Synthesis and realization of novel triple-passband filter based on frequency transformation

An analytical synthesis method for triple-passband microwave filters is presented in this paper. This method is based on a frequency transformation from the normalized frequency domain to the actual frequency domain. The coupling coefficients and the external quality factors can be calculated analytically with no need of optimization. A triple-band resonator is utilized to replace each capacitance in the low-pass prototype circuit as the building block of the triple-passband filter. To validate the synthesis theory, an example triple-passband filter is designed and fabricated with three passbands at 3.0-3.1 GHz, 3.2-3.3 GHz and 3.4-3.46 GHz, respectively, which has a maximum return loss of 20 dB in each passband.