A K-Band Variable-Gain Phase Shifter Based on Gilbert-Cell Vector Synthesizer With RC–RL Poly-Phase Filter

A K-band variable-gain phase shifter using a Gilbert-cell vector synthesizer with a current digital-to-analog converter (DAC) and an RC-RL poly-phase filter (PPF) is presented, which is fabricated with a 65-nm RF CMOS process. It controls both gain and phase with low phase and gain variations as well as a low insertion loss. The Gilbert cell synthesizes a vector by summing in-phase (I) and quadrature (Q) phase signals of which the amplitudes are decided by the tail currents of the summing cells, respectively. The tail currents are given by current DACs, which are particularly designed to make the synthesizer have a constant output impedance in all phase and gain states. Because the gain and phase control resolutions depend on those of the current DACs, the size and insertion loss of the variable-gain phase shifter are not dependent on the control resolutions. The total root-mean-square (rms) phase and gain errors are measured to be 0.488° and 0.098 dB for 4096 states of the 7-bit 360° phase and 5-bit 17.8-dB gain controls, respectively, which are calibrated with extra 2-phase and 1-gain control bits. It shows -3.5 dB maximum gain including ON-chip balun losses with 6.6-mW dc power consumption.

I. Introduction

To overcome the high path losses of millimeter (mm)-wave frequencies and to use beam space multiple inputs and multiple outputs (MIMOs) in wireless communication systems, the phased-array beamforming technique has been widely studied. High-bit phase and gain controls are required to support massive MIMO and to perform phased-array calibrations. Active phase shifters based on vector-summing synthesizers are more advantageous than passive phase shifters based on switches. This is because the active phase shifters allow small insertion loss with a gain control function and high-bit phase and gain resolutions without increased chip size. Passive phase shifters require a chip area that is commensurate with the number of phase control bits and an additional chip size to implement a variable-gain function. Therefore, active phase shifters with gain controls are being intensively studied. Dual vector synthesizers are applied to control gain and phase independently without output impedance changes [1], but they have a small gain control range of ~8 dB, and the gain is deteriorated by the parasitic capacitances of the complex RF structure. An attenuator-based vector sum-type phase shifter [2] provides a high gain control range, but it has large rms phase and gain errors due to discrete attenuations. To drive vector synthesizers, various I/Q generators have also been studied. The delay line-based I/Q generation provides the widest bandwidth, but it has a too large size to take advantage of a high degree of integration of active phase shifters [3]. A classical two-stage RC poly-phase filter (PPF) with small loading effects and wideband characteristics is used as an I/Q generator of the active phase shifter in mm wave bands [4]–[6], but it has a large insertion loss due to impedance mismatches between the stages and the impedance mismatch with the Gilbert cell.