A Fully Integrated VLSI Circulator for 915-MHz ISM Band RFID and IoT

This paper presents a highly integrated three-port standard 45nm node CMOS VLSI based circulator for UHF 915-MHz ISM band, RFID, and other IoT applications. The motivation is to lower the cost of these rapidly growing applications. The circuit is designed using differential amplifiers with medium grade input coupling to obtain effective bidirectionality and quasi-broadband characteristics at the ports of the overall system. This results in both high directivity and isolation at the appropriate ports. The designed circulator has return losses at all ports better than 1 dB and reverse isolation of above 17 dB in the frequency neighborhood of 915 MHz, i.e., the frequency range of 0.3 to 1.2 GHz. The proposed circuit thus can be used in RFID and IoT applications designed for the UHF 915-MHz ISM band.

I. Introduction

RF and microwave transceivers generally use circulators to couple a single antenna to the transmitting and receiving parts of the system. These devices should have minimal loss from the port connected to the transmitter to the port connected to the antenna, and from the port connected to the antenna to the port connected to the receiver. They also should have high isolation from the port connected to the transmitter to the port connected to the receiver, and from the port connected to the antenna to the port connected to the transmitter. Several integrated active circulators have been reported recently for different frequencies, with different levels of performance, sizes, and power consumption [1]-[5]. However, not enough attention has been devoted to the use case of low cost and low power active integrated circulators for RFID and IoT applications in the UHF 915-MHz ISM band.