1. F. Bertinetto, A. Godone, M.P. Sassi and E. Bava, Alta Frequenza,LIII-4, 231 (1984).
2. E. Bava et al., IEN Report No. 282 (1981).
3. P.T. Woods and B.W. Jolliffe, Journal of Phys. E: Scient. Instrum.,8, 395 (1976).
4. Metrologia,19, 163 (1984).
5. T.G. Blaney, G.J. Edwards, B.W. Jolliffe, D.J.E. Knight and P.T. Woods, J. Phys. D.: Appl. Phys.,9 (1976).