Design Techniques for Dual-Passband Filters

This paper introduces two possible approaches to the design of microwave filters presenting two passbands separated by one rejection band, which is created by suitably placing transmission zeros in the stopband. In the first method the two passbands have almost equal extension and the transmission zeros are symmetrically placed in the stopband; the second method allows instead the synthesis of filters with different passbands width and all the transmission zeros in the stopband placed at the same frequency. Both design methods are based on suitably defined frequency transformations and employ well established prototype synthesis techniques; the practical implementation is performed through the classical multiplecoupled cavity topology. Two test filters operating in the GSM 800 MHz band, designed with the described procedures and actually fabricated, have validated the novel design methods.