Low-cost, digital lock-in module with external reference for coating glasstransmission/reflection spectrophotometer

Abstract

A versatile, low-cost, digital signal processor (DSP) based lock-in module withexternal reference is described. This module is used to implement an industrialspectrophotometer for measuring spectral transmission and reflection ofautomotive and architectonic coating glasses over the ultraviolet, visible andnear-infrared wavelength range. The light beams are modulated with an opticalchopper. A digital phase-locked loop (DPLL) is used to lock the lock-in to thechop frequency. The lock-in rejects the ambient radiation and permits thespectrophotometer to work in the presence of ambient light. The algorithm thatimplements the dual lock-in and the DPLL in the DSP56002 evaluation modulefrom Motorola is described. The use of a DSP allows implementation of thelock-in and DPLL by software, which gives flexibility and programmability to thesystem. Lock-in module cost, under 300 euro, is an important parametertaking into account that two modules are used in the system. Besides, thealgorithms implemented in this DSP can be directly implemented in the latestDSP generations. The DPLL performance and the spectrophotometer arecharacterized. Capture and lock DPLL ranges have been measured andchecked to be greater than the chop frequency drifts. The lock-in measuredfrequency response shows that the lock-in performs as theoretically predicted.