iSource
®
StarLPRO-1500
Manual
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2.2
PHYSICS PACKAGE
The main design characteristics of the physics package are its low power consumption, small size and mass,
along with minimal environmental sensitivities and mechanical ruggedness.
All parts of the physics package are incorporated in a aluminium tube surrounded by magnetic shields. Inside
this tube, lamp and cell sections form two separate blocks which operate at well-defined but different
temperatures. The cylindrical tube is filled with polyurethane foam for low thermal conductivity. The lamp and
cell sections are separated with a glass window. This configuration greatly reduces the thermal flow between
the blocks and the tube envelope. It allows a very compact design with low power consumption, short warm-up
time and minimal environmental sensitivities.
Other design features which contribute to the compact design are:
-
use of the integrated filter technique (IFT)
-
use of a magnetron-type microwave resonator
The integrated filter technique which combines the optical filtering and pumping in one cell contributes also to
the reliability since the configuration is simplified and the number of components reduced. The thermal
capacitance of the cell assembly is relatively low. As a consequence, the necessary power during warm-up is
greatly reduced.
The magnetron resonator is a cylindrical cavity loaded with a concentric capacitive-inductive structure (annular
metal electrodes). It allows smaller cavity dimensions and concentrates the microwave field at the right region
of the cell.
The Rb lamp is an electrode-less RF-discharge lamp: a heated glass bulb which contains Rb and a starter gas
surrounded by an RF-coil.
Although the atomic clock transition frequency is inherently quite stable, there are second order influences
which affect the frequency, i.e. temperature (buffer gas), light intensity (light shift = optical Stark effect),
magnetic field (2nd order Zeeman effect). As a consequence, the temperatures of lamp and cell, the power of
the lamp oscillator and the current in the C-field coil have to be carefully stabilized.
2.3
ELECTRONICS PACKAGE
2.3.1 PRINCIPAL FUNCTIONS OF THE ELECTRONIC CIRCUITS
The clock transition of a Rb resonator is a microwave transition at 6.834 .. GHz.
The microwave resonance occurs as a dip in the optical signal; i.e. in the Rb lamp light which, after transiting
the cell, is detected by a photodiode.
The basic purpose of the electronics package is to synchronize the ingoing microwave frequency, derived from
a quartz crystal oscillator, to this absorption dip. This is achieved by tuning the microwave frequency to
maximum optical absorption.
Frequency variations of the microwave signal are transformed into DC current changes at the photodetector.
The dip, visualized in the photocurrent versus microwave frequency curve of Fig. 2-3, is very small: on the order
of 1% of the total photocurrent which is however approximately 10 times higher compared to other commercial
rubidium standards on the market.
Since DC detection of the dip is not feasible , an AC detection method is used for the following reasons:
- The dip amplitude is very small compared to the total photocurrent.
- The slope of the derivative of the dip photocurrent versus microwave frequency corresponds to roughly 1
nA/Hz. AC detection is the only solution to have a good signal/noise ratio since the photo-detector with
associated amplifier are affected by flicker noise.
The AC method involves square wave frequency modulation of the microwave signal at a rate of fm~330 Hz. As
shown in Figure 2-3 the modulated microwave frequency flips between 2 discrete frequency values f1 et f2. The
resulting photo- current i(t) appears then also (after the transient )at 2 discrete values i1 and i2 .
The difference between i1 and i2 produces the error signal used for the quartz crystal center frequency
adjustment until the mean value of f1 and f2 is exactly equal to the rubidium hyperfine frequency.
