulums
are NOT in phase.

They may well be 'a bit closer' in phase, but the kick just moves the
difference a fixed small amount in one direction, which may be sufficient to
bring the phases closer, or it may be too much and go through the in phase
point. With the design there is no time where the 2 pendulums are *held* in
phase.

The design in fact relies on the fact that the phase of the 2 pendulums is
constantly changing.

As is true for any PLL.


Rubbish, the function of a phase locked loop is to keep the phase of the 2
signals the same, within the constraints of the loop filter.

The clock *never* achieves this, it is open loop and applies a 'kick' to one
pendulum the amplitude of which is NOT related to the difference in phase of
the 2 pendulums.

A fixed kick is given without any knowledge that it will be of the correct
amplitude to achieve an in phase or near in phase condition. There is NO
feedback of an error signal that relates to the phase difference between the
2 pendulums.

The only time phase comes into the picture is the timing of when the 'kick'
is given, so as not to disrupt the normal swing of the pendulum, and whether
or not to give a kick at all.

It is and ingenious system, but not a phase locked loop.

I guess it could be closer to a PLL if the kick had its amplitude varied by
the phase difference between the 2 pendulums, but you still have the problem
that if you were in the state where no kick was required there is no way of
slowing the second pendulum without waiting for it to drift back, so it is
still open loop.

You are making pointless distinctions. A phase locked loop is not defined
by its mechanics but by the nature of its control. The Shortt clock
maintains the relative *phase* of the two clocks by brief adjustments to the
frequency via a spring. This is controlled by measuring the relative
*phase* of the two clocks.

It's that simple. You are just making things more complicated by talking
about the details of how the adjustment works and the time function of the
frequency. NO PLL can keep the two clocks perfectly in sync.

Calling it open loop is just absurd. The loop is closed because it
*measures* the phase of the clocks and adjusts the phase according to the
measurement. It may be binary, but the adjustment is controlled by the
measurement.

--

Rick C