Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then
use use used to 'pull it' to reduce the error- which is how a true phase
lock loop works.

The system seems to operate more as follows, the slave is designed to
run 'very nearly right'. It receives precise pulses from the master
which it will naturally sync to. The same will happen if you have two
oscillators on nearly the same frequency if you 'feed' the output of one
to the tuned circuit of the other. (Including harmonics.) This is used,
for example, by some amateurs to lock radio oscillators to GPS locked
references.

Still, it is an clever system and of interest.

Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then use
use used to 'pull it' to reduce the error- which is how a true phase lock
loop works.

The system seems to operate more as follows, the slave is designed to run
'very nearly right'. It receives precise pulses from the master which it
will naturally sync to. The same will happen if you have two oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks to see
if the phase is fast or slow. In one case it invokes a spring that tweeks
the phase of the slave. In the other case it does not invoke the spring
allowing the clock to continue running unadjusted. The default behavior of
the slave clock is to run a bit slow and the adjustments speed it up (or the
other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that does
not make it anything other than a phase locked loop.

--

Rick C

On 01/08/17 13:49, rickman wrote:
Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then
use
use used to 'pull it' to reduce the error- which is how a true phase lock
loop works.

The system seems to operate more as follows, the slave is designed to run
'very nearly right'. It receives precise pulses from the master which it
will naturally sync to. The same will happen if you have two
oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for
example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks to
see if the phase is fast or slow. In one case it invokes a spring that
tweeks the phase of the slave. In the other case it does not invoke the
spring allowing the clock to continue running unadjusted. The default
behavior of the slave clock is to run a bit slow and the adjustments
speed it up (or the other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that
does not make it anything other than a phase locked loop.


Hmm, I half see your point but I'm not entirely convinced.

I'm just not convinced that the description truly 'maps' to that of a
true PLL.

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.

Brian Reay wrote on 8/1/2017 1:19 PM:
On 01/08/17 13:49, rickman wrote:
Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then use
use used to 'pull it' to reduce the error- which is how a true phase lock
loop works.

The system seems to operate more as follows, the slave is designed to run
'very nearly right'. It receives precise pulses from the master which it
will naturally sync to. The same will happen if you have two oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks to see
if the phase is fast or slow. In one case it invokes a spring that tweeks
the phase of the slave. In the other case it does not invoke the spring
allowing the clock to continue running unadjusted. The default behavior
of the slave clock is to run a bit slow and the adjustments speed it up
(or the other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that
does not make it anything other than a phase locked loop.


Hmm, I half see your point but I'm not entirely convinced.

I'm just not convinced that the description truly 'maps' to that of a true PLL.

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.

The only difference between that and the Shortt clock is the Short clock
only has one polarity of impulse and is adjusted to run a bit off so the
required intermittent impulses will keep it in phase with the master.

If you are interested in mechanical clocks (the Shortt clock uses
electricity to isolate the master and slave even though the master is purely
mechanical) you can read about the Fedchenko AChF-3 time piece. It came
well after the Shortt clock and not long before quartz and atomic clocks,
but was amazingly accurate without any fancy footwork with master slave
complexity.

Fedchenko used a compound spring for want of a better name. I've read that
it corrects for the parabolic distortion introduced in the timing of a
circular pendulum swing. This is a second order effect in that the
coefficient in the term is rather small. But in these clocks it makes a
difference. The way most clocks correct for it is to keep the amplitude of
the pendulum swing as constant as possible minimizing the second order
deviation. The Fedchenko clock uses a pendulum spring with two distinct
lengths. This causes a different rate of spring over the range of angle.
Some descriptions seem to say it actually causes the pendulum to swing in a
parabolic arc. Either way it corrects for the second order term in the time
equation of the pendulum making it less sensitive to variations in the
amplitude of oscillation.

--

Rick C

rickman wrote on 8/1/2017 11:59 PM:
Brian Reay wrote on 8/1/2017 1:19 PM:
On 01/08/17 13:49, rickman wrote:
Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

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

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then use
use used to 'pull it' to reduce the error- which is how a true phase lock
loop works.

The system seems to operate more as follows, the slave is designed to run
'very nearly right'. It receives precise pulses from the master which it
will naturally sync to. The same will happen if you have two oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks to see
if the phase is fast or slow. In one case it invokes a spring that tweeks
the phase of the slave. In the other case it does not invoke the spring
allowing the clock to continue running unadjusted. The default behavior
of the slave clock is to run a bit slow and the adjustments speed it up
(or the other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that
does not make it anything other than a phase locked loop.


Hmm, I half see your point but I'm not entirely convinced.

I'm just not convinced that the description truly 'maps' to that of a true
PLL.

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.

The only difference between that and the Shortt clock is the Short clock
only has one polarity of impulse and is adjusted to run a bit off so the
required intermittent impulses will keep it in phase with the master.

If you are interested in mechanical clocks (the Shortt clock uses
electricity to isolate the master and slave even though the master is purely
mechanical) you can read about the Fedchenko AChF-3 time piece. It came
well after the Shortt clock and not long before quartz and atomic clocks,
but was amazingly accurate without any fancy footwork with master slave
complexity.

Fedchenko used a compound spring for want of a better name. I've read that
it corrects for the parabolic distortion introduced in the timing of a
circular pendulum swing. This is a second order effect in that the
coefficient in the term is rather small. But in these clocks it makes a
difference. The way most clocks correct for it is to keep the amplitude of
the pendulum swing as constant as possible minimizing the second order
deviation. The Fedchenko clock uses a pendulum spring with two distinct
lengths. This causes a different rate of spring over the range of angle.
Some descriptions seem to say it actually causes the pendulum to swing in a
parabolic arc. Either way it corrects for the second order term in the time
equation of the pendulum making it less sensitive to variations in the
amplitude of oscillation.

Thought I'd mention John Harrison's 'Clock B' too. It was designed 250
years ago, but never built that I am aware of until recently. It has proved
to be nearly as accurate as the Shortt and Fedchenko clocks even though it
was a much, much earlier design. I don't know any details of why it is so
good other than that Harrison took into account every source of error and
included a compensating factor to balance it out. I haven't see any further
detail. Pretty impressive. Clearly the man was a genius.

--

Rick C

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