Synthesis of Star-Junction Multiplexers

This paper illustrates a general approach to the synthesis of microwave multiplexers (RF combiners) presenting a star-junction topology (with a resonating junction). The channel filters composing the multiplexer can be arbitrarily specified; in particular, the attenuation characteristic may include transmission zeros (imaginary and/or complex). The synthesis procedure, developed in a low-pass frequency domain (suitably defined), is based on an iterative algorithm which evaluates the characteristic polynomials of both the overall multiplexer and the composing filters. Using these latter polynomials, the normalized coupling matrices of the filters can be computed using one of the methods available in the literature and the dimensioning of the channel filters is carried out through the usual de-normalized parameters (coupling coefficients and external Q). The detailed design steps illustrating the synthesis of a triplexer for base stations of mobile communications are presented. The designed device was then built, and the comparison between measurements and simulations has validated the new synthesis approach.