Manuscript received September 22;
1994. This work was financially sup-ported by the Swedish Research Council for Engineering Sciences (TFR) and the Swedish Board for Industrial and Technical pevelopment(NUTEK). The authors are with the Department of Optoelectronics and Electrical Measurements;
Chalmers University of Technology;
S-412;
96;
Goteborg;
Sweden. IEEE Lag Number 9408158;