Ultra-Wideband Switched-Capacitor Delays and Circulators—Theory and Implementation

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 (BW). On the other hand, our recent work on quasi-electrostatic wave propagation in switched-capacitor networks has proposed 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 corroborate these findings by implementing prototype devices of the quasi-electrostatic delay element. Our measurements of the delay element reveal that delays of tens of nanoseconds over hundreds of MHz BW are realizable on-chip in a compact form factor. In addition, we apply synthetic rotation across these switched-capacitor networks to realize an ultra-broadband N-port circulator with ultra-compact form factor. We implemented a wideband three-port circulator showcasing this new architecture in a standard 65-nm CMOS process. The circulator exhibits symmetric performance across all three ports and dc-1-GHz operation for a modulation frequency of 500 MHz. The measured transmission losses of the circulator range between 3.1 and 4.3 dB, matching is-15 dB, isolation is >18 dB, and noise figure (NF) is consistent with the insertion loss. This device occupies an area of 0.19 mm² ( λ_center²/1.9×10⁶), representing about 100-1000 × higher miniaturization compared to the prior art.

I. Introduction

Devices with non-reciprocal transmission, such as gyrators, isolators, and circulators, are often required in wireless communication devices and radar/imaging sensors. The gyrator, a component with a 180° non-reciprocal phase response, was postulated as the fifth fundamental circuit element after the resistor, capacitor, inductor, and transformer [1]. It had been shown that any passive non-reciprocal network with arbitrary transmission parameters can be realized using these five fundamental components [2]. Isolators are commonly used in optical systems [3] and high-power base stations [4] to protect signal sources (lasers and high-power power amplifiers) from back reflections. Circulators are critical to single-antenna simultaneous-transmit-and-receive (STAR) systems, such as frequency-modulated continuous-wave (FMCW) radars [5], [6] and full-duplex wireless radios [7]–[9]. Circulator and isolators are also crucial to cryogenic quantum computing systems to excite and read-out the qubits [10].