A high-speed, high-precision data acquisition and processing system for experiments producing steady-state periodic signals

A wide variety of experiments investigate the response of a sample to a steady-state periodic excitation. We describe a computerized data acquisition and processing system for experiments of this type assembled from relatively inexpensive, commercially available components which has been optimized for highly precise determinations of the magnitude and relative phasing of drive and response signals having frequencies from 1 μHz to approximately 1 MHz. Through the use of timing control with a precision of 1 part in 109 and high speed on-line signal processing, the system combines a very high rate of data acquisition with extended signal averaging times, enabling it to acquire an uninterrupted data stream consisting of at least 109 samples. Furthermore, the system acts as a digital filter which can be tuned to completely reject unwanted signals of any frequency other than the exact signal frequency. This allows accurate characterization of sample response even when obscured by other, extraneous signals of substantially larger magnitudes as long as they have at least somewhat different frequencies. Oscillatory experiments on very dilute polymer solutions, including oscillatory flow birefringence, dynamic viscoelasticity, oscillatory electric birefringence and dielectric measurements, are important examples of this type of measurement situation, in which the experiment depends critically on accurate determination of the magnitude and phasing of weak response signals, frequently under extremely poor signal-to-noise conditions. The system has an aggregate sampling rate of 1 MHz and up to four input channels. The potential for distortion of the results by aliasing frequencies may limit the input signal frequency to less than 500 kHz for one channel. © 1994 John Wiley & Sons, Inc.