Tapped-line interdigital filter equivalent circuits

An exact equivalent circuit for the tapped-line interdigital filter geometry is deduced from an exact equivalent circuit of multiple-coupled lines. For a third order filter an analytical expression of the insertion loss function was obtained and compared to the one computed from Cristal´s proposed exact equivalent circuit. Cristal´s circuit behaved considerably different in the stopband and in particular it does not predict the transmission zero closest to the passband.