LRS Micro-g LaCoste FG5, Руководство пользователя, Страница: 79 / 149

Adjustment and Maintenance
4
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 TIA. 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
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-oltage bias is zener-limited to 600 volts (on-board). A
potentiometer is used to set the voltage supplied to the APD.
The analog output is buffered by an OP AMP directly from the APD. The zero-
crossing of the fringe signal is determined by the high speed comparator,
which
in turn drives a 50-
Ω
line driver chip. A 40-mV hysteresis is implemented on the
discriminator to avoid multiple triggering. Positive zero-crossings of the fringe
signal are detected and begin the leading edge of the TTL fringe signal. This TTL
fringe
signal leaves the board through an SMA connector, and is available
outside the interferometer on a BNC connector labeled TTL. The TTL signal is
used for timing by the gravimeter electronics.
All on-board voltages are derived from the
±
15V supply with linear regulators:
±
6 volts for the APD , and
±
5 volts for the discriminator and 50
Ω
TTL driver.
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