A new active quasi-circulator with wideband performance

Abstract

IntroductionIn modern RF/microwave systems, non-reciprocal components such as isolators and circulators [1]- [4] play an important role because it provides isolation between ports in both time and frequency. A conventional ferrite circulator [4] can be replaced by an active quasicirculator [1]-[3] even though it is not quite an ideal active circulator. The quasi-circulator is similar to the conventional except that there is no signal flow from port 3 to port 1. This active quasi-circulator has the same S-parameter matrix as those reported in [1]- [3]. It have numerous advantages as well as disadvantages when compared with conventional ferrite circulators, its main advantages are small size, light weight, and compatibility with monolithic microwave integrated circuit (MMIC) technology. Active quasi-circulators with large bandwidth will have potential applications in the future because it allows signal flow in one direction without interaction between ports. Each port can also transmit and receive signals at the same time and at different frequencies without the need for filters, making it suitable for both FDMA and TDMA. This work describes a simple active quasi-circulator that gives wideband operation, and differs from previous structures [1]-[3] which were limited to a narrow frequency range. Experimental results shows good return loss and isolation within the expected operating frequency range, making it practical to implement. Active Circulator DesignThe circuit diagram of the proposed active quasi-circulator is shown in Fig. 1. It is formed by connecting three single stage distributed amplifiers (DAs) together using a T-network phase shifter. The gate and drain impedances of the transistors are combined with external series inductors to form three artificial transmission lines (ATL1, ATL2 and ATL3). All ATLs were realized using designs based on constant-K filters [5], i.e. o Z C L K = = / where L and C are determined by the external series inductor and the total shunt capacitors respectively. In order to achieve flat wideband gain, the L and C of all ATLs should have identical characteristics. In general, the gate-source capacitance (C gs ) is different from the drain-source capacitance (C ds ); therefore, a series capacitor C s and a parallel C p are added to maintain identical characteristics for all ATLs to achieve unity gain for each single stage DA [6]. The additional phase shifter is formed using a T-network in which the series inductance and shunt capacitance Cǯ are equal to the external inductance L/2 in the ATLs and junction capacitance C respectively. This ensures that the image impedance [5] of this additional Tnetwork matches to that of the ATL3 and maintains the characteristic impedance, Z o required by constant-K filters. This T-network has the same propagation factor as the π-network using the same series inductance L and shunt capacitance C, [5].A block diagram of the proposed active quasi-circulator is shown in Fig. 2 and shows two parallel paths from input port 1 to output port 3. For path 1...