Absolute accelerometer calibration is performed over the extended range of 80 Hz to 50 000 Hz using a quadrature Michelson laser interferometer, a beryllium electromagnetic shaker and a unique signal conditioning algorithm. High-speed digitization and an advanced air bearing for the armature allows conventional fringe counting at the lower range of frequencies. A different technique is used at higher frequencies, where vibration displacements are much smaller than a wavelength. By effectively calibrating and numerically normalizing the output of the optical detectors, this system uses a digital tracking filter to pick out sub-wavelength components of motion. The beryllium armature provides adequate stiffness to allow a back-to-back arrangement, with the Unit Under Test on top, and the mirror and laser underneath.
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