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Kirt's Cogitations™ #226

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WWVB Atomic Clock Signals

WWVB - National Bureau of Standards (Kirt's Cogitations #226) - RF CafeAs is typical for Gary Breed, editor of High Frequency Electronics magazine, he wrote a great general interest article about the time and frequency broadcasts put out by the National Institute of Standards and Technology (NIST) – originally National Bureau of Standards (NBS). His piece concerned the history of the WWV and WWVH signals that have been used since their origin in 1920 by amateur and professional radio operators to calibrate and synchronize time and frequency standards. I highly recommend reading the one-page article.

There is yet another standard signal that is broadcast continually from the NIST site in Boulder, Colorado, that just about everyone is familiar with whether they know it or not. That would be the WWVB signal used by all of the "radio-controlled" or "atomic" clocks that people have sitting on their mantels, hanging on their walls, built into Wal-Mart electronics weather stations, or are wearing on their wrists as a watch.

WWVB Antenna Array at NIST in Boulder, ColoradoThe WWVB radio signal is broadcast continually at 60 kHz, from an antenna array located on the plains just east of the foothills of the Rocky Mountains, near Boulder, CO (I have been by there when living in Colorado). The two sets of antennas are spaced 2,812 feet (857 m) apart. Each antenna is a top loaded monopole consisting of four 400 foot (122 m) high towers arranged in a diamond shape. A "top hat" system of cable is suspended between the four towers. This top hat is electrically isolated from the towers, and is electrically connected to a downlead suspended from the center of the top hat, with the downlead serving as the radiating element. A lot of other tricks that any Ham would love are employed to make the antenna as efficient as possible (about 65% between the two antennas) for a system that operates at a wavelength of about 2.7 miles (5 km).

The effective radiated power is 50 kW. A computer system is used to keep the system tuned under all meteorological conditions (rain, ice, wind, etc.). The frequency uncertainty of the WWVB signal as transmitted is less than 1 part in a trillion (1012). When the path delay to the remote receiver is removed, UTC time can be determined with an uncertainty of less than 100 microseconds. A full description is provided on the WWVB website.

The 60 kHz signal is broadcast continuously at a rate of 1 bit per second using pulse width modulation to produce a binary coded decimal (BCD) format. The time code contains the year, day of year, hour, minute, second, and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds. WWVB identifies itself by advancing its carrier phase 45° at 10 minutes after the hour and returning to normal phase at 15 minutes after the hour. Again, visit the website for details.

Coverage area maps are provided by NIST that plot two-hour increments and show where the receive signal strength is expected to be 100 microvolts per meter or greater. As you would expect for 60 kHz, the area varies throughout the 24-hour period, contracting during daylight hours and expanding at night. The western half of the United States is pretty much fully covered all the time, but the east coast falls well below the threshold for a large part of the day. This explains why the directions that come with most radio-controlled clock products says that it might take until the next morning after power-up to be able to synchronize with Boulder time.

WWVB is yet another one of those ubiquitous technologies that has been so absorbed into our world that we barely notice or appreciate it. Sort of like GPS, which nowadays inhabits not only aircraft and ship cockpits but extends to cell phones, laptop computers, car dashboards and dog collars, WWVB-based clocks are all around us. My house has a WWVB Radio-Controlled or Atomic Clock Symbol (Icon, Logo)digital weather station with a built-in "atomic clock." I see WWVB-synchronized clocks sitting on peoples' desks at work. Some types of test equipment, including calibration lab time standards, are now advertising integrated WWVB receivers. Look around and you will certainly see something with the familiar radiating antenna symbol on it.

If you are really interested in learning what it takes to design a product that uses the WWVB signal, here is a link to the complete designer's specification published by NIST.

 "WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers"

This pub, "NIST Time and Frequency Services," thoroughly explains the NIST time and frequency standard system.

Additional Info:  See WWV Moves to Colorado (Part 1) and WWV Moves to Colorado (part 2), in the ARRL's QST magazine.



Posted October 14, 2021
(updated from original post on 12/29/2009)

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