A problem in RF switches of multiplexing BPM system

At SPring-8, we have been developing a new detection circuit for beam position measurement with a resolution of submicron and a measuring interval of a few milliseconds with high stability. In the circuit, a multiplexing method using RF switches is employed in order to cancel a drift of the circuit. To achieve design performance, the switches must have the following features: a short switching time, a high repeatability and a long lifetime. During the evaluation of the components for the new detection circuit, we found that some RF switches made of GaAs had a problem that the output signal changed for a few mdB for seconds after the switches were turned on. A few mdB, which is 1/10000 in voltage ratio, corresponds to a beam position error of a few microns in SPring-8. Such position error is out of the required specifications. We investigated several kinds of RF switches and decided to adopt a CMOS RF switches as the multiplexer. In this paper, we report switching properties of several RF switches and the demonstration of beam position measurement by using the switches. (authors)