What is an Atomic clock?
An atomic clock is the most accurate man-made time device. In an atomic clockthe atom is the pendulum of the clock. When an electron inside an atom transitsbetween two energy levels, it either emits or absorbs an energy quantum of aprecise frequency, which is related to the energy difference between the twoatomic levels (according to Planck’s formula E = hv).Thus it creates an atomic oscillator.
Four major facts underlie the accuracy of Atomic clocks :
1. The output frequency is locked to the energy level difference of free atoms.
2. The atoms are isolated from their surroundings (minimum interaction/ collisions with other atoms, photons, fields, etc.)
3. Long interrogation time reduces frequency uncertainty (by Heisenberg principle: (f ~ 1/(t).
4. The atoms’ thermal motion is reduced as much as possible (eliminating Doppler shift of the 1st and 2nd order).
A rubidium clock is an atomic clock which uses rubidium atoms and an “optical pumping” technique to measure the atomic transition frequency. A rubidium Atomic clock includes 87Rb isotope confined in a vapor cell.
The atoms are excited to a high energy level (in figure #1) by light emitted by other 87Rb atoms. The light is filtered to achieve the required wavelength,thereby increasing the population of level 2 compare to level 1.
An injected microwave frequency of ~6.84GHz induces transition from level 2 to level 1 increasing the number of atoms in level 1, and yielding more absorption of light. A photodiode detects the reduction in the light intensity. This reduction occurs only when the injected microwave frequency equals precisely the transition frequency of the atoms. The injected frequency is derived from a crystal oscillator. The photodiode detection signal is then used to tune the crystal oscillator frequency. In other words, the crystal oscillator frequency is locked to the frequency of the waves emitted by the atoms.
1. Light from a 87Rb lamp contains both 3-1 and 2-1 lines.
2. A 85Rb cell filters the 2-1 line out, and transmits the 3-1 line only.
3. The filtered light “optically pumps” the 87Rb atoms in the resonance
4. cell from level 1 to 3.
5. A microwave signal at a frequency of 6.84 GHz is injected into the cell and induces transition from level 2 to 1.
6. When the injected wave frequency matches the difference of the 2-1 line, the
7. photo-diode detects a reduction in the intensity of the transmitted light.
AccuBeat’s Digital Frequency Locked Loop
AccuBeat’s rubidium frequency standards use a unique Digital FrequencyLocked Loop (DFLL) to implement the disciplining of the crystal oscillator tothe atomic resonance frequency. A microprocessor is embedded in the DFLL, witha unique patented algorithm. This algorithm reduces the sensitivity to externaldisturbances, allows easy lock at start, and enables the implementation of aunique hold-over mode. This keeps the internal oscillator running with the lastmemorized frequency when the lock is lost. These features make the rubidiumfrequency standard a very smart clock.
In addition, the microprocessor enables user access to internal parameters allowingvery fine control of the digital frequency over a wide range.