Beam position monitor systems for the SNS linac

The 402.5- and 805-MHz beam position and phase monitors for the Spallation Neutron Source linac are based on standard PC technology. The SNS beam is to be injected into a storage ring with a 1-MHz circulation frequency requiring 650-ns-long minipulses. The injection cycle takes 1 ms and the machine can run at rates up to a 60 Hz. The RF input signals are down converted to 50 MHz and sampled at 40 MHz with 14-bit ADCs to produce I and Q data streams. A custom PCI module has been designed to accept modular digital front end (DFE) and analog front end (AFE) circuit cards. This hardware is installed in a standard rack-mounted 1U computer chassis running Windows 2000, Lab VIEW and custom DLL software. The system continuously self-calibrates by generating 300-ns long RF pulses which are switched between the AFE inputs and the cables going to the BPM pickups. This provides a TDR-like calibration of the entire system. The system is designed to provide a position and phase resolution of 0.1% of the beam pipe aperture and 0.1 degrees RMS respectively over a 50 μs period. The design of the system is described as well as the initial performance measuring beam during the commissioning of the SNS MEBT.