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.

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.


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.

In a pll, there is continuous feedback from the vco to the phase
detector, closing the loop and keeping the phase offset constant,
The phase is continuously updated every cycle, whereas the Shortt
clock can have significant accumulated error in the time between
corrections...

Chris

On 05/08/2017 14:34, Chris wrote:

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.

Untrue.

The matter starts off when the slave signals to the master and drops
the gravity link in the master, then, when the master pendulum is in
a position to accept the impulse from that dropped gravity link, it
signals back to the slave

But ... I'm still trying to google for the exact mechanisms because
most URLs only hint at what is happening. (I'm also awaiting delivery
of a couple of hope-jones' books about electric clocks)

On 05/08/2017 14:57, Gareth's Downstairs Computer wrote:
On 05/08/2017 14:34, Chris wrote:

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.

Untrue.

The matter starts off when the slave signals to the master and drops
the gravity link in the master, then, when the master pendulum is in
a position to accept the impulse from that dropped gravity link, it
signals back to the slave

But ... I'm still trying to google for the exact mechanisms because
most URLs only hint at what is happening. (I'm also awaiting delivery
of a couple of hope-jones' books about electric clocks)


I hope you have more success getting a copy of those books than getting
the PA tuning instructions for a mass produced amateur TX.

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

Chris wrote on 8/5/2017 9:34 AM:
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.

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.


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.

You aren't making sense. The reference is never adjusted in a PLL. That's
why it's the *reference*.


In a pll, there is continuous feedback from the vco to the phase
detector, closing the loop and keeping the phase offset constant,
The phase is continuously updated every cycle, whereas the Shortt
clock can have significant accumulated error in the time between
corrections...

There is no requirement in a PLL for continuous action or even frequent
action.

--

Rick C

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.



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.

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 ..

Gareth's Downstairs Computer wrote on 8/5/2017 9:57 AM:
On 05/08/2017 14:34, Chris wrote:

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.

Untrue.

The matter starts off when the slave signals to the master and drops
the gravity link in the master, then, when the master pendulum is in
a position to accept the impulse from that dropped gravity link, it
signals back to the slave

But ... I'm still trying to google for the exact mechanisms because
most URLs only hint at what is happening. (I'm also awaiting delivery
of a couple of hope-jones' books about electric clocks)

What you are describing is how the phase measurement of the master is made.
The gravity lever is simply a remontoire providing a consistent push to
overcome the force of friction. It is designed to be invariant of small
changes in timing of its release. You can see that in the animation linked
below. The gravity arm is released at the point when the wheel is directly
under the end of the gravity lever. A small change in timing changes the
force only a tiny amount. This is critical to maintaining the swing of the
free pendulum without affecting its period.

http://www.chronometrophilia.ch/Elec...cks/Shortt.htm

The animation happens in real time so it is hard to see the details of what
is going on. The gravity lever and accompanying control is the magic of the
clock. The rest is pretty straight forward. You need Flash to view this
page. There is a button to see the wires.

--

Rick C

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

In rec.radio.amateur.homebrew Jeff wrote:

https://en.wikipedia.org/wiki/Shortt...sync hronizer

"This feedback loop functioned as an electromechanical version of a
phase-locked loop..."



..and of course everything on Wikki is correct!!!

Jeff

The usual cry of those who have not bothered to do any research on a
subject and are shown a Wiki article that contradicts their position
is that Wiki can be edited by anybody.

Wiki is more correct than most of the babble on USENET and the Wiki article
has 18 external references to back it up.

Where is your annotated list of references?

Here's another site that says the same thing:

http://www.meccanotec.com/shortt.html

"The slave is kept in synchrony with the master in a phase locked loop."



--
Jim Pennino

On 05/08/17 16:19, rickman wrote:
Peter Fairbrother wrote on 8/5/2017 11:01 AM:
On 05/08/17 14:34, Chris wrote:
[..
The slave pendulum is kicked from time to time, and 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.

Yup.

Compare with pwm (pulse width modulation) or ppm (pulse position
modulation) - I forget what the actual modulation in the clock is
called, but it is just another modulation, despite being binary and
fixed in amplitude.

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.

Indeed.. in some ultimate sense, perhaps that is the final purpose of a
PLL.

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.

Yes, in a typical PLL - however I was considering a more theoretical one
where eg the phase offset was known to be positive or negative.



On reflection, is a system where the phases are several full cycles
out-of-phase, but where the system over time adjusts the slave to (close
to) the actual phase of the master, still a PLL?

On further reflection, I think it must be - so perhaps a better
definition might be that the integral of the phase difference remains
close to zero over long periods time (while leaving how close and how
long as an exercise for the reader) .

-- Peter F