Packaged Quasi-Circulators With Low NF and High Isolation Considering the Fabrication Tolerance

A packaged quasi-circulator (QC) with low noise, low insertion loss (IL), high isolation, and ease of manufacturing is proposed in this letter. The proposed QC is constructed by two couplers, three connecting transmission lines (TLs), and a common-emitter amplifier. High isolation between transmitter (TX) and receiver (RX) ports can be obtained using two-way signal cancellation by designing the amplitude and phase of the amplifier, while the noise from the amplifier is suppressed by the isolation of the coupler. The negative group delay (NGD) technique is introduced to change the slope of the loop phase to improve the TX-ANT transmission bandwidth. Furthermore, a reconfigurable load is employed to ensure high isolation within a wide frequency range since the isolation performance is sensitive to the fabrication tolerance. For design validation, a QC is designed and fabricated, which achieves the in-band TX-ANT IL of 2.1 dB, ANT-RX IL of 1.7 dB, and noise figure (NF) of 2.3 dB. By tuning two controlling voltage of varactors, the proposed circulator can obtain >40 dB isolation in the frequency range of 1.925–2.09 GHz.

I. Introduction

Circulators are critical for miniaturization in allowing the transmitter (TX) and the receiver (RX) to share the same antenna (ANT), as the ANTs occupy the most size of the systems, such as full-duplex (FD) wireless systems and frequency-modulated continuous-wave (FMCW) radars [1], [2], [3]. When used in a transceiver, the circulator is cascaded before the low-noise amplifier (LNA), making its noise figure (NF) directly add up to the total NF of the RX. In addition, the high isolation at the ANT interface is necessary to prevent the LNA from compression due to the self-interference signal from TX [4]. Therefore, there is a strict requirement on the noise and isolation performance of the circulator. Conventional circulators rely on ferrite materials to break the reciprocity [5], [6], which have higher requirements during manufacturing and are incompatible with integrated circuits. These disadvantages have pushed the development of magnetless circulators and quasi-circulators (QCs).