Design of compact Ka-band monolithic branch-line coupler on silicon substrate

This paper presents a compact Ka-band monolithic branch-line coupler using synthetic transmission line (TL). The prototype, which is fabricated by the standard 0.18-¿m 1P6M CMOS technology, shows the circuit size of 300.0 ¿m × 360.0 ¿m (1.48 × 10^-3 ¿₀ ² ) without contact pads, achieving 95.1% size reduction. The measured results show the excellent agreements with those of the simulations. The measured insertion loss is 4.9 dB and the return loss is kept higher than 10.0 dB from 28.8 GHz to 39.1 GHz. The isolation of the coupler is higher than 15.0 dB and has a maximum value of 23.0 dB at 37.0 GHz. The measured frequency range of phase difference (90⁰ ± 10⁰) between two output ports is 4.5 GHz from 31.1 GHz to 35.6 GHz.