WWVB Technology

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Radio Controlled Clock Technology
a brief discussion about broadcast time synchronization

Do you know exactly what time it is? Well, the most accurate clock in the U.S. is an atomic (cesium) clock located at the U.S. National Institute of Standards and Technology (NIST) in Boulder, Colorado. Atomic clocks keep time using natural characteristic frequencies of atoms such as cesium, hydrogen and rubidium and are extremely stable because the effect of factors such as temperature, pressure and humidity on these frequencies is negligible. The extremely accurate time measured at this facility is used by scientists, radio and television stations, telephone companies, space program personnel, air traffic control systems and others whenever a precise knowledge of the time is essential. This same technology is available in a receiver that connects to your PC, our Radio Sync.

Radio Broadcast
The
Wide area radio time broadcast station, WWVB, is  a long wave station with a 50kW transmitter operated by the NIST near Fort Collins, Colorado. It transmits a continuous time-coded signal at 60kHz. This signal can be picked up throughout the United States, Southern Canada and quite often much farther a field. The signal is normally transmitted with an accuracy better than 1 part in 100 billion and when received, is generally accurate to within 100 microseconds.

NIST radio station WWVB is located on the same site as WWV near Ft. Collins, Colorado. The WWVB broadcasts are used by millions of people throughout North America to synchronize consumer electronic products like ExactSet clocks. In addition, WWVB is used for high level applications such as ClockWatch Radio Sync network time synchronization and frequency calibrations.

Signal Description
WWVB continuously broadcasts time and frequency signals at 60 kHz. The carrier frequency provides a stable frequency reference traceable to the national standard. There are no voice announcements on the station, but a time code is synchronized with the 60 kHz carrier and is broadcast continuously at a rate of 1 bit per second using pulse width modulation. The carrier power is reduced and restored to produce the time code bits. The carrier power is reduced 10 dB at the start of each second, so that the leading edge of every negative going pulse is on time. Full power is restored 0.2 s later for a binary “0”, 0.5 s later for a binary “1”, or 0.8 s later to convey a position marker. The binary coded decimal (BCD) format is used so that binary digits are combined to represent decimal numbers.

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. If you plot WWVB phase, this results in a phase step of approximately 2.08 microseconds.

Antenna and Transmitters
WWVB uses two identical antennas that were originally constructed in 1962, and refurbished in 1999. The north antenna was originally built for the WWVL 20 kHz broadcast (discontinued in 1972), and the south antenna was built for the WWVB 60 kHz broadcast. The antennas are spaced 857 m apart. Each antenna is a top loaded dipole consisting of four 122-m towers arranged in a diamond shape. A system of cables, often called a capacitance hat or top hat, 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. The downlead serves as the radiating element.

Ideally, an efficient antenna system requires a radiating element that is at least one-quarter wavelength long. At 60 kHz, this becomes difficult. The wavelength is 5000 m, so a one-quarter wavelength antenna would be 1250 m tall, or about 10 times the height of the WWVB antenna towers. As a compromise, some of the missing length was added horizontally to the top hats of this vertical dipole, and the downlead of each antenna is terminated at its own helix house under the top hats. Each helix house contains a large inductor to cancel the capacitance of the short antenna and a variometer (variable inductor) to tune the antenna system. Energy is fed from the transmitters to the helix houses using underground cables housed in two concrete trenches. Each trench is about 435 m long.

A computer is used to automatically tune the antennas during icy and/or windy conditions. This automatic tuning provides a dynamic match between the transmitter and the antenna system. The computer looks for a phase difference between voltage and current at the transmitter. If one is detected, an error signal is sent to a 3-phase motor in the helix house that rotates the rotor inside the variometer. This retunes the antenna and restores the match between the antenna and transmitter.

There are three transmitters at the WWVB site. Each transmitter consists of two identical power amplifiers which are combined to produce the greatly amplified signal sent to the antenna. One transmitter delivering an amplified time code signal into the north antenna system, and one transmitter feeds the south antenna system. The time code is fed to a console where it passes through a control system and then is delivered to the transmitters.

Using two transmitters and two antennas allows the station to be more efficient. As mentioned earlier, the WWVB antennas are physically much smaller than one quarter wavelength. As the length of a vertical radiator becomes shorter compared to wavelength, the efficiency of the antenna goes down. In other words, it requires more and transmitter power to increase the effective radiated power. The north antenna system at WWVB has an efficiency of about 50.6%, and the south antenna has an efficiency of about 57.5%. However, the combined efficiency of the two antennas is about 65%. As a result, each transmitter only has to produce a forward power of about 38 kW for WWVB to produce its effective radiated power of 50 kW.

Performance
The frequency uncertainty of the WWVB signal as transmitted is less than 1 part in 1012. If the path delay is removed, WWVB can provide UTC with an uncertainty of less than 100 microseconds. The variations in path delay are minor compared to those of WWV and WWVH. When proper receiving and averaging techniques are used, the uncertainty of the received signal should be nearly as small as the uncertainty of the transmitted signal.

Technical details are from the NIST

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Last reviewed March 19, 2008