Fully Integrated Ultra-Wideband Differential Circulator Based on Sequentially Switched Delay Line in 28-nm FDSOI CMOS

In this paper, a non-magnetic circulator, which realizes non-reciprocal signal flows by sequentially switching delay lines, is presented in 28-nm fully depleted silicon-on-insulator (FDSOI) CMOS process. The proposed circulator is designed differentially to increase power handling capability and bandwidth. The bandwidth of insertion loss and isolation can be extended by latticely coupled inductors used in the differential synthetic delay lines. The characteristic impedance of the delay lines is determined by considering the channel resistance of CMOS transistor. The measured insertion losses of transmitter (TX) to antenna (ANT) and ANT to receiver (RX) are 2.5 dB and 2.6 dB, respectively. TX to RX isolation is $> 20\ \text{dB}$ up to 7 GHz. The measured TX input power 1 dB compression point is 4.7 dBm at 3.5 GHz. The chip size of the differential circulator is $1.33\times 0.72\ \text{mm}^{2}$, which is as small as a single ended version, thanks to the coupled inductors.

I. Introduction

Conventional ferrite circulators are too bulky, expensive, and incompatible with on-chip integration. Thus, there has been considerable attention on the integrated non-magnetic circulators [1]–[6]. Circulators, which are based on capacitance modulation in resonators and gyrator using N-path filter or switched transmission line, have shown a non-reciprocity without ferrite, but have a limited bandwidth due to the use of LC resonators or quarter-wavelength transmission lines around the gyrator [2]–[4]. A switched capacitor based circulator overcomes the reliance on the quarter-wavelength transmission lines [5]. However, the required multiple clock phases still limit the bandwidth. The circulator, which separates the operation frequency of transmitter and receiver using a bilateral frequency converter, also has a limited bandwidth due to the use of duplexer [6]. Meanwhile, the sequentially switched delay line (SSDL) circulator has theoretically infinite bandwidth, but has not been integrated in a Si process [1].