New Wideband Miniature Branchline Coupler on IPD Technology for Beamforming Applications

In this paper, we present a new wideband miniature branchline coupler as a key circuit to be integrated in 60-GHz packaged beamforming networks for phased-array antennas. First, the integrated passive device (IPD) technology from STMicroelectronics is investigated in the mm-wave range through the simulation, fabrication, and measurements of a microstrip line and a simple hybrid coupler. Then, a novel coupler topology with emphasis on miniaturization and broadband operation is theorized. Analytical equations are derived and a 60-GHz coupler is optimized on IPD technology. Measurement results are discussed and compared with state-of-the art publications. The whole 57-66-GHz bandwidth is efficiently covered with the three following performance: -10-dB impedance matching, ±1-dB amplitude imbalance, and ±5° phase imbalance. As an application example, the novel coupler is integrated into a 4 × 4 Butler matrix suitable for an array-antenna demonstrating stateof-the art performance in terms of insertion loss and phase error. The measurement of different samples shows low variation of the IPD process because of very good reproducibility making it a suitable candidate for circuits operating in the 60-GHz band.