Design method for a lumped-element bandpass filter with two resonators and its application to multiplexers

A design method for a lumped-element bandpass filter is presented that simplifies construction of a multiplexer. Each filter consists of a series resonator and a shunt resonator. The input impedance of each filter designed in this study is near open-circuited at the resonance frequencies of the other channels. Owing to this property, all the filters can be connected together at a common port in parallel without any additional peripheral circuitry to build a multiplexer. This property is because of the combination of a series resonance circuit and a shunt resonance circuit of the filter. To verify the design method, a multiplexer with three channels is fabricated with lumped elements together with distributed elements. Measured results are in good agreement with prediction. The measured insertion loss is less than 0.5 dB at each channel, whereas return loss at each port is better than 15 dB and suppression of other channel signals is better than 20 dB. The isolation between channels is better than 20 dB.