The Skyscan Atomic Clock
Radio-Controlled Atomic Clocks
What Is A Radio-Controlled Atomic Clock
The atomic clocks we’ve been talking about may be the most accurate timekeepers on the planet, but they aren’t very useful unless their information (time signals) are widely communicated.
So the U.S. government has mandated for its uses that the official standard time is to be maintained by the Time and Frequency Division of NIST and by the U.S. Naval Observatory.
These time signals are broadcast by radio and over the internet through these official (primary) timekeepers and relayed through a series of secondary timekeepers.
Since you are reading this online right now, you can visit the Official U.S. Time web site and choose a U.S. time zone when you get there. For a more global view, try the World Time Server and pick your location.
At several broadcast frequencies, NIST operated radio transmitters located near Fort Collins, Colorado spread the time signal for all to use.
As long as you are within about 2000 miles of their location, you can pick up and interpret the time information.
These broadcasts are critically important for the function of the Global Positioning Satellite system (GPS), among other navigational tools.
Since 1962, WWVB has been broadcasting this time signal at an operating frequency of 60 kHz, well below the standard AM radio station frequencies in the U.S.
The time code is synchronized with the 60 kHz carrier frequency, and contains information for the year, day of year, hour, minute, second, and flags to indicate the status of Daylight Saving Time, leap years, and leap seconds.
By tuning in to this signal, all the information you could want about the current time can be obtained.
Finally, we know the secret of those amazingly accurate timepieces, the radio-controlled atomic clock.
Using a miniature radio receiver tuned to WWVB and built in to each radio controlled clock or watch, these marvels are programmed to try to receive the WWVB broadcast, usually once per day.
Many of them can also be manually told to “listen” for the time code.
When it is successfully received and interpreted, the time code is used to reset the displayed time – date, daylight saving time, and all.
Then, these timepieces use their own, internal quartz oscillators to keep a good level of accuracy until the next time code comes in.
Of course, not all locations are equally suited to receiving the WWVB signal.
Generally, it is easiest to pick up the time code at night – just the way reception of distant AM stations improves at night for regular radio (there is less solar interference with the ionosphere).
But, you need to be within the effective range of the broadcast which is about 2000 miles.
The WWVB signal blankets most of the continental U.S. and Canada, even reaching parts of South America at times.
But it will not reach Alaska or Hawaii.
There can also be natural or man-made obstructions to the radio signal.
Deep valleys tend to have more trouble receiving the broadcast.
And modern buildings with steel reinforced construction can act as radio barriers (known as Faraday cages), blocking the WWVB signal just the way they do standard radio reception.
Still, a bit of planning can help to solve the difficulty.
Decide in advance where you will leave your radio controlled clock during the night, when it will try to pick up the time code.
Try for a high location in the building if you are in a valley, and set it near a large window if you are in a steel building.
Keep it away from other sources of broad band radio interference, like televisions or computers.
You just might succeed!
The Skyscan Atomic Clock
Since 1967, the International System of Units (SI) has defined the second as the duration of 9192631770cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom.
This definition makes the caesium oscillator the primary standard for time and frequency measurements, called the caesium standard.
Other physical quantities, e.g., the volt and the metre, rely on the definition of the second in their own definitions.
The actual time-reference of an atomic clock consists of an electronic oscillator operating at microwave frequency.
The oscillator is arranged so that its frequency-determining components include an element that can be controlled by a feedback signal.
The feedback signal keeps the oscillator tuned in resonance with the frequency of the electronic transition of caesium or rubidium.
The core of the atomic clock is a tunable microwave cavity containing the gas. In a hydrogen maser clock the gas emits microwaves (the gas mases) on a hyperfine transition, the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude.
Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate.
For both types the atoms in the gas are prepared in one electronic state prior to filling them into the cavity.
For the second type the number of atoms which change electronic state is detected and the cavity is tuned for a maximum of detected state changes.
Most of the complexity of the clock lies in this adjustment process.
The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the spreading in frequencies caused by ensemble effects.
One way of doing this is to sweep the microwave oscillator’s frequency across a narrow range to generate a modulated signal at the detector.
The detector’s signal can then be demodulated to apply feedback to control long-term drift in the radio frequency.
In this way, the quantum-mechanical properties of the atomic transition frequency of the caesium can be used to tune the microwave oscillator to the same frequency, except for a small amount of experimental error.
When a clock is first turned on, it takes a while for the oscillator to stabilize. In practice, the feedback and monitoring mechanism is much more complex than described above.