Wide Dynamic Range Front-end Electronics for Beam Current and Position Measurement

Abstract

An Analog Devices log detector, AD8306, and a Digital Signal Processor (DSP), ADSP-21992, have been found useful for building wide dynamic range, accurate and inexpensive front-end electronics to measure and process the RF signals from TRIUMF's beam monitors. The high-precision log detector has a useful dynamic range of over 100 dB. The 160 MHz mixed-signal DSP is used to digitize the log detector output, linearize it via a lookup table, perform temperature compensation, and remove the variable duty cycle 1 kHz pulse structure of the beam. This approach has been applied to two types of devices in a 500 MeV proton beamline. The 0.1% DC to CW total current monitor is based on a capacitive pickup resonant at 46.11 MHz, the second harmonic of the bunch frequency. The DSP software provides low pass filtering, calculates the antilog of the data and passes the output to a CAMAC input register. The BPM electronics process data from inductive pickup loops. The DSP controls a GaAs switch which multiplexes signals from four adjacent pickups to a single log detector. The DSP performs difference-over-sum or log-ratio data analysis along with averaging over an arbitrary number of samples.