Single-chip integration of electronically switchable bandpass filter for 3.5GHz WiMAX application

This paper demonstrates the design and implementation of a miniaturized bandpass filter integrated with switch function in commercial GaAs pHEMT process for 3.5GHz WiMAX application. A tapped-input, capacitive-loaded hairpin-comb bandpass filter is proposed to benefit from the high-density metal-insulator-metal capacitor in the GaAs process, such that the required electrical length of coupled-line is only 13.3 degree at the center frequency and the filter size can be largely reduced. The proposed integrated bandpass filter features low in-band insertion loss as well as a wide 30-dB rejection-band up to 33.8 GHz. The filter size is only 1.22 mm×0.61 mm, and is realized in a chip size of 1.5 mm×1.0 mm. Then, by adding two D-mode FETs in the pHEMT process, the proposed filter can be switched on and off electronically. Low-loss bandpass response with wide rejection band in the “filter on” state and wideband 35-dB isolation from DC up to 28.7 GHz in the “filter off” state are achieved. The chip size of this switchable bandpass filter is only 2.0 mm×1.0 mm.