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.