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The face of an analog quartz watch.

Radio-controlled and atomic clocks

by Chris Woodford. Last updated: March 30, 2016.

You might have the most expensive watch in the world, but if it's set to the wrong time to begin with, it's no use to you at all. Even really good quartz clocks struggle to keep time to better than a second a day; if they wander out by just a couple of seconds in 24 hours (an amazing accuracy of 99.998 percent), and the errors don't cancel out, that could add up to a minute a month or almost a quarter of an hour a year. That's why most people regularly check their watches against a reliable time signal—like the ones you hear before news broadcasts on radio stations. Now wouldn't it be neat if your watch could listen to those broadcasts and set itself to the right time automatically without you ever needing to worry? That's the basic idea behind radio-controlled clocks and watches, which set their time by super-accurate atomic clocks. Let's take a closer look at what these things are and how they work!

Photo: Once, only very wealthy people could afford superb Swiss watches like this. Now quartz watches synchronized with radio signals mean anyone can own a watch as accurate as an atomic clock. Radio-controlled clocks and watches were popularized by such companies as Junghans in the late 1980s and early 1990s. Today, many different manufacturers make them and there are millions in use all over the world.

What is an RCC?

Conceptual idea of how a RCC radio-controlled clock works: a radio sends a signal to a quartz clock to keep it in time.

An ordinary clock or watch is a time-counting device that adds up the number of seconds, minutes, hours, and days that have passed. But it doesn't actually know what time it is until you tell it: it's not a time-keeping device unless you set it to the right time to start with. A radio-controlled clock (RCC) is different. It's similar to an ordinary electronic clock or watch but it has two extra components: an antenna that picks up radio signals and a circuit that decodes them. The circuit uses the radio signals to figure out the correct time and adjusts the time displayed by the clock or watch accordingly. Unlike an ordinary clock or watch, an RCC always knows what time it is—you never have to tell it!

Photo: The basic concept of RCC radio-controlled clocks: a radio transmitter hundreds or thousands of km/miles from your home (represented here by the ordinary silver radio) beams regular signals to your quartz clock or watch to keep it in time.

The radio signals come from a unique radio "station" that doesn't broadcast any words or music. There's no DJ and no irritating advertisements for car insurance. All the station broadcasts is the time—over and over again—in the form of a special code that only radio-controlled clocks can understand. In the United States, these time signals are broadcast by a station called WWVB operated by the National Institute of Standards and Technology (NIST) from a base near Fort Collins, Colorado. (Other countries have equivalent radio stations. In the UK, for example, the station is called MSF and operated by the National Physical Laboratory, while China's station is called BPC and broadcast by the National Time Service Center.) The NIST time code contains the basic time and date, whether it's a leap-year, whether it's daylight-saving time, and so on and takes about a minute to broadcast in its entirety.

Most RCCs synchronize themselves with a time broadcast signal once a day, at night, although some check themselves every few hours. Generally, that gives them an accuracy of better than plus or minus a half second (±0.5s) a day. Another advantage is that they automatically correct themselves for daylight-saving time, leap years, months with different numbers of days, and so on.

It's pretty obvious that an RCC is only going to be as accurate as the time signals it uses to regulate itself. How can you be sure those are accurate? The time-signal radio stations operated in different countries broadcast UTC (Coordinated Universal Time), the officially agreed time used worldwide that's informally known as GMT (Greenwich Mean Time). UTC is maintained by hundreds of atomic clocks (the world's most accurate timekeeping devices) around the world, all of which are synchronized with one another. It's because RCC radio signals are based on time kept by atomic clocks that you'll sometimes see RCC manufacturers describing their products as "atomic" clocks and watches (even though they're really no such thing).

What are atomic clocks?

Atomic clocks are actually quartz clocks—just like the ones you have at home. The difference is that an ordinary quartz clock relies purely on the oscillations of its quartz crystal to count seconds. As we've already seen, the rate at which quartz vibrates is affected by things like ambient temperature, so although a quartz clock is generally very accurate, it doesn't necessarily keep time as well as you might think. By contrast, an atomic clock has an extra mechanism—pulsating atoms—that it uses to keep an ordinary quartz clock to time.

Cesium atomic clocks

This atomic mechanism is based on the idea that atoms have electrons in particular energy states. When an atom absorbs energy, electrons leap to higher energy states and become unstable. They then give out the same energy as photons of light (or some other kind of electromagnetic radiation such as X rays or radio waves), returning to their original or ground state. The cesium atoms used in many atomic clocks have 55 electrons arranged in orbitals. The very outermost electron can oscillate between two different energy states by spinning in two slightly different ways. When it shifts from the higher to the lower of these states, it gives out a photon that corresponds to microwaves with a frequency of exactly 9,192,631,770 Hz (roughly 9.2 billion hertz or 9.2 gigahertz). That means it can be stimulated from its lower to its higher state by exactly the same microwaves. We can use this neat fact to keep a quartz clock to very precise time.

NIST-F1 United States atomic clock

Photo: The NIST-F1 Cesium fountain atomic clock: the amazingly accurate clock by which pretty much every other clock and watch in the United States is set! Photo by courtesy of National Institute of Standards and Technology (NIST) Physics Laboratory.

