Insertion loss and effective phase shift in composite filters at cut-off frequencies

Expressions are derived for computing the insertion loss and insertion phase shift at the cut-off frequencies of a composite filter designed by the conventional Zobel technique and working between equal resistive terminations. The results obtained are exact if dissipation is neglected. The calculations are simple and are in terms of the m-values for the filter sections and the ratio of image impedance to termination. Different expressions apply for the cases of log. frequency symmetrical and log. frequency dissymmetrical filters.