On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
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
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
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
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.
Oh yes, I recall the B clock- I have an interest in clocks (actually
more watches) - and read up on Harrison's history, partly due to his
work on clocks / watches directly but also as much of my engineering
work was navigation related.
I recall reading of the building of the modern version of the B clock -
it must have been in the 70s or early 80s.
As you say, Harrison was a genius- albeit an largely unrecognised /
unappreciated one in his own time- at least by the Gov. of the day. I've
seen the examples of his work in the National Maritime Museum- the
quality is unbelievable, especially when you consider the technology of
Suspect someone is claiming a benefit under false pretences? Incapacity
Benefit or Personal Independence Payment when they don't need it? They
I've had an interest in clocks as well. Working in computing, was
interested in the IBM master clocks, which have a Graham deadbeat
escapement and either an electrically wound spring, or weight
driven mechanism, + an Invar pendulum. Found a mid 1930's
example some time ago, which has been running now for about a
year. Stripped down completely and rebuilt. IBM claim around 15
seconds a month error, but after rating for a few weeks, it shows
an error of less than a second a month. There's noise on the
stability, drifting +/- half a second or so from day to day, but
was quite amazed at the accuracy of such an old clock...
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.
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.
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
There is always jitter in the output of the PLL that is independent of the
Please review your PLL materials. There is no such thing as a PLL that
aligns perfectly with the reference.
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
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.
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.
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.
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