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Eight Amazing Objects: How Engineers Use the Elements to Create Extraordinary Technologies
ISBN 978-0-9839661-3-5 (paperback)
978-0-9839661-4-2 (electronic)
978-0-9839661-5-9 (hardback)

204 pages plus front and back matter

 

How an Atomic Clock Works This chapter lays out the essential principles and operation of an atomic clock. Specifically, it includes:

  • A discussion of the basics of timekeeping using resonance, focusing on the pendulum of a grandfather clock at first and then the vibrations of a quartz crystal oscillator, explaining briefly the piezoelectric effect in quartz.
  • A clear description of how the “atomic” part of an atomic clock functions as a feedback loop for a quartz oscillator. It goes in detail on the workings of the first cesium-based atomic clock.
  • A short discussion of the uses of accurate time.
  • The basic details and elementary mathematics underlying the GPS (Global Positioning System).

How an Atomic Clock Works

(This video is from EngineerGuy series #4)

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Transcript

I want to show you the most amazing thing.

The world's first commercially available chip scale atomic clock. Symmetricon's CSAC. That's right: This tiny device, about the size of a quarter, is an atomic clock. The most accurate atomic clocks lose about a second over 138 million years.

The way that atomic clocks work amazes me, let me explain how the very first one worked.

I'll start with Jell-O. Tap a block of Jell-O and it wiggles back and forth. Just like the swings of a pendulum in a grandfather clock, the oscillations of this Jell-O keep time.

Now, Jell-O isn't very good for this, but inside an atomic clock there's a chunk of quartz in a similar shape that it if we tap it - which we do it with a jolt of electricity - it will oscillate some five millions time per second. It keeps time to about 1 second in 90,000 years - a fraction of the accuracy needed for an atomic clock.

Quartz loses time because it slows down and needs to be "nudged" to restore its oscillation. There's where the “atomic” part of an atomic clock comes into play. We use cesium atoms to control those nudges very accurately. Every time the quartz's oscillations slow down just the tiniest bit we give it a tap - an electrical jolt at just the right time so essentially its oscillations never decay. Let me show you how we use cesium to do this.

The atoms in pure cesium exist mostly in two slightly different forms: A low energy form and one with just a bit more energy. For an atomic clock these two states have two properties critical to making a clock. One, they can be separated by a magnet. And two, the lower energy atoms can be converted to the higher energy ones if we bombard cesium with the right radiation.

Engineers tie the slowing down of the quartz vibrations to the precise wavelength of the bombarding radiation to create a feedback loop. Let me show you how.

In an oven we heat cesium chloride to create a gaseous stream of cesium ions. The stream contains both the low and high energy ions. We first flow it through a magnet separating the two types, discarding the high energy ones, allowing the lower energy ions to pass into a chamber. Inside the chamber we bombard the ions with just the right wavelength radiation to make them jump to higher energy. As these gaseous ions leave the chamber they pass through another magnet that directs high energy ions toward a detector, this time discarding any lower energy ones. The detector converts the arriving ions to a current

.

The trick here is to tie that current from the detector to the quartz oscillator. When the quartz's oscillations decay, that is it slows down a little, then the energy bombarding the cesium ions in the chambers changes and fewer high energy ions exit the chamber, so current decreases or stops. This tells the electronics to "zap" the quartz oscillator and correct the period of oscillation. It does this by applying the proper voltage that, via the piezoelectric effect, taps the quartz and restores its oscillations. Thus creating a clock that loses less than a second over many millions of years.

Our world runs off such accuracy. For example, the global position system - GPS - requires it.

The global positioning system consists of 24 satellites orbiting the earth. A GPS receiver uses the position of four of these satellites to locate itself. One to correct the time on the receiver and three to locate its position. Here's how it works. A signal is sent to the receiver from the first satellite that contains that satellites location and the signal's time of departure. The receiver then multiplies the signal's travel time by the speed of light to calculate its distance from the satellite. With one satellite the receiver knows that it's located on a sphere around that satellite with a radius equal to the calculated distance. So, it does the same calculation with a second satellite. The intersection of these two spheres narrows the location to the circumference of a circle. Then with a third satellite the receiver can reduce the location to a single point.

Since signals are traveling at the speed of light, being off by even a millisecond means an error of about a million feet, or 300 kilometers. But with atomic clock accuracy, the receiver can locate itself to about 3 feet.

I'm Bill Hammack, the engineerguy.

This video is based on a chapter in the book Eight Amazing Engineering Stories. The chapter features more information about this subject. Learn more about the book at the address below.