High-speed data acquisition system and receiver configurations for time-domain radiofrequency electron paramagnetic resonance spectroscopy and imaging

Design strategies, system configuration, and operation of a dual-channel da ta acquisition system for a radiofrequency (RF) time-domain electron parama gnetic resonance (EPR) spectrometer/imager operating at 300 MHz are describ ed. This system was configured to incorporate high-speed analog-to-digital conversion (ADC) and summation capabilities with both internal and external triggering via GPIB interface. The sampling rate of the ADC is programmabl e up to a maximum of 1 GS/s when operating in a dual-channel mode or 2 GS/s when the EPR data are collected in a single-channel mode, By using high-sp eed flash ADCs, a pipelined 8-bit adder, and a 24-bit accumulator, a repeti tion rate of 230 kHz is realized to sum FIDs of 4096 points. The record len gth is programmable up to a maximum of 8K points and a large number of FIDs (2(24)) can be summed without overflow before the data can be transferred to a host computer via CRIB interface for further processing. The data acqu isition system can operate in a two-channel (quadrature) receiver mode for the conventional mixing to baseband, For detection using the single-channel mode, the resonance signals around the center frequency of 300 MHz were mi xed with a synchronized local oscillator of appropriate frequency leading t o an intermediate frequency (IF) which is sampled at a rate of 2 GS/s, Comp arison of quadrature-mode and an IF-mode operation for EPR detection is pre sented by studying the FID signal intensity across a bandwidth of 10 MHz an d as a function of transmit RF power, Imaging of large-sized phantoms accom modated in appropriately sized resonators indicates that IF-mode operation can be used to obtain distortion-free images in resonators of size 50 mm di ameter and 50 mm length. (C) 1999 Academic Press.