1. Christian Doppler Laboratory for Measurement Systems for Harsh Operating Conditions, A-8010 Graz, Austria
2. Institute of Electrical Measurement and Sensor Systems, Graz University of Technology, Inffeldgasse 23/II, A-8010 Graz, Austria
3. Department of Physics, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand