Micro-g LaCoste FG5, Manual

Page: 84 / 168

Adjustment and Maintenance
4
The scaler-counter keeps track of how many fringes have gone by and
measures the absolute time of occurrence of these fringes. The scaler-counter
scales (divides) the fringes by a number which can be set on the scaler-
counter board (typically preset to 4000).
The 10-MHz signal from the rubidium oscillator is also divided by a preset
factor in the scaler-counter (usually 2000). The time interval between the
occurrence of each scaled fringe and the next scaled clock pulse from the
rubidium oscillator is measured with the UTIC. The computer records each
time interval, the number of clock signals, and the number of scaled fringes
since the last time interval measurement. This information is used to
construct the absolute time of occurrence of the scaled fringes.
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.
4.4.1.1.Avalanche Photo Diode Printed Circuit Board
This circuit detects the optical fringes produced in the interferometer. The
FG5 uses a 50-MHz APD which is powered by a high-voltage module
mounted inside the interferometer base.
A high-speed comparator and 50-
Ω
driver on the APD board minimize noise
problems on the long cables between the interferometer base and the scaler-
counter board. The analog and digital versions of the fringe signal are both
available on BNC connectors mounted on the interferometer.
4.4.1.2.APD Board and Photo Diode Supply Module
4-26
The APD board is mounted inside the interferometer base, along with the
high voltage power supply module. The APD is mounted directly on the
board. The APD's high-voltage bias is zener-limited to 600 volts (on-board).
Two potentiometers are used to set the temperature coefficient and voltage
supplied to the APD.