A technique for improving the accuracy and dynamic range of beam position-detection equipment

Beam position detection equipment often uses circuits that convert amplitude-modulated position signals to phase modulation. These signals are limited to remove amplitude variations, and phase-detected to obtain analog position information. The phase-modulated signals are clipped by limiters that must be closely phase-matched in pairs over the signal intensity range of interest. Accuracy and dynamic range are determined principally by the phase match between the limiters. When the duration of the beam is a few microseconds or more, significant improvements can be obtained by down-converting the RF signals to an intermediate frequency near 10 MHz. Limiter circuits are available that can be closely matched in pairs at 10 MHz over a 60-dB signal amplitude range. For example, the peak phase deviation measured among 30 matched pairs was <0.4°, corresponding to a phase tracking error of <0.1 dB. This exceptional matching enabled beam position measurements over a 50-dB dynamic range of signal intensity. A description of the detection technique and the limiter is given