How Do You Know the Correct Time?
Jon Titus - March 22, 2012
In September 2011, researchers at the CERN research labs in Geneva, Switzerland revealed
neutrinos sent from Geneva to detectors in Gran Sasso, Italy might have traveled faster than light.
Now, though, the 60-nanosecond discrepancy between the arrival of the neutrinos and the speed of
light might come down to faulty timing. So how to do know what time it really is?
A standards organizations such as the US National Institute of Standards and Technology (NIST)
owns many atomic clocks that use an electron transition between two energy levels in cesium-133
atoms to define a second, which comprises 9,192,631,770 cycles of radiation between these energy
levels. Hewlett-Packard (now Agilent Technologies) manufactured a primary cesium-based atomic
clock called the HP-5071A. In 2005, Agilent sold its time-standards product line to Symmetricom,
which still manufactures the 5071A standard. A search on ebay turned up two of the original
HP/Agilent standards for about $US 20,000. So that type of timing standard seems out of reach of
many labs.
About eight years ago, researchers at NIST created a chip-scale atomic clock shown in the image
below. Information provided at the time noted, "The chip-scale clock is less accurate than larger
atomic clocks... However, the clock’s small size, low power dissipation and potentially low cost make
it ideal for a variety of commercial and military applications. Compared to quartz-crystal oscillators,
the most precise time and frequency references of equivalent size and power, chip-scale atomic
clocks potentially offer a 1,000-fold improvement in long-term timing precision."
Image courtesy of NIST.
Now you can buy a small atomic clock at modest cost ($US 1500) and mount it on a circuit board.
The SA.45s Chip Scale Atomic Clock (CSAC) comes from Symmetricom, mentioned above. The small
module weighs 35 grams, and measure 1.6 x 1.4 x 0.5 inches (4 x 3.5 x 1.1 cm). The module
consumes less than 120 mW of power and can switch into a 50-mW low-power mode with a 3.3-volt
supply. The manufacturer suggests possible applications in undersea exploration that uses seismic
waves to detect oil or gas deposits as well as uses in military communication equipment, unmanned
aerial vehicles, and GPS receivers. I can see the appeal to manufacturers and users of test-an-measurement equipment, too. I bet engineers will find other ways to use this clever clock module.
Image courtesy of Symmetricom.
The SA.45s module produces a 10-MHz and a 1-Hz logic-level output that provides short-term
stability of one part in 1.5 x 10-10 over one second and long-term aging of one part in 3 x 10-10 per
month. An input signal lets equipment synchronize clocks and control and calibrate them. A serial
port lets equipment, or a terminal, communicate with the clock module to obtain information about
clock status and to "discipline" the clock--essentially bring it into phase with an external clock. Serial
commands also let you set the length of the cable used to supply an external 1-Hz signal, probably
from a GPS receiver.
So if you need more accuracy than that furnished by a quartz-crystal oscillator, but not enough to
require a rack-mounted timing standard, this small chip-scale device deserves your attention. For
more information and a link to the 29-page user's guide, visit:
http://www.symmetricom.com/products/quantum-atomic-oscillators/chip-scale-atomic-clock-csac/SA.
45s-CSAC/.
Engineers have other options such as timing information from GPS satellites, each of which includes
an atomic clock. These clocks have an accuracy of about 10-13 seconds. But the satellites and their
signals experience errors that include satellite position, the clocks experience small time
differences, signal reflections, and ionospheric changes affect signal-transit times. And the positions
of the satellites in a constellation "seen" by a receiver can cause a dilution of precision, or DOP.
Timing techniques that use four or more satellites let base stations account for small errors and
produce accurate timing information. But base-station receivers and GPS-locked timing standards
can get expensive. For a good GPS tutorial, visit: http://www.trimble.com/gps/index.shtml.