A Very Accurate X-Band Rotary Attenuator with an Absolute Digital Angular Measuring System

The precision rotary attenuator described in this paper has an 11-in-diameter optical grating attached to its rotor and the angle of the rotor is displayed to a resolution of 0.001° on a row of numerical indicator tubes, using techniques that were developed by Russell in 1966 at the National Engineering Laboratory in Scotland. According to measurements made at 10 GHz against the British National Standard, this rotary attenuator follows the 40 log10 (sec θ) law to within ± 0.0015 dB up to 16 dB. A small but important cyclic source of error has been discovered in the angular measuring system and the theory of it is presented. Two ways of reducing it are discussed.