Universal Active Coupler Design With Arbitrary and Continuous Phase Adjustment

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Abstract
This paper presents the design of an integrated wideband active coupler, using a universal structure designed for 45° and 180°, to realize arbitrary phase shifts below 90° and above 90°, respectively, by adjusting a pair of capacitors in the coupler. Furthermore, continuous phase shifts within the ranges of 180°–135° and 40°–90° are achieved. The variable capacitor in the passive network (PN) is implemented using variable capacitance, implemented by a transistor and by adjusting dimensions and gate‐source biasing. Each PN in the coupler is configured as a fifth‐order low‐pass filter, designed by transfer matrix analysis. A systematic design procedure based on this analysis is introduced, illustrating the combination of staggering technique, lumped‐element compensation, and multisection impedance transformation to provide low output phase error and improved directivity. Measurements conducted on fabricated chips in GaAs technology for the 180° (45°) coupler within the frequency range of 10–20 GHz reveal an output phase of 180° ± 1.5° (45° ± 1°), return loss better than 15 dB (15 dB), directivity greater than 30 dB (35 dB), and a coupling gain at the center frequency equal to 6 dB (5.2 dB). Continuously tunable output phase couplers for the phase ranges of 40°–90° and 180°–135° are achieved, based on the proposed universal structure, for each 2 GHz bandwidth in 12–14 GHz band. Within this frequency range, impedance matching around –10 dB and phase error of ± 1° are obtained. Comparable performance is observed in each 1 GHz sub‐band within the 14–20 GHz range. Additionally, simulation of the above couplers in 180 nm CMOS technology domonstrates similar performance.

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