On 08/03/17 13:44, Brian Reay wrote:


While I prefer mechanical watches, I favour Rolex (originally English,
BTW), I would quite like to get one of the 'tuning fork' watches,
ideally the version with the clear dial. Another classic.



Bulova Accutron. You can find them on US Ebay, not cheap, but even
the example with the exposed internals. Pretty neat watches, but not
sure how accurate they would be by now. Also like the early Junghans
Mega msf clocks. Bought one of those around 1990. Still keeps spot
on time and use it to rate the IBM clock and others...

Chris

Jeff wrote on 8/3/2017 5:32 AM:

This is another false dichotomy. The aspect of the Shortt clock you are
referring to is that it is *discrete* rather than continuous.

Not correct the phases of the 2 pendulums are *never* in phase. Even when a
kick is given, as of course if they were in phase there would be no need for
a kick.

You don't understand the meaning of "phase". If you said the two
frequencies were never the same I would agree. The slave pendulum runs
slower than the master with the intermittent impulse to adjust the phase.
The relative phase varies with time as a sawtooth function and so at some
point the phase *must* be aligned as the slave passes from being ahead to
being behind. On the next adjustment the phase is adjusted or not. When
properly adjusted the phase of the slave will only be "bumped" every other
adjustment time. On the adjustment times when the slave phase is *not*
adjusted the phase will be in alignment ideally.


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.

When a electronic phase lock loop is locked there is no error as the 2
signals are perfectly in phase. There will only be a change in locked
control voltage if the phase drifts.

You need to go back to PLL 101 class. When the PLL is "locked" it simply
means the error in phase is small enough that the loop can compensate by
varying the VCO frequency. If you understand the math you will see that
this means it will *always* hunt for the perfect alignment. If there is no
integral term in the feedback loop, there will always be a phase error
dependent on the dF/dV slope of the VCO. If there *is* an integral term in
the feedback loop the loop will have small fluctuations as the frequency
adjusts to correct the phase, but when the phase error reaches zero the
frequency error will *not* be zero and the phase error will immediately
become non-zero.


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.

No, a phase locked loop has the same accuracy, or tolerance if you wish, as
the reference.

There is always jitter in the output of the PLL that is independent of the
reference 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.

No it will only vary in sympathy with the reference signal, or with signals
that are not damped by the loop filter due to being faster than the loop
filer can deal with.

Please review your PLL materials. There is no such thing as a PLL that
aligns perfectly with the reference.

--

Rick C

Jeff wrote on 8/3/2017 1:17 PM:

You don't understand the meaning of "phase". If you said the two
frequencies were never the same I would agree.

Phase is fundamentally linked to frequency.

The slave pendulum runs slower than the master with the intermittent
impulse to adjust the phase. The relative phase varies with time as a
sawtooth function and so at some point the phase *must* be aligned as the
slave passes from being ahead to being behind.

That is a ridiculous statement, if it were true you could say that any 2
random signals were 'in phase' just because at some point in time they both
had the same phase angle.

Not sure if you are referring to the Shortt clock or the PLL. But the
statement applies equally to both. There is no magical stability in the
PLL. It is a control loop and as such the thing being controlled will
*never* remain in phase or at the same frequency as the reference.

--

Rick C

On 08/03/17 17:27, rickman wrote:


Not sure if you are referring to the Shortt clock or the PLL. But the
statement applies equally to both. There is no magical stability in the
PLL. It is a control loop and as such the thing being controlled will
*never* remain in phase or at the same frequency as the reference.


I think the difference is that while a pll always has a phase offset
the reference and vco are in phase lockstep once the loop has aquired
lock. It's a closed loop system whereas the Shortt clock is an open
loop system, only getting a kick back into sync from time to time.

Like a hit and miss governor ?...

