Reconfigurable GaAs Bandpass Filters Using Cascading of Lowpass and Highpass Filters

Designs of on-chip reconfigurable band-pass filters are proposed based on GaAs process. The filters are constructed by cascading elliptic low- and high-pass filters, and reconfigurable center frequency and bandwidth are achieved by controlling the cut-off frequencies of the low- and high-pass filters. Varactors and switched capacitor arrays formed by the pHEMT transistors are used as tuning element to achieve the tunability. By introducing series LC tank resonator in the low- and high-pass filters, transmission zeros are formed outside the passband to improve the selectivity of the filer. One on-chip bandpass filter with tunable center frequency and one bandpass filter with controllable bandwidth are designed, fabricated and tested using the 0.15um GaAs pHEMT process. The filters realize a frequency tuning range of 1.9~2.53GHz, and a bandwidth tuning range of 330~750MHz, respectively, with bandstop suppression larger than 30dB.

I. Introduction

Various wireless standards have emerged in military, industrial, and consumer applications, which have put forward higher requirements for wireless devices that need to support multiple frequency bands. Traditional receivers use multiple discrete filters to select signals in different desired frequency bands. Since these filters are not tunable, each frequency band used by the device requires a separate filter, which will take up a lot of area and increase cost. In order to minimize circuit size and reduce cost, different tunable on-chip filters have been proposed in the past few years [1]–[2]. Due to the high loss in semi-conductor substrate, the quality factor (Q) for the on-chip filter is usually very low. To increase the Q value of the on-chip filter, Q-enhancement technology was proposed, but it will simultaneously reduce its linearity and increase noise[3]. The frequency conversion BPF composed of low-pass filter and switch, called N-path filter, can have high Q characteristics [4], [5], but its additional clock circuit makes the overall structure complicated, limiting its application.