Synthesis of Strictly Inline Filters With Transmission Zeros

In this article, we discuss the synthesis of a strictly inline filter configuration allowing the placement of up to four transmission zeros (TZs) in the frequency response. This configuration here referred to as extracted-zero (EZ) is often adopted in the practice, but the dimensioning generally resorts to empirical strategies and numerical optimization, being no synthesis-based approaches available till now. The topology of these filters is represented by an array of coupled resonators without cross couplings, with source and load coupled to internal resonators (instead of the first and last as in the classical all-pole filters). We show here how to compute the equivalent circuit parameters (coupling coefficients and resonating frequencies) once a suitable approximating frequency characteristic has been devised. Several synthesis examples are presented, and it is shown how filters of this type, previously empirically designed, and actually fabricated, can be obtained with the new design approach.