Stopwatch and Timer Calibrations - National Institute of Standards ...

The Direct Comparison Method
Please note that local “time and temperature” telephone services are not traceable
references and should not be used. For traceable calibrations, use only sources
that originate from a national metrology institute, such as those listed in Table
6. The following sections briefly describe the various radio and telephone time
signals, and provide information about the types of clock displays that can and
cannot be used.
5.A.1. Audio Time Signals Obtained by Radio
The radio signals listed in Table 6 include a voice announcement of UTC and
audio ticks that indicated individual seconds. WWV, the most widely used station,
features a voice announcement of UTC occurring about 7.5 s before the start of
each minute. The beginning of the minute is indicated by a 1500 Hz tone that
lasts for 800 ms. Each second is indicated by a 1 kHz tone that lasts for 5 ms.
The best way to use these broadcasts is to start and stop the stopwatch when the
beginning of the minute tone is heard.
Most of the stations listed in Table 6 are in the high-frequency (HF) radio
band (3 MHz to 30 MHz), and therefore require a shortwave radio receiver. A
typical general-purpose shortwave receiver provides continuous coverage of the
spectrum from about 150 kHz, which is below the commercial AM broadcast
band, to 30 MHz. These receivers allow the reception of the HF time stations
on all available frequencies. The best shortwave receivers are designed to work
with large outdoor antennas, with quarter-wavelength or half-wavelength dipole
antennas often providing the best results. However, in the United States, adequate
reception of at least one station can usually be obtained with a portable receiver
with a whip antenna, such as the one shown in Figure 10. This type of receiver
typically costs a few hundred dollars or less.
HF radio time stations normally broadcast on multiple frequencies because
some of the frequencies are not available at all times. In many cases, only one
frequency can be received, so the receiver might have to be tuned to several
different frequencies before finding a usable signal. In the case of WWV, 10
MHz and 15 MHz are probably the best choices for daytime reception, unless the
laboratory is within 1000 km of the Fort Collins, Colorado station, in which case
2.5 MHz might also suffice. Unless the receiver is near the station, the 5 MHz
signal will probably be the easiest to receive at night [13].
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