TheEC: Atomic Clocks

Aug 1, 2012

A collection of "atomic" clocks

A somewhat radio-related topic this time on the Engineer's Corner: Atomic Clocks! At NPR stations, like Rhode Island Public Radio, we rely on precision timekeeping because we coordinate our local programming with NPR's national programming via each end paying attention to a very specific clock. Everything is timed down to the second and, in some cases, to half-seconds! We use a special clock system from ESE to synchronize all our clocks in the studios. Such systems are expensive, costing thousands of dollars, because of their very high precision. But at home you can get pretty close by using an "atomic clock", available at most Radio Shacks, Walmarts and other, similar stores.

Atomic clocks keep their time by listening for a specific signal on 60 Hertz (Hz) from a special radio station: WWVB located in Fort Collins, Colorado. Because of the nature of signals at that frequency, and the physics of Earth's atmosphere at night vs. during the day, the signals from WWVB can reach much, much further late at night. In fact, it can - and does - cover all of the continental US. It's the same physics that allow AM radio
stations (which broadcast from 530 to 1700 kilohertz/kHz) to be heard hundreds or thousands of miles away at night; think of WBZ 1030 AM and its famous slogan of being "heard in 38 states!"

However, one thing to keep in mind is AC power "alternates" at 60 Hz as well. That means anything that actively uses AC power can interfere with the WWVB signal. That's especially true in Rhode Island, where even with WWVB's massive signal, we're still over 1700 miles away from Fort Collins!

How to maximize your atomic clock's reception?

1. Mount your clock in a place at least a few feet away from anything that uses power. Watch out for modern electronics that often use power even when "off:" things with clocks, TV's on standby, laptop computers, cellphone chargers, etc.

2. Your clock has an antenna inside, usually parallel to the face of the
clock. Angle the clock itself so it faces towards almost due west. If you
have a compass, it's 278 degrees. If not, the direction of the setting sun ought to do it.

3. The WWVB signal is usually strongest around 2 or 3 AM local time. Most
clocks will automatically check for a WWVB sync around then. But if you want to manually force a synchronization, wait until at least three hours after it's fully dark in Rhode Island.

If all else fails, try leaving the clock overnight in a west-facing window, or in your car on the dashboard. You don't need to sync an atomic clock every night; most will keep pretty good time (perhaps a second or two off) for at least three or four days.