Institute of Electronics and Information Technology, Lublin University of Technology, 38A Nadbystrzycka Str., 20-618 Lublin, Poland and Nuclear Energy Division, National Centre for Nuclear Research, 7 A. Sołtana Str., 05-400 Otwock/Świerk, Poland, Tel.: +48 22 273 1307, Fax: +48 22 779 3481