Performance of BPM electronics for the LEP spectrometer

At the LEP e+/e− collider at CERN, Geneva, a Spectrometer is used to determine the beam energy with a relative accuracy of 10−4. The Spectrometer measures the change in bending angle in a well-characterized dipole magnet as LEP is ramped. The beam trajectory is obtained using three beam position monitors (BPMs) on each side of the magnet. The error on each BPM measurement should not exceed 1 micron if the desired accuracy on the bending angle is to be reached. The BPMs used consist of an aluminum block with an elliptical aperture and four capacitive button pickup electrodes. The button signals are fed to customised electronics supplied by Bergoz. The electronics use time multiplexing of indidvidual button signals through a single processing chain to optimize for long-term stability. We report on our experience of the performance of these electronics, describing measurements made with test signals and with beam. We have implemented a beam-based calibration procedure and have monitored the reproducibility o...