[time-nuts] Mechanical 1PPS Oscillator Disciplining

Tom Van Baak tvb at LeapSecond.com
Fri Jan 9 16:51:25 EST 2015


Yes, Neal Stephenson's Mother Earth Mother Board article is a classic that every time nut should read at some point.
The "one page" version is at http://archive.wired.com/wired/archive/4.12/ffglass_pr.html

Prior to quartz, pendulum clocks and tuning fork oscillators were the standard. Even until the 1950's or early 1960's tuning fork oscillators were used when one needed accurate frequency in the audio range. That's because dividing down high frequency quartz oscillators to, say, 60 Hz or 400 Hz required lots of circuitry. Not sure if Neal's reference to "vibrating reed" is what we would call a tuning fork, or if it's something else.

Here in the US General Radio made precision tuning fork oscillators. Model numbers 213, 723, 813, 815. One example is at http://leapsecond.com/museum/gr815b/
Also check out old issues of "General Radio Experimenter" magazine for details on these wonderful instruments.

Pendulum clocks were also used in power plants around the world to keep the grid synchronized. There is occasional discussion about this on clock collecting or horology forums. They are precious and can be extremely accurate, as good as a second a year.

Since pendulum clocks were better long-term timekeepers and generated only 0.5 or 1 Hz signals, a PDTF (Pendulum Disciplined Tuning Fork) made sense. Has a nice ring to it, doesn't it.


----- Original Message ----- 
From: "Andy Bardagjy" <andy at bardagjy.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Friday, January 09, 2015 9:22 AM
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 indefinitely."

Does anyone know any other history about that particular piece of equipment?


Andy ◉ Bardagjy.com ◉ +1-404-964-1641

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