Radio Controlled Clock Technology
a brief discussion about broadcast time
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.
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
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
More about the NIST
Frequently Asked Questions
Computer Clock Synchronization
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