Schottky Signal From Distributed Orbit Pick-Ups

In the CERN Extra Low ENergy Anti-proton (ELENA) ring, intended for the deceleration of antiprotons, the longitudinal Schottky signal is obtained by summing the multiple electrostatic pick-up (PU) signals that are also used to measure the closed orbit. The signals from the individual PUs are phase-compensated to a single, common longitudinal location in the machine and added in the time domain. In this contribution, the related theoretical phase compensation is calculated and compared to measurements. We show how the cross correlation between the Schottky noise from the individual PUs can be used to find the correct phase-compensation for an optimal signal-to-noise ratio (SNR). This improvement in terms of SNR is, as expected, proportional to the square root of the number of PUs. The capability of the system to measure both, the bunched and the un-bunched low intensity (~3·10⁷ H⁻ @ 100keV / 144kHz) beams is confirmed by the experimental results presented. Furthermore, the inter-bunch phase correlation is briefly addressed and, for the case of bunched beams, the Schottky signal levels once down converted to different harmonics of the revolution frequency (f_{r}ev) are presented. In applications where the coherent beam signal dominates the spectrum and limits the dynamic range of the signal processing system, a down-conversation to a non-integer multiple of the RF harmonic is proposed as a way to reduce the coherent signal level.