Chris

Chris wrote on 8/3/2017 3:05 PM:
On 08/03/17 17:27, rickman wrote:


Not sure if you are referring to the Shortt clock or the PLL. But the
statement applies equally to both. There is no magical stability in the
PLL. It is a control loop and as such the thing being controlled will
*never* remain in phase or at the same frequency as the reference.


I think the difference is that while a pll always has a phase offset
the reference and vco are in phase lockstep once the loop has aquired
lock. It's a closed loop system whereas the Shortt clock is an open
loop system, only getting a kick back into sync from time to time.

Like a hit and miss governor ?...

I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.

The phase comparison signal from a PLL is typically "grainy" in the same way
and has to be filtered to become a control signal. The only reason you say
they are in "lock step" is because the grain is very fine. The Shortt clock
grain is very fine as well typically adjusting only every other 30 second
period.

I guess the difference is the Shortt clock is adjusting the instantaneous
phase and the average frequency while a typical PLL adjusts the
instantaneous frequency to try to keep the phase aligned. Both will see
variations in phase over time.

--

Rick C

On 08/03/17 21:31, rickman wrote:

I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.

The phase comparison signal from a PLL is typically "grainy" in the same
way and has to be filtered to become a control signal. The only reason
you say they are in "lock step" is because the grain is very fine. The
Shortt clock grain is very fine as well typically adjusting only every
other 30 second period.

I guess the difference is the Shortt clock is adjusting the
instantaneous phase and the average frequency while a typical PLL
adjusts the instantaneous frequency to try to keep the phase aligned.
Both will see variations in phase over time.


I would see the Shortt clock as a frequency locked loop, not the same
thing as a pll. Different level of instantaneous precision.

Semantics, semantics :-)...

Chris

Chris wrote on 8/3/2017 6:33 PM:
On 08/03/17 21:31, rickman wrote:

I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.

The phase comparison signal from a PLL is typically "grainy" in the same
way and has to be filtered to become a control signal. The only reason
you say they are in "lock step" is because the grain is very fine. The
Shortt clock grain is very fine as well typically adjusting only every
other 30 second period.

I guess the difference is the Shortt clock is adjusting the
instantaneous phase and the average frequency while a typical PLL
adjusts the instantaneous frequency to try to keep the phase aligned.
Both will see variations in phase over time.


I would see the Shortt clock as a frequency locked loop, not the same
thing as a pll. Different level of instantaneous precision.

Not sure why you say that. What is measured and adjusted is the phase.
Either the slave is a bit ahead or a bit behind and it is either spurred on
a bit or it is not. The frequency of the pendulum is not impacted other than
at the moment of phase adjustment.

--

Rick C

On 8/3/2017 3:05 PM, Chris wrote:
On 08/03/17 17:27, rickman wrote:


Not sure if you are referring to the Shortt clock or the PLL. But the
statement applies equally to both. There is no magical stability in the
PLL. It is a control loop and as such the thing being controlled will
*never* remain in phase or at the same frequency as the reference.


I think the difference is that while a pll always has a phase offset
the reference and vco are in phase lockstep once the loop has aquired
lock. It's a closed loop system whereas the Shortt clock is an open
loop system, only getting a kick back into sync from time to time.

Like a hit and miss governor ?...

Chris




In this case I have to (surprise!) agree with Rickman. A phase locked
loop is never in lockstep with the reference - there is always a bit of
drift in the oscillator.

It's no different than driving down a highway. You can aim your car
straight down the road - but you need to continually make small
adjustments to account for things like the road and the wind.

The Shortt clock is not that much different, except that it purposely
runs at a slightly lower frequency than the reference, and the frequency
at which the comparison occurs is much lower.

But the result is the same - a signal that is accurate due to
compensation based on the instantaneous phase at specific times.

--
==================
Remove the "x" from my email address
Jerry Stuckle

==================

Jeff wrote on 8/4/2017 4:58 AM:


I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.


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.

--

Rick C

In rec.radio.amateur.homebrew Jeff wrote:


I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.


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.

Jeff


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

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


--
Jim Pennino