Department of Electrical Measurements, LTH, P,O Box 118, SE-221 00, Lund, Sweden. lars.malmstrom@elmat.lth.se|Department of Electrical Measurements, LTH, P,O Box 118, SE-221 00, Lund, Sweden. lars.malmstrom@elmat.lth.se|Department of Electrical Measurements, LTH, P,O Box 118, SE-221 00, Lund, Sweden. lars.malmstrom@elmat.lth.se|Department of Electrical Measurements, LTH, P,O Box 118, SE-221 00, Lund, Sweden. lars.malmstrom@elmat.lth.se|Department of Electrical Measurements, LTH, P,O Box 118, SE-221 00, Lund, Sweden. lars.malmstrom@elmat.lth.se