LRS Micro-g LaCoste FG5, Manual, Page: 77 / 149

Adjustment and Maintenance
4
4.4. Timing System and Data Acquisition
4.4.1.TIMING
The timing system consists of the avalanche photo diode (APD) board, the
rubidium oscillator, and the Time Interval Analyzer (TIA). Data are taken by
two different interface cards inside an IBM-compatible Pentium II (or better)
personal computer. The computer processes the data, compiles statistics, and
computes a gravity value, including certain corrections (e.g., for the tides and the
gradient).
Optical fringes are produced in the interferometer by combining the portion of
the laser beam hitting the freely falling and reference retroreflectors (the corner
cubes) with the portion traveling directly through both beam splitters. A fringe
is produced every time the falling object traverses a distance equal to the
wavelength of the laser, lambda, over two (
λ
/2). As the velocity of the falling
object changes, the frequency (f) of the fringe signal is swept according to f =
2gt/
λ
, where g = gravity and t = time. The optical fringes are detected by an
APD mounted in the interferometer base.
The zero-crossings of the fringes provide very good fiducial marks which can be
used for timing. The zero-crossing points of the a.c.-coupled fringe signal are
determined using an ultrafast comparator. The comparator outputs a square-
wave version of the frequency-swept fringe signal. The comparator is located on
the APD circuit board.
The TIA scales the fringes using a software-specified scale factor (usually 1000),
then times each scaled fringe. The TIA times events from t=0 at trigger.
The time of occurrence of each scaled fringe and the distance derived from the
number of fringes that have passed can be expressed as a time and distance pair.
The data are then fit to a parabola by the computer to determine a best value for
the acceleration—a gravity value.
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