Active MMIC Circulator Performance in a Phased-Array-Like Environment

This paper presents an analysis of a 8-12 GHz active 0.25 μm GaAs pHEMT MMIC circulator intended for integration into a phased array. The architecture is based on a closed-loop connection of three asymmetric -5.6-dB Lange couplers and resistive-feedback gain-matched amplifiers. The nominal simulated performance shows an isolation of 20 dB over a 40% bandwidth, with a return loss of better than 10 dB and an insertion gain of 2.4 dB across the band. The degradation in circuit performance is analyzed as a function of variations in port impedance. A loop-gain analysis demonstrates the circuit stability for port impedances in the VSWR = 2 circle, simulating coupling in a phased array. We show in simulation and measurements that the bandwidth, match on all ports and insertion gain are not sensitive and agree with simulations. However, the isolation degrades from 20 dB when the port impedances change within a VSWR=1.1 circle. Additionally, we show that the deviation of the coupler design from the nominal case affects the isolation. The statistical analysis gives insight into design criteria of this type of circulator for insertion into phased array.

I. Introduction

Non-reciprocal behavior of transistors can be used to achieve circulation and was first presented in 1965 [1]. In transistor-based devices, the distinction is made between active 3-way circulators with full rotational symmetry (as in a ferrite device), and quasi-circulators that have isolation between two of the ports but do not have full circulation. Different topologies of active MMIC circulators and quasi-circulators with the use of quadrature hybrids have been demonstrated in [2], [3] and [4]. Noise performance and power limitation analysis, as well as design techniques for simultaneous optimization are described in [5], [6], and [7]. In addition, integration with antennas is discussed in [8], [9].