Investigation on a binary-weighted reflection-coefficient generator for broadband multiport transmitters

A reflection-coefficient (gamma, Γ) generator is a key component in multiport transmitters. The binary-weighted gamma generator consists of power dividers and switchable impedance loads to present the different magnitudes of the reflection coefficients. Among different gamma generators, the binary-weighted configuration has many advantages such as no need for additional digital-to-analog converters and avoiding nonlinearity effects caused by nonlinear devices. However, for broadband operation the correct relative amplitude and phase imbalances in the different branches of the binary-weighting network become critical and must be taken into account. Thus, in this paper the broadband capability is elaborately investigated. Based on this study, a broadband gamma generator which has specially designed curved compensation transmission lines for minimal amplitude and phase imbalances is proposed. This design is implemented and compared against another, first design by experiment to demonstrate the improvement in error-vector-magnitude performance. pHEMT devices are used as switches to present high and low impedance to provide a time-varying Γ. The influence of amplitude and phase imbalances in the binary-weighting network on the error-vector-magnitude is evaluated. The proposed design relieves the imbalances leading to better error-vector-magnitude performance compared to the original one. It achieves a fractional bandwidth of 96% and 105% for an error-vector-magnitude below 5% and 10%, respectively.