A 5-6-GHz polyphase filter with tunable I/Q phase balance

A tunable polyphase filter with integrated input and output buffers was designed and fabricated in a 0.4 μm SiGe BiCMOS technology with a 5-6-GHz bandwidth. Series tunable capacitors (varactors) provide phase tunability for the differential quadrature outputs of the polyphase filter. The tunable phase can be used to improve image rejection in Weaver or Hartley architectures, or mitigate in-phase and quadrature (I/Q) phase error in direct conversion or low-IF receivers. The die area of the fabricated circuit with pads is 920 μm× 755 μm. Based on measurements, approximately 15/spl deg/ of I/Q phase imbalance can be tuned out using the fabricated polyphase filter, proving the concept of tunable phase. To the authors' knowledge, this is the first reported tunable I/Q balance polyphase network.