Ultra Compact, Ultra Wideband, DC-1GHz CMOS Circulator Based on Quasi-Electrostatic Wave Propagation in Commutated Switched Capacitor Networks

Recent research has revealed the possibility to achieve non-magnetic non-reciprocity using time-variance. However, prior CMOS-based circulators rely on the interference between non-reciprocal switched-capacitor/transmission-line gyrators and reciprocal transmission-line rings, which increases form factor and restricts frequency tunability and bandwidth. On the other hand, recent works on quasi-electrostatic wave propagation in switched-capacitor media have demonstrated a new regime in multipath switched-capacitor network operation that enables an ultra-broadband, ultra-compact reciprocal/non-reciprocal true-time-delay element. In this work, we apply synthetic rotation across this quasi-electrostatic medium to realize an ultra-broadband N-port circulator with ultra-compact form-factor. This new architecture is showcased in a wideband 3-port circulator implemented in a standard 65nm CMOS process. This circulator exhibits symmetric performance across all 3 ports and DC-1GHz operation for a modulation frequency of 500MHz. The measured transmission losses range between 3.1-4.3dB, matching is <; -15dB, isolation is >18dB and NF is consistent with the insertion loss. This device occupies an area of 0.19mm² (λ_center²/1.9 × 10⁶), representing about 100-1000× higher miniaturization compared to the prior art.

I. Introduction

Devices with non-reciprocal transmission (gyrators, isolators and circulators) are often required in wireless communications and radar/imaging sensors. There has been a recent surge in research to realize integrated CMOS non-reciprocal components that do not rely on magnetic (ferrite) materials [1]-[6]. Techniques that have been explored include varactor modulation in resonant rings [1], switched-capacitor N-path filter approaches [2], [3] and switched-transmission-line approaches [4], [5]. While these efforts have been successful in enhancing linearity and power handling [5], their bandwidths and frequency tunability have been limited to a fraction of the center frequency and their footprints have been large due to the use of LC resonators or wavelength-scale transmission lines.