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KF5JRV > TECH 17.04.16 03:37l 107 Lines 7093 Bytes #999 (0) @ USA
BID : 1628_KF5JRV
Read: GUEST
Subj: WWVB Format
Path: IW8PGT<CX2SA<N9PMO<N0KFQ<KF5JRV
Sent: 160417/0051Z 1628@KF5JRV.#NWAR.AR.USA.NA BPQ1.4.65
Enhanced WWVB Broadcast Format Change
Since October 29, 2012 at 1500 UTC (9:00 AM MDT), NIST Radio Station WWVB has been
broadcasting a phase modulated (PM) time code that has been added to the legacy
AM/pulse-width-modulation signal. This enhancement to the broadcast, which has been tested
throughout 2012, provides significantly improved performance in new products that are designed
to receive it. Existing radio-controlled clocks and watches are not affected by this enhancement
and continue to work as before.
Disciplined oscillator products that track and lock to the 60 kHz WWVB carrier and were
designed to work as frequency standards, will not work with the PM signal and have become
obsolete. Radio-controlled clocks that are based on synchronous AM demodulation
(lock to the carrier), such as the Spectracom NetClock and receivers manufactured by True
Time during the 1970s and 1980s, have also become obsolete.
Station Information
NIST radio station WWVB is located on the same site as NIST HF radio station WWV near
Fort Collins, Colorado. The WWVB broadcasts are used by millions of people throughout North
America to synchronize consumer electronic products like wall clocks, clock radios and
wristwatches. In addition, WWVB may be used in other consumer timekeeping applications,
such as appliances, cameras, and irrigation controllers, as well as in high level applications
such as accurate time synchronization.
Signal Description
WWVB continuously broadcasts digital time codes on a 60 kHz carrier that may serve as a
stable frequency reference traceable to the national standard at NIST. The time codes are
synchronized with the 60 kHz carrier and are broadcast continuously in two different formats
at a rate of 1 bit per second using pulse width modulation (PWM) as well as phase
modulation (PM).
In the first of the two formats, based on PWM, which has been in use for several decades,
the carrier power is reduced by 17 dB at the start of each second and restored to full
power 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 pulse-width modulated time code contains the year, day of year,
hour, minute, UT1 time correction and flags that indicate the status of Daylight Saving
Time, leap years, and leap seconds, as listed in the legacy WWVB time code format
description and detailed in NIST Special Publication 432 (NIST Time and Frequency
Services).
In the second of the two formats, based on PM, which has been in use since
October 29, 2012, binary-phase-shift-keying (BPSK) modulation is used, wherein the
carrier's phase is unaffected when conveying a "0" and is inverted (i.e. 180-degree shifted)
when conveying "1". This time code, also operating at a rate of 1 bit/sec, is delayed by
0.1 s with respect to the first time code described above, such that 180-degree transitions
in the carrier phase can only occur 0.1 s after the 17 dB power reduction that is created
by the pulse-width-modulation. The phase-modulated information may take several different
forms, with the basic one having a frame duration of one minute, as in the legacy AM/PWM
broadcast.
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 monopole 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.
North antenna coordinates: 40° 40' 51.3" N, 105° 03' 00.0" W
South antenna coordinates: 40° 40' 28.3" N, 105° 02' 39.5" W
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. Two are in constant operation and one serves
as a standby transmitter that is activated if one of the primary transmitters fail. 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 more transmitter power to
increase the effective radiated power. The north antenna system at WWVB has an efficiency
of about 56.3%, and the south antenna has an efficiency of about 54%. However, the
combined efficiency of the two antennas is about 68.8%. As a result, each transmitter
only has to produce a forward power of about 51 kW for WWVB to produce its effective
radiated power of 70 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 about 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.
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