5, -0.54 eV

How Does an Atomic clock work?

radioclocksBy now, you have probably seen or own a radio controlled clock. These clocks are sold in all forms: as wall clocks, desk clocks, travel alarms, and wristwatches. They have a tremendous advantage over conventional clocks, they are always right! When working properly, radio controlled clocks always display the correct time, down to the exact second. This means that you should never have to adjust them. During the transition from standard time to daylight saving time (DST) they "spring forward" one hour, and when DST is finished they "fall back" one hour.

Due to technology advances and the economies of scale, radio controlled clocks are now very inexpensive, often costing just a few dollars more than conventional clocks. This page provides information about radio controlled clocks, including how they work, where they work, and what to do when they don’t work.

How They Work

Some manufacturers refer to their radio controlled clocks as "atomic clocks", which isn't really true. An atomic clock has an atomic oscillator inside (such as a cesium or rubidium oscillator). A radio controlled clock has a radio inside, which receives a signal that comes from a place where an atomic clock is located.

In the United States, the signals received by radio controlled clocks originate from NIST Radio Station WWVB, which is located near Fort Collins, Colorado. WWVB broadcasts on a frequency of 60 kHz. Your radio controlled clock actually has a miniature radio receiver inside, which is permanently tuned to receive the 60 kHz signal.

The 60 kHz signal is located in a part of the radio spectrum called LF, which stands for low frequency. This is an appropriate name, because the FM radio and TV broadcasts that we are accustomed to listening to use frequencies thousands of times higher. The lowest frequency received by any of the other radios in your house is probably 530 kHz, the bottom of the AM broadcast band. Even that frequency is nearly 10 times higher than the WWVB signal.

At 60 kHz, there isn't enough room on the signal (bandwidth) to carry a voice or any type of audio information. Instead, all that is sent is a code, which consists of a series of binary digits, or bits, which have only two possible values (0 or 1). These bits are generated at WWVB by raising and lowering the power of the signal. They are sent at a very slow rate of 1 bit per second, and it takes a full minute to send a complete time code, or a message that tells the clock the current date and time. When you turn a radio controlled clock on, it will probably miss the first time code, so it usually takes more than one minute to set itself (sometimes 5 minutes or longer) depending on the signal quality and the receiver design.

Once your radio controlled clock has decoded the signal from WWVB, it will synchronize its own clock to the message received by radio. Before it does so, it applies a time zone correction, based on the time zone setting that you supplied. The time broadcast by WWVB is Coordinated Universal Time (UTC), or the time kept at the Prime Meridian that passes through Greenwich, England. While a few users like their clocks to display UTC (ham radio operators, for example), most prefer to display local time. This means that the time in your area is corrected by the number of hours shown in the table.

Time Zone Difference from UTC During Standard Time Difference from UTC During Daylight Time
Pacific -8 hours -7 hours
Mountain -6 hours
Central -5 hours
Eastern -4 hours

Once your radio controlled clock has synchronized, it won't decode the signal from WWVB again for a while. Most clocks only decode the signal once per day, but some do it more often (for example, every 6 hours). Those that decode the signal just once per day usually do it at midnight or in the very early hours of the morning, because the signal is easiest to receive when it is dark at both WWVB and at the site where the clock is located. In between synchronizations, the clocks keep time using their quartz crystal oscillators. A typical quartz crystal found in a radio controlled clock can probably keep time to within 1 second for a few days or longer. Therefore, you shouldn't notice any error when you look at your clock display, since it will appear to be on the right second, even though it has probably gained or lost a fraction of a second since the last synchronization.

Where They Work

WWVB radio controlled clocks should be able to work in most places in North America. The red areas on the coverage maps below show where a WWVB radio controlled clock should be able to synchronize. Note that the red area is largest at night, and smallest in the daytime (click on the map to see a larger image). For example, 0600 UTC is about midnight in the central United States.


These maps are based on a field strength of 100 microvolts per meter, which in theory should be a large enough signal for most receivers to work with. In fact, some receivers have much better sensitivity (20 or 30 microvolts per meter). However, simply having a large signal doesn't mean that the receiver will work. What really matters is the signal-to-noise ratio, or the size of the signal compared to the size of the electrical noise near the same frequency. Raising the noise level is just as harmful as reducing the signal level. For example, if the radio controlled clock is near a source of interference (like a computer monitor) the noise level will increase, and the clock might not be able to synchronize. If the radio controlled clock is in a building with a metal roof, much of the signal will be blocked. Therefore, the signal level will be reduced, and the clock might not be able to synchronize.

Source: www.nist.gov
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