[time-nuts] Mechanical 1PPS Oscillator Disciplining

Bill Hawkins bill at iaxs.net
Fri Jan 9 23:22:02 EST 2015

Wow! That reference is a nugget of pure gold amongst the chaff. As a
mechanical engineer by training (BSME MIT 1960) and experience
(industrial process control), I am deeply interested in
electromechanical things.

Experience suggests that it is impossible for widely separated pendulum
clocks to "remain synched with each other indefinitely." RS-232
communication does not require synchronized clocks because the message
contains a string of bits that synchronizes the clocks adequately before
the message begins. Certainly computer crystal clocks are not
synchronized. Is it true that there have been "no new ideas since the
turn of the century"? Sadly, there seems to be some hype in Stephenson's
beautifully written article.

On New Year's Day as I set a Seth Thomas pendulum clock that has been
running for over a hundred years, I thought about disciplining a 10 MHz
oscillator with the pendulum's beat in order to measure the variations
in ADEV and phase noise through the seasons. I have some of the older
Lucent boxes (with Rb) that could be modified to provide the PCDO
(pendulum clock), but I don't know what I need to calculate and record
those measurements. For that matter, is it feasible or is there too much
noise? Am I the 420,000th person to try this?

Any answers appreciated.

Bill Hawkins

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Andy
Sent: Friday, January 09, 2015 11:22 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Mechanical 1PPS Oscillator Disciplining

>From a fascinating (albeit long) article about transatlantic
communication cables


On the bottom of page 45 to the top of page 46

"Each piece of equipment on this tabletop is built around a motor that
turns over at the same precise frequency. None of it would work - no
device could communicate with any other device - unless all of those
motors were spinning in lockstep with one another. The transmitter,
regenerator/retransmitter, and printer all had to be in sync even though
they were thousands of miles apart.

This feat is achieved by means of a collection of extremely precise
analog machinery. The heart of the system is another polished box that
contains a vibrating reed, electromagnetically driven, thrumming along
at 30 cycles per second, generating the clock pulses that keep all the
other machines turning over at the right pace. The reed is as precise as
such a thing can be, but over time it is bound to drift and get out of
sync with the other vibrating reeds in the other stations.

In order to control this tendency, a pair of identical pendulum clocks
hang next to each other on the wall above. These clocks feed steady,
one-second timing pulses into the box housing the reed. The reed, in
turn, is driving a motor that is geared so that it should turn over at
one revolution per second, generating a pulse with each revolution. If
the frequency of the reed's vibration begins to drift, the motor's speed
will drift along with it, and the pulse will come a bit too early or a
bit too late. But these pulses are being compared with the steady
one-second pulses generated by the double pendulum clock, and any
difference between them is detected by a feedback system that can
slightly speed up or slow down the vibration of the reed in order to
correct the error. The result is a clock so steady that once one of them
is set up in, say, London, and another is set up in, say, Cape Town, the
machinery in those two cities will remain synched with each other

Does anyone know any other history about that particular piece of


Andy ? Bardagjy.com ? +1-404-964-1641

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