High-Isolation and High-Rejection Microstrip Diplexer With Independently Controllable Transmission Zeros

A high-isolation and high-rejection microstrip diplexer based on the stub-loaded resonators with independently controllable transmission zeros has been presented. The transmission zero is an inherent characteristic of the stub-loaded resonator. By allocating properly these transmission zeros at the specific frequencies, the output isolation and out-of-band rejection of the diplexer can thus be improved simultaneously. For demonstration, a diplexer with third-order Chebyshev bandpass response has been designed and fabricated with microstrip technology. The measured results are in good agreement with the simulated predictions, showing that output isolation and out-of-band rejection both exceeding 70 dB are achieved.