1. Dpto. Electricidad y Electronica, University of Valladolid, 47011 Valladolid, Spain
2. Dpto. Fisica Aplicada, University of Salamanca, 37008 Salamanca, Spain
3. Department of Engineering, University of Perugia, 06123 Perugia, Italy
4. Department of Mathematical and Computer Sciences, Physical Sciences and Earth Sciences, University of Messina, 98122 Messina, Italy