Digital Signal Processing for Optical Particle Counters in Order to Improve the Signal-to-Noise Ratio

ABSTRACT

Especially In clean technology optical particle counters with high flow rates, low detection limits with respect to particle size and a low false count rate are needed. The lower detection limit and the false count rate can be reduced if the SNR can be increased. In this paper, we present a method to increase the SNR for any given optical particle counter using electronics means. First, the process of signal formation is investigated and the quantities determining the signal are described. The task is then to extract the particle pulses from the mixture of signals. Current methods for signal detection are briefly described. Using the signal theory, a correlation receiver is designed which provides a maximum SNR at the filter exit. This receiver has built-in digital technology. It offers the possibility of on-line filtering of particle signals. The results show a distinct improvement of the SNR. The actual value is different for each individual optical particle counter. In the case of the clean room optical particle counter (TSI LPC 3751), the SNR increased by a factor of 2.06. This means that the lower detection limit can be reduced from 500 to 420 nm or that, without lowering of the detection limit, the security of false counting can be improved to greater than 6σ.