Peter Fairbrother wrote on 8/5/2017 11:01 AM:
On 05/08/17 14:34, Chris wrote:
On 08/05/17 09:45, Jeff wrote:


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.

The amplitude is not, but the frequency is - why do you think the amplitude
should be related to the difference in phase?

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.

Ah, yes there is, see below.

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.

Are you referring to the kick given to the master pendulum? That is not part
of the PLL system. The kicks given to the master pendulum are specifically
designed not to affect the phase of the master pendulum at all.

If not, if you are referring to the kick given to the slave pendulum (these
are quite different kicks) that is not how the clock works.

The slave pendulum is kicked from time to time, ad kicked a little more
often when the phases get too far apart - the difference in phases is the
error signal mentioned above - and these kicks do affect the phase of the
slave pendulum.

What they fail to see is that the amplitude of the kick *is* adjusted. It's
just the adjustment is binary, on or off. But that is still *adjustment*
and is in response to the measured phase.


Exactly. The control is single path, master to slave, with no feedback
to the reference, making it an open loop design. The master has no
knowledge of the state of the slave at any time.

That is exactly what a PLL is - and it is almost (though not quite) what
this clock does. It is certainly what the slave does.

In a pll, there is continuous

Not necessarily continuous - a bang-bang action is allowable, and does not
prevent a system from being a PLL.

feedback from the vco to the phase
detector, closing the loop and keeping the phase offset constant,

A PLL does not necessarily keep the phase offset constant, just within the
interval =/- 2pi.

Not only that, but if you examine the equations for a PLL you will find it
is *impossible* to maintain a constant phase offset with any variations in
the reference or noise in the system.


The phase is continuously updated every cycle,

Not necessarily continuously updated, or updated every cycle - as long as
the offset is continuously within the range -2pi to 2pi, the phases are locked.

whereas the Shortt
clock can have significant accumulated error in the time between
corrections...

Yes - but that doesn't mean it is not a PLL, as long as the error is less
than +/- 2pi.



A phase-locked loop is a system which produces a (slave) vibration the
integral of whose phase in comparison to the phase of another (master)
vibration is continuously between -2pi and 2pi over long periods.

A last requirement is that the phase-locked loop system should have no
effect whatsoever on the master vibration. That's it.

If it does that, the phases are locked - they may not be tightly locked, but
the vibrations do not skip or add beats.

More advanced PLLs might keep the difference between phases much smaller, as
in this clock - but that is not a requirement of a PLL. There is no such
thing as absolutely tightly locked, there is only unlocked or locked.

Neither is continuous updating necessary, though the integral should be
continuously in that interval.

In this clock the hit-and-miss synchroniser action undoubtedly does act as a
PLL.

However it might be argued that the slave does subsequently have some (very
small) input to the master, when it operates the gravity drive (whuzzat? I
am not a clockmaker).

That certainly has an effect on the amplitude of the master; although as the
idea an intention and practical effect is that it has no effect whatsoever
on the phase of the master, thus the slave clock action overall most
definitely should be considered a PLL.

-- Peter Fairbrother

ps; the +/- 2pi bit is not really a requirement either, as long as the
system can keep count of the missing/extra beats - but as most systems don't
do that we shall just gracefully ignore that for now ..

In a typical PLL isn't the requirement to be within +/- pi rather than 2 pi?
If you exceed a range of +/- pi from the intended alignment the feedback
will start to push the controlled oscillator further out of alignment
potentially aligning with another cycle of the master.

--

Rick C