Zhengxiang Ma

RF measurement technique for characterizing thin dielectric films

We have developed a novel technique for measuring the dielectric constant and loss tangent of a thin film dielectric material up to 5 GHz. The dielectric film needs to be deposited on a metal layer and capped with a metal electrode layer. The bottom metal layer does not have to be very conductive, as long as its sheet resistance is uniform and known. Only one step lithography on the top metal layer is required. No dc electrical contact to the bottom metal layer is necessary. The measurement is taken with a Vector Network Analyzer and a coplanar-wave-guide miniature wafer probe.

Compensation of Frequency-Dependent Gain/Phase Imbalance in Predistortion Linearization Systems

Impairments in the analog quadrature modulator can adversely impact the performance of a digital predistortion linearization system. In this paper, we focus on modeling and compensation of frequency-dependent gain/phase imbalance and dc offset when the input and output of the direct upconverter are available. We first propose channel models to describe the effects of these impairments on transmitted I/Q data streams. We then develop algorithms to estimate the parameters of these channel models and to construct I/Q compensators. Performance of the algorithms and the I/Q compensators is shown in conjunction with digital predistortion.

A least-squares/Newton method for digital predistortion of wideband signals

Power amplifiers (PAs) are essential in communication systems, but are inherently nonlinear. To achieve linearity with good efficiency, PA linearization is necessary. Digital baseband predistortion is a highly cost-effective way to linearize PAs, but most existing architectures assume that the PA has a memoryless nonlinearity. For wider bandwidth applications, such as wideband code-division multiple access, PA memory effects can no longer be ignored. Therefore, in order to achieve good linearization performance, the predistorter needs to also have memory structure. In this paper, we propose a new model for the wideband predistorter and a least-squares(LS)/Newton algorithm to estimate the model parameters. Performance of the LS/Newton algorithm is studied through computer simulations. Good linearization performance is achieved by using the new model in an experimental testbed.