A Compact and Wideband mmWave Passive CMOS Circulator Based on Switched All-Pass Networks

This letter presents a compact and wideband passive CMOS circulator for mmWave phased array transceivers. The letter focuses on achieving a compact die area while still offering competitive performance in terms of loss, isolation, and linearity. Our implemented circulator includes two reciprocal phase shift branches as well as a single-path nonreciprocal phase shift branch. We propose to use first-order lattice all-pass filters with coupled inductors to create the required phase shifts, which offer more compact, wideband, and predictable results compared to conventional lattice all-pass filters with two separate inductors. We also propose to use four identical first-order lattice cells in reciprocal and nonreciprocal branches. This can further reduce the size of the nonreciprocal branch due to fewer inductors compared to a typical second-order all-pass filter like bridged-T. The circuit is implemented in a 28 nm CMOS process, and the active die area is only $0.17~\text {mm}^{{2}}$ . Our measurements demonstrate that the implemented circulator operates over a 1 dB insertion loss bandwidth of 14–28 GHz that achieves 66% fractional bandwidth with an insertion loss of 3.8 dB, isolation of over 20 dB, and input third-order intercept point of +19 dBm.