Resumen de Integrated Transformers and its Application to the RFIC Design.
Josep Cabanillas Costa
The context of this thesis is the design of CMOS low power RFICs suitable for direct conversion architectures. Our starting point is the analysis of the characteristics of the integrated passive components (inductors and mainly transformers) from a circuit designer perspective. Then, the achieved understanding of these components is exploited in order to optimize the performance of some of the building blocks of a RF front-end.
This thesis is divided in three main chapters. Chapter 1 is dedicated to the analysis of integrated transformers. It starts with a revision of the state of the art of the integrated inductors in order to set up the basis for the analysis of the monolithic transformers. Due to their influence on the component quality factor, a special insight is dedicated to the analysis of the eddy currents. Then, we introduce the integrated transformers and revise the topologies used in their implementation and their characteristics and electrical equivalent models. However, it is important to remark that the goal of this thesis is not the modeling of the component. Instead, we will use EM simulators to reproduce their behavior and point out some physical mechanisms present in integrated transformers that have not been properly considered in the equivalent electrical models presented in the literature so far. In particular, we will study eddy currents in integrated transformers and will demonstrate their dependence on the operating mode of the transformer as well as on the loads connected to the primary and the secondary. As we will see this phenomenon has a direct influence on the component model as well as on the component optimization. Thus, different optimization procedures should be applied to minimize the component losses depending on the application.
Finally, this chapter finishes with the design of a double balanced mixed that uses two differential transformers to increase the isolation between ports. It is demonstrated that a differential driving minimizes the effects of the parasitic capacitances and increases noticeably this isolation. It is also shown that the etching of the silicon underneath of the integrated transformers that reduces these parasitic capacitances also increases this isolation (even if a single-ended excitation is applied).
Chapter 2 investigates the design of oscillators having low phase noise and large tunning range. After identifying the quality factor of the integrated inductors as one of the main factors limiting the oscillator phase noise, a novel transformer-based (parallel) resonator is introduced. This new topology overcomes the performance of the common inductor-based resonators in terms of tuning range and effective quality factor.
A detailed description of the resonator is then performed in order understand its potentially and limitations. Thus, its properties are firstly studied from an electrical circuit perspective using the equivalent models discussed in the previous chapter. Then, it is realized the analysis in the EM domain in order to compare its performance with the standard inductor-based resonator. Once again, eddy currents are identified as the main factor limiting the effective quality factor of the transformer-based resonator. Thanks to the already achieved understanding of integrated transformers, the layout optimization method proposed by López-Villegas [35] for integrated inductors can be extended to the optimization of the transformer (when working in common-mode). The extension of the parallel design to N-resonators is also shortly discussed. Finally, the proposed resonator is used in a low phase-noise 1.7 GHz CMOS VCO.
Chapter 3 presents a new topology of quadrature oscillator based on the differential coupling at the second harmonic of two differential oscillators. As in the previous design, an integrated transformer plays a relevant role in this circuit being used to establish the coupling between oscillators. In order to obtain a basic understanding of the phenomena involved in the generation of quadrature signals using the proposed method and set up the design procedure several concepts must be revised. In particular, the theory on forced (or injected) and coupled oscillators is discussed and applied to the proposed quadrature oscillator. Finally, to show the feasibility and potentiality of this methodology, different designs in integrated and hybrid implementations are presented.
