Straightforward approximation of bandstop transmission function synthesized as the reflection function of bandpass wave digital ladder structure

This contribution introduces a direct design method for bandstop wave digital ladder filters. The application of this method provides several advantages which can not be obtained by applying frequency transformation techniques. The approximation process starts by extracting the properties of the scattering matrix of the bandstop reference ladder structures. Consequently, the transmission function is formulated in the reference frequency domain. Then, this function is designed through the construction of its squared - absolute. This is achieved by applying iterative interpolation techniques such that equiripple characteristics are obtained in the three bands. The resulting bandstop transmission function is synthesized in the reference domain by considering it as the reflection function of a corresponding bandpass structure. Then, by applying three-port series and parallel adaptors, the filter is realized in the digital domain.