Sample Averaging and Rectify, Integrate, and Dump for Synchronous Demodulation Measurement of Mean Radiation

A subsystem which implements two approaches for the synchronous demodulation and measurement of the mean intensity of chopped solar radiation has been designed and built as part of a microprocessor-controlled data acquisition system for the Laser Heterodyne Spectrometer (LHS) instrument built at NASA Langley Research Center, Hampton VA. One approach is based on a commonly used analog technique consisting of full-wave rectification followed by a fixed-interval integrate and dump. The alternate approach consists of a simple amplifier/filter whose output is synchronously sampled, digitized, and averaged. The two approaches are presented and the noise performance compared analytically and empirically. Pertinent issues of performance and implementation are discussed. The complete measurement subsystem is described briefly.