Is a radio controlled clock an atomic clock?

No. It contains an ordinary quartz movement and a longwave receiver that resets it once or twice a day from a broadcast originating at a national laboratory. The atomic clock is at the transmitter, so the wall clock is atomically traceable rather than atomic in itself.

Why does my radio clock only update at night?

Longwave signals travel much farther after dark, because the ionospheric layer that absorbs them during daylight disappears. Most clocks therefore schedule their reception attempts for the small hours, when a distant transmitter can be received reliably even at the edge of its coverage area.

What frequency does WWVB use?

WWVB broadcasts on 60 kilohertz from Fort Collins, Colorado. Britain's MSF signal also uses 60 kilohertz, from Anthorn in Cumbria, while Germany's DCF77 uses 77.5 kilohertz from Mainflingen. The formats differ, so a clock made for one region cannot decode another's signal.

Why will my clock not pick up the signal?

Interference is the usual cause. LED lighting, monitors, televisions and switch-mode power supplies all radiate noise in the same band and easily overwhelm a distant transmitter. Move the clock to an outside wall away from electronics, rotate it, and leave it overnight to try again.

Radio Controlled Clock and WWVB

The clock on the kitchen wall that has never once been adjusted, and that fixes itself when the clocks change, is almost certainly a radio controlled clock. It is usually sold as an atomic clock, which is misleading in an interesting way: there is no caesium inside it, only an ordinary quartz movement and a small receiver listening for a longwave time signal broadcast from a transmitter that may be hundreds of miles away.

This article explains how a radio controlled clock gets the time from the WWVB, MSF and DCF77 signals, why reception is better in the small hours than at midday, what to do when a clock refuses to set itself, and why the atomic label is a marketing convenience rather than a description. For a reference you can check it against, the live atomic time display reads out server-synchronised time.

What Is a Radio Controlled Clock?

It is a quartz clock with a longwave radio receiver attached. The quartz movement keeps time in the ordinary way, and once or twice a day the receiver picks up a coded broadcast from a national time station and resets the movement to match.

The broadcasts themselves originate at national laboratories, so the time in the signal really is derived from atomic standards. Three transmitters cover most of the radio clock market: WWVB in the United States, MSF in the United Kingdom and DCF77 in Germany, with JJY performing the same service in Japan. All of them work at very low frequencies, between 40 and 80 kilohertz, where signals follow the curve of the Earth and penetrate buildings far better than the VHF and UHF bands used for broadcasting. The data rate is correspondingly tiny, typically one bit per second, but a full date and time needs only sixty bits, so a complete frame arrives every minute.

How the WWVB Signal Works

WWVB transmits from Fort Collins, Colorado on 60 kilohertz, carrying time from NIST at a radiated power of tens of kilowatts. Each second the carrier is reduced in power for a moment, and the length of that reduction encodes a bit.

Sixty of those bits make one frame, and the frame carries the minute, the hour, the day of the year, and flags for daylight saving and for any pending leap second. A radio clock waits for the marker that identifies the start of a minute, reads the frame that follows, and sets the movement. Because a single frame can be corrupted by noise, most radio clocks demand agreement between successive minutes before accepting a change. Since 2012 the station has added a phase modulation on top of the older amplitude coding, which lets a modern receiver recover the signal at a considerably lower signal-to-noise ratio and improves reception near the edge of the coverage area. On a good night the transmitter reaches most of the continental United States and beyond.

MSF and DCF77: Britain and Germany

The European signals work on the same principles with different details:

MSF: broadcast on 60 kilohertz from Anthorn in Cumbria, carrying the time kept by NPL. It moved there from Rugby in 2007 and reliably serves the British Isles and a useful part of north-western Europe.
DCF77: broadcast on 77.5 kilohertz from Mainflingen, near Frankfurt, carrying the time kept by PTB. Its usable range of roughly two thousand kilometres covers most of continental Europe, which is why so many European radio clocks are set to it.
WWVB: 60 kilohertz from Fort Collins, covering North America, with a signal format that is not compatible with the European ones.
JJY: two Japanese transmitters, on 40 and 60 kilohertz, serving Japan and parts of neighbouring countries.

The formats differ, so a radio controlled clock bought in Germany will not set itself in Colorado and vice versa. A few travel clocks include multiple decoders, but most do not, and a clock carried across the Atlantic simply becomes an ordinary quartz clock that must be set by hand.

Why Does Reception Improve at Night?

Because the layer of the ionosphere that absorbs longwave signals by day disappears after dark. During daylight a receiver depends mainly on the ground wave, which weakens steadily with distance; at night the signal can also bounce off the ionosphere and travel much farther. A radio controlled clock therefore hears a far weaker signal at midday than at three in the morning.

This is why almost every radio clock attempts its update in the small hours rather than during the afternoon, and why a clock that sits stubbornly wrong all day will often be correct by breakfast. It also explains why moving one a few feet can matter so much. Interference is the other half of the story: LED lighting, computer monitors, televisions, switch-mode power supplies and solar inverters all radiate noise in the same band, and a transmitter hundreds of miles away cannot compete with a cheap power supply on the same shelf.

Getting a Difficult Radio Clock to Set Itself

Move it to an outside wall, ideally facing the transmitter, and keep it clear of electronics. The internal antenna is a ferrite rod that is deaf along its own axis, so rotating the clock changes reception dramatically: it should lie across the line to the transmitter rather than pointing at it. Then leave it overnight, because a forced daytime search will usually fail where an unattended night-time one succeeds. Reinforced concrete, metal foil insulation and basements defeat the signal entirely, and in those places a network-synchronised device is the better answer.

Why a Radio Controlled Clock Is Not an Atomic Clock

The atom is at the transmitter, not on your wall. What sits in the clock is a quartz oscillator no better than any other, corrected once a day from a broadcast that happens to originate at a national laboratory, which makes the device atomically traceable rather than atomic.

The practical consequence is small but real. Between updates a radio controlled clock drifts exactly as any quartz movement would, typically by a fraction of a second a day, and it accumulates that error until the next successful reception clears it. If reception fails for a week, the error grows for a week. The signal also takes a measurable time to travel, a few milliseconds across a continent, which no wall clock cares about but which rules these broadcasts out for precision timing. Genuine atomic instruments are described in how atomic clocks work, and the laboratories behind the broadcasts in national time standards. The underlying quartz behaviour is the subject of why computer clocks drift.

Conclusion

A radio controlled clock is an ordinary quartz clock with a longwave receiver, kept honest by a daily correction from WWVB, MSF, DCF77 or JJY. The signals carry a minute-long frame at one bit per second, reception is best overnight when the ionosphere stops absorbing longwave, and interference from nearby electronics is the usual reason a clock will not set. Calling it atomic is a shorthand for where the time came from, not for what is inside. To compare any clock against a live reference, open the atomic time display, measure your device on the clock accuracy check, or start at the atomicclock.now homepage.

Radio Controlled Clocks and WWVB: How They Really Work