In a cesium atomic clock, there's a quartz oscillator tuned to exactly the same frequency, 9,192,631,770 Hz, which makes microwaves and fires them at a bunch of cesium atoms. If its frequency is correct, and hasn't drifted at all, these microwaves will have exactly the right amount of energy to shift the electrons in the atoms to their higher energy state. A magnetic detector in the clock measures how many atoms are in the higher and lower energy states. If most are in the higher state, it means most have been excited by the waves from the quartz oscillator. And that means those waves are exactly the right frequency, so the quartz oscillator must be telling time correctly. However, if the atoms are mostly in the lower state, it means the oscillator has drifted away from its correct frequency and isn't giving out the right amount of energy to promote electrons in the cesium atoms. A feedback mechanism in the clock detects this and adjusts the frequency of the oscillator so it's correct again. In this way, the quartz oscillator is constantly regulated so it's always exactly set to 9,192,631,770 Hz. An electronic circuit converts this exact frequency into one-per-second pulses that can be used to drive a relatively ordinary quartz clock mechanism with amazing accuracy. "Amazing" in this case means just that: the best atomic clocks are accurate to within 2 nanoseconds per day, or one second in 1.4 million years!

Other types of atomic clocks

Other atomic clocks work in broadly the same way but using atoms of different gases to regulate the quartz oscillator. In a hydrogen clock, atoms of hydrogen gas are stimulated with a microwave-frequency laser (maser), but they're less practical because hydrogen is a fairly hard gas to contain. Rubidium clocks are simpler, and therefore more compact and portable; they use microwaves to excite the atoms in rubidium glass. The world's most advanced atomic clocks, such as NIST-7 at the National Institute of Standards and Technology in Boulder, Colorado, use what are called atomic fountains. They use six laser beams to contain cesium atoms, cool them almost to absolute zero, bounce them upward, and let them fall back down through gravity (hence the name "atomic fountain"). This process makes them oscillate between two precise energy states that can be measured, in a broadly similar way to how we explored above, and used to keep a quartz clock to time.

How do atomic clocks work?

Step-by-step diagram showing how atomic clock works

  1. At one end of the clock, an oven (red) heats a lump of cesium metal so cesium atoms boil off it. The atoms are either in their unexcited ground state (orange) or their excited state (yellow).
  2. A magnetic filter at one end of the oven allows only unexcited atoms (orange) to pass through.
  3. The unexcited atoms enter a chamber called a microwave cavity. Here, they are bombarded with microwaves controlled by a quartz oscillator (green, 6), theoretically tuned to the magic frequency of 9,192,631,770 Hz.
  4. If the quartz oscillator is working at exactly this frequency, most of the unexcited cesium atoms will be converted to their excited state. Otherwise, far fewer will be excited. A second magnetic filter lets only the excited atoms pass through.
  5. A detector measures the number of excited atoms.
  6. The detector feeds back a signal to the microwave oscillator (green), constantly adjusting its frequency to ensure that a maximum number of atoms are excited. This ensures that the frequency of the oscillator is always as close as possible to 9,192,631,770 Hz.
  7. An electronic divider circuit (blue) converts this high frequency signal down to a lower frequency signal that can drive a fairly ordinary quartz clock mechanism.
  8. A digital display (gray) connected to the circuit shows the precise atomic time.

Who invented radio-controlled clocks?

Early electromagnetic radio-controlled clock from 1921, patent drawing by Thaddeus Casner.

According to Michael Lombardi of the NIST, one of the world's authorities on radio-controlled clocks: "There is no true consensus on who invented the first RCC that could synchronize to a wireless signal." He suggests the first such device may have been the Horophone invented by Frank Hope-Jones (1887–1950) and sold from 1913.

I looked through numerous patents covering RCCs on the US Patent and Trademark Office database and the earliest one I found was filed on March 24, 1921 (granted February 5, 1925) by Thaddeus Casner for the Radio Electric Clock Corporation of New York City. Casner explains that his invention covers a "... mechanism by means of which a clock may be periodically corrected by electrical impulses transmitted through space... [by] Hertzian waves [what we now call radio waves]..." You can read a full description and browse numerous detailed drawings (including the one shown here) in US Patent: # 1,575,096: Mechanism for Synchronizing Clocks (via Google Patents).

Artwork: One of the drawings of Thaddeus Casner's early radio-controlled clock. It keeps time using a traditional gear mechanism (which I've roughly indicated in blue), but also uses electromagnets (red) controlled by radio signals to keep the time correct. Artwork courtesy of US Patent and Trademark Office.

Can clocks really be this accurate?

We can now tell time with an incredible degree of accuracy—a nanosecond or two each day. While you might think that's wonderful, it really just swaps one problem for another. In past times, the problem was that we couldn't tell time well enough to keep up with the "natural accuracy" of the real world. So while the heavens turned and the planets whizzed round the Sun, our clocks struggled to keep time as effectively as the natural clock high in the sky.

Today, ironically, it's just the opposite. We now define time not in terms of moving planets but using oscillating atoms. Since 1967, the second has been defined as 9,192,631,770 oscillations of the atom cesium-133 between two energy states (for reasons we saw up above). Keep time with an atomic clock (as the world's various national standards organizations now do) and you'll find that the stars and planets gradually get out of step—because they're not moving accurately enough! Earth's rotational speed isn't constant, for example: it has random blips and it's gradually slowing down. All this means that we have to "correct" our super-accurate atomic clocks, from time to time, so they keep step with the much less accurate world around us. We do that by adding occasional "leap seconds" to the official, scientifically measured world time (International Atomic Time, TAI) so that it always agrees with the official time that people actually use (Coordinated Universal Time, UTC).

A brief history of atomic time

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