Abstract
A high precision rotator with accurate mechanical phase control has been constructed. The system is primarily designed for the rotation of polarizers, diameters up to 100 mm, to produce amplitude modulation of broad band radiation in acousto-optical instruments for radiation intensity measurement. Rotation speed may be phase-locked at 2, 4 or 8 Hz to a crystal oscillator or over the range 0.625-10.5 Hz to an external signal. Air bearings are used for low friction and noise: acoustic output, at 80 mm, is only 16 mu Pa at 4 and 8 Hz. A high torque motor gives fast rotor acceleration, 57 rad s-2, and broad-band phase control electronics (analogue and digital) utilizing encoding marks on the rotor circumference give good stability over the range of speeds without external adjustment. Outputs at 128 and 4 pulses rev-1 are provided for synchronization of external apparatus such as phase sensitive detectors. The standard deviation, at 4 Hz (240 RPM), in phase determined between pi /2 pulses (4 per rev) is 1 mrad and in frequency, measured over 100 s, is 0.8 mu Hz (2*10-7).
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