503 4th DAAAM International Conference on Advanced Technologies for Developing Countries September 21-24, 2005 Slavonski Brod, Croatia CHOPPING FREQUENCY OPTIMIZING IN THE NIR MOISTURE METER Mirko Filic, Venco Corluka and Zdravko Valter Moisture meter, NIR, near infrared, chopping frequency 1. Introduction The moisture measuring is realised via reflected radiation at specific wavelengths in near infrared region (NIR). Because it has been noted that NIR although being inappropriate for the qualitative spectrometry, can at the same time be very acceptable for the quantitative analytics of some individual substance kinds including water [2]. The substance containing it has a great impact on that kind of moisture measuring and it makes the measuring to be more complex. The optoelectronic part of the NIR moisture meter designed and realised by the authors is analysed in this paper. The infrared radiation measurement system is based on the cooled PbS photoconductive detector and a mechanical chopper. The signal obtained from the detector is very small and mixes with the noises from various sources. However, the basic function of the proposed measuring system is to separate the useful signal from the overall signal, to amplify it to the level which can be acquired and post processed by the matching PC digital system. Partially it is effectuated by the mechanical chopper which in conjunction with a detector converts the NIR beam to the alternating photocurrent. Its frequency can be chosen by the rotary speed of the chopper control with the electric drive. In addition, the noise reduction and increasing signal to noise ratio are realised by the AC coupled analogueue preamplifier with the power line noise filter. The optimum frequency and rotary speed of the chopper drive are at the same time disposed by the theoretical analysis and with the experiments on the proposed measuring system. 2. Conditioning PbS detector signal in the infrared moisture meter The NIR radiation is produced by the halogen lamp. The narrow band of reference beam at 1800nm and of the absorbed beam at 1940nm are selected by the optic filters [2] and directed to the sample. The rotational wheel of the mechanical chopper has nine symmetric distributed circular sockets and is situated between lamps and sample. So the selected beam, the reference at the first and the absorbed at the second time are chopped with the rotational wheel and the alternating reflected radiation falls into the photodetector. 2.1 Electronic part of the measuring system The characteristics of PbS photoconductive detectors such as high detection levels in wavelengths of between 1000nm and 3200nm, good response speed and reasonable price, mean that they can be used in various applications for the moisture detection due to one distinctive absorption wavelength of water at 1940nm [2]. Thanks to the radiation chopping and the use of alternating signal, the response time is limited by the chopping frequency. The block diagram of the proposed measuring system is shown in Fig.1. The bias voltage for the supply detector and the other active elements for the conditioning signal are obtained by the stabilized rectifier supply module from the AC power line. The small AC signal from the detector is conditioned by the AC