
Shortt–Synchronome free pendulum clock - segfaultbuserr
https://en.wikipedia.org/wiki/Shortt–Synchronome_clock
======
segfaultbuserr
> _Using modern optical sensors which detected the precise time of passage of
> the pendulum without disturbing it, he compared its rate to an atomic clock
> for a month. He found that it was stable to 200 microseconds per day (2.31
> ppb), equivalent to an error rate of one second in 12 years, far more
> accurate than the 1 second per year that was previously measured._

This is incredible. Its performance matched or exceeded quartz oscillators in
modern electronic systems [0]. It's the ultimate (electro-)mechanical clock
[1].

A good general-purpose quartz crystal oscillator is only stable to 20,000 ppb,
a good temperature-compensated oscillator is stable to 1000 ppb, and a crystal
oven (a quartz crystal in a sealed, temperature-controller oven, the best
quartz oscillator you can ever made) is stable to 1 ppb. And this ultimate
mechanical clock is stable to 2.31 ppb [0], I'd happy to use it as the time
base for my SDR receiver or spectrum analyzer if I was given the chance...
It's only measured for a month, so long-term stability can be worse (and
crystal oven's short-term stability can be better), but you get the idea.

> _Shortt clocks kept time with two pendulums, a master pendulum swinging in a
> vacuum tank and a slave pendulum in a separate clock, which was synchronized
> to the master by an electric circuit and electromagnets. The slave pendulum
> was attached to the timekeeping mechanisms of the clock, leaving the master
> pendulum virtually free of external disturbances._

The basic principle of accurate timekeeping - using a free-running primary
oscillator as a reference, and lock a less-accurate secondary oscillator to
the primary, is vital in electronic systems. Atomic clocks, radio/GPS time
signal receivers, or NTP, all use this principle. This clock basically
implemented a rudimentary form of PLL mechanically.

[0]
[https://en.wikipedia.org/wiki/Crystal_oven#Comparison_with_o...](https://en.wikipedia.org/wiki/Crystal_oven#Comparison_with_other_frequency_standards)

[1] Now electro-mechanical clocks are making the comeback, in the form of MEMS
oscillators in consumer electronics. But MEMS oscillators are often slightly
worse than its equivalent crystal oscillators, and their main advantage is
size (i.e. chip-scale integration), not accuracy.

~~~
petertodd
> Now electro-mechanical clocks are making the comeback, in the form of MEMS
> oscillators in consumer electronics

Note that quartz crystals are also electro-mechanical devices, as the quartz
crystal is physically vibrating. This is particularly easy to see in lower-
frequency quartz oscillators, such as the common 32.768KHz ones, that
physically look and act just like tuning forks:
[https://www.youtube.com/watch?v=_2By2ane2I4](https://www.youtube.com/watch?v=_2By2ane2I4)

This also means that mechanical vibration/shock can throw off quartz crystals:
[https://www.youtube.com/watch?v=M4Z7CjxfUU8](https://www.youtube.com/watch?v=M4Z7CjxfUU8)

~~~
segfaultbuserr
Good point.

> _This also means that mechanical vibration /shock can throw off quartz
> crystals_

This is one strength of MEMS oscillators, they are more tolerate to shocks and
vibrations. [https://www.sitime.com/sites/default/files/gated/SiTime-
MEMS...](https://www.sitime.com/sites/default/files/gated/SiTime-MEMS-Timing-
Improves-Motor-Control-Applications.pdf)

------
emerged
Do pendulums lose accuracy from seismic activity, people stomping on the
floor, etc? Or does the nature of a pendulum naturally absorb that sort of
interference?

~~~
fanf2
Here’s an example of a magnitude 7.0 earthquake in Anchorage disrupting a
synchronome clock near Seattle -
[http://leapsecond.com/pend/synchronome/quake.htm](http://leapsecond.com/pend/synchronome/quake.htm)
(Not a fancy Shortt-Synchronome tho)

~~~
mackman
Wow interesting. I would have bet (and lost) that the seismic activity would
have been at a such higher frequency that it wouldn’t have affected the 1Hz
pendulum. I suppose either that is wrong or there’s also some much lower
frequency energy in an earthquake.

~~~
fanf2
Here’s a page about that earthquake which has a lot of data including the
power spectrum. It peaks around 1Hz. [https://www.monitorseis.net/seismo-
blog/m70-in-alaska-and-so...](https://www.monitorseis.net/seismo-blog/m70-in-
alaska-and-some-quakes-in-the-pnw)

------
tantalor
[https://en.wikipedia.org/wiki/Master/slave_(technology)#Term...](https://en.wikipedia.org/wiki/Master/slave_\(technology\)#Terminology_concerns)

~~~
segfaultbuserr
The terminology in this particular system was chosen in 1921, it's unlikely to
be revised again.

