Page 361 - The Mechatronics Handbook
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HF Radio Signals (Including WWV and WWVH)
                                 High frequency (HF) radio broadcasts occupy the radio spectrum from 3 to 30 MHz. These signals are
                                 commonly used for time and frequency transfer at moderate performance levels. Some HF broadcasts
                                 provide audio time announcements and digital time codes. Other broadcasts simply provide a carrier
                                 frequency for use as a reference.
                                   HF time and frequency stations include NIST radio stations WWV and WWVH. WWV is located near
                                 Fort Collins, Colorado, and WWVH is on the island of Kauai, Hawaii. Both stations broadcast continuous
                                 time and frequency signals on 2.5, 5, 10, and 15 MHz, and  WWV also broadcasts on 20 MHz.  All
                                 frequencies broadcast the same program, and at least one frequency should be usable at all times. The
                                 stations can also be heard by telephone; dial (303) 499-7111 for WWV or (808) 335-4363 for WWVH.
                                   WWV and WWVH signals can be used in one of three modes:
                                     • The audio portion of the broadcast includes seconds pulses or ticks, standard audio frequencies,
                                       and voice announcements of the UTC hour and minute. WWV uses a male voice, and WWVH
                                       uses a female voice.
                                     • A binary time code is sent on a 100 Hz subcarrier at a rate of 1 bit per second. The time code
                                       contains the hour, minute, second, year, day of year, leap second and Daylight Saving Time (DST)
                                       indicators, and a UT1 correction. This code can be read and displayed by radio clocks.
                                     • The carrier frequency can be used as a reference for the calibration of oscillators. This is done
                                       most often with the 5 and 10 MHz carrier signals, since they match the output frequencies of
                                       standard oscillators.
                                   The time broadcast by WWV and WWVH will be late when it arrives at the user’s location. The time
                                 offset depends upon the receiver’s distance from the transmitter, but should be <15 ms in the continental
                                 United States. A good estimate of the time offset requires knowledge of HF radio propagation. Most
                                 users receive a signal that has traveled up to the ionosphere and was then reflected back to earth. Since
                                 the height of the ionosphere changes throughout the day, the path delay also changes. Path delay variations
                                                                           9
                                 limit the received frequency uncertainty to parts in 10  when averaged for 1 day.
                                   HF radio stations such as WWV and WWVH are useful for low level applications, such as the manual
                                 synchronization of analog and digital clocks, simple frequency calibrations, and calibrations of stop
                                 watches and timers. However, LF and GPS signals are better choices for more demanding applica-
                                 tions [2,7,15].


                                 LF Radio Signals (Including WWVB)
                                 Before the advent of satellites, low frequency (LF) signals were the method of choice for time and
                                 frequency transfer. While the use of LF signals has diminished in the laboratory, they still have two major
                                 advantages—they can often be received indoors without an external antenna and several stations broad-
                                 cast a time code. This makes them ideal for many consumer electronic products that display time-of-
                                 day information.
                                   Many time and frequency stations  operate in the LF band from 30  to 300 kHz  (Table 17.7).  The
                                 performance of the received signal is influenced by the path length and signal strength. Path length is
                                 important because the signal is divided into ground wave and sky wave. The ground wave signal is more
                                 stable. Since it travels the shortest path between the transmitter and receiver, it arrives first and its path
                                 delay is much easier to estimate. The sky wave is reflected from the ionosphere and produces results
                                 similar to those obtained with HF reception. Short paths make it possible to continuously track the
                                 ground wave. Longer paths produce a mixture of sky wave and ground wave. And over very long paths,
                                 only sky wave reception is possible.
                                   Signal strength is also important. If the signal is weak, the receiver might search for a new cycle of the
                                 carrier to track. Each time the receiver adjusts its tracking point by one cycle, it introduces a phase step
                                 equal to the period of the carrier. For example, a cycle slip on a 60 kHz carrier introduces a 16.67 µs
                                 phase step. However, a strong ground wave signal can produce very good results. An LF receiver that


                                 ©2002 CRC Press LLC
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