Active quasi-circulators using quadrature hybrids for simultaneous transmit and receive

This paper presents a new type of quasi-circulator that consists of three quadrature hybrids with broadband operation, high isolation, and suppression of noise figure (NF) for simultaneous transmit and receive (STAR) phased-array systems. The operation principal of the circulator is based on phase cancellation and combination techniques to enhance isolation and suppress noise figure (NF). The circulator is implemented using three MMIC Lange couplers and active elements to provide quasi-circulator operation. The preliminary measured data shows that it is possible to obtain 35 dB isolation* between the transmit and receive port with 1.5 GHz operational bandwidth at X-band, while the NF at the receive port is < 5.5 dB. Using an external integrated power amplifier to partially implement an active quasi circulator, the performance of the device shows 25 dB isolation with 17% bandwidth, with NF less than 4.7 dB at the receive port. The proposed architecture is suitable for MMIC integration and system-on-chip (SOC) applications. (*The isolation is defined by the S-parameter difference, in dB, between forward transmission from the transmit to receive port and from the transmit to the antenna port).