Development of a digital lock-in amplifier for open-path light scattering measurement

A digital lock-in amplifier has been developed for improving the quality of the measurements of scattered light from fine particles, for example, diesel particulate emissions. The special features of this digital lock-in amplifier are: to be able to produce consistent signals from a very noisy signal background; to be able to produce accurate measurements from a fast moving signal source; to have the advantage of a low restriction of reference frequency and the ability to cope with changing reference frequency easily, which an analogue lock-in amplifier does not have; and to make it possible to produce a low-cost multiple-channel lock-in amplifier for measuring scattered light from a wide range of scattering angles. A series of lab tests has been carried out. The results of the tests show that this digital lock-in amplifier has much lower noise level and more stable signals than an analogue amplifier. It has the potential capability to measure the scattered light from moving particles.