Trisection Bandpass Filters With All Mixed Couplings

In this letter, new patterns of symmetric trisection bandpass filters (BPFs) of two types with all mixed couplings are discussed. The condition under which the first-type filter with a central reflection-type resonator transforms into a delay line filter is considered. A second-type filter with a central through-type resonator becomes a quasi-elliptic filter under the same condition. As a result, the two transmission zeros are located equidistant regarding s = 0 on the complex plane s = sigma + j Omega, Omega = omega/omega(0) - omega(0)/omega)/FBW, for both types of filters, when this condition is fulfilled. A position of these transmission zeros with respect to s = 0 is expressed in terms of the mixed coupling coefficients. Two compact microstrip trisection BPFs of different types with all mixed couplings were proposed, fabricated, and measured.