NIST Changes WWVB Modulation to Improve ‘Atomic Clock’ Reception
Adds phase modulation to existing AM signal
March 15, 2013
The wristwatches and clocks that advertise accurate “atomic time” rely on the 60 kHz signal from WWVB in Boulder Colo. to synchronize them to official U.S. time. In the days of analog TV, some broadcasters used the 60 kHz signal to provide an accurate frequency reference for transmitters and sync generators, or for accurate studio time. While broadcasters have largely switched to GPS as the precision reference for time and frequency, those “atomic” consumer clocks still rely on WWVB, as it can be received indoors with a fairly simple receiver.
As with other devices using the lower frequency bands, harmonics from switching power supplies and noise from other electronic devices is making it more difficult to receive the 60 kHz WWVB signal indoors. To reduce the impact of this noise, NIST is now implementing a new phase modulation WWVB signal
. The existing AM modulation will be retained so that old timekeepers will still work, but newer devices--expected to show up in the marketplace later this year--will benefit from the more robust phased-modulated signal.
NIST has a Web page, WWVB Radio Controlled Clocks
that shows WWVB coverage and offers some tips on how to improve reception. Another Webpage, NIST Radio Station WWVB
shows the transmit antennas and describes the time format. The paper Enhanced WWVB Broadcast Format
explains the new modulation method and how the data is encoded.