LRS Micro-g LaCoste FG5, Manual, Page: 26 / 149

Design: Components and Function
2
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Rubidium Oscillator
•
Time Interval Analyzer (TIA)
The Avalanche Photo Diode (APD) is located in the interferometer (Figure 2-8).
It detects the fringes created when the test and reference beams are recombined.
An ultrafast comparator chip located on the APD board detects the zero-
crossings of the sinusoidal fringes and outputs a TTL (square wave) version of
the frequency-swept fringe signal. During a drop, the fringe signal sweeps from
DC to about 6 MHz.
The Rubidium Oscillator is an atomic resonance-controlled oscillator (or
equivalent) which outputs a stable sinusoidal signal of 10 MHz.
The Time Interval Analyzer (TIA) scales the fringes using a software-specified
scale factor and outputs the time of occurrence for each scaled fringe.
Figure 2-11
Timing diagram
2-16