System and Circuit Design Aspects for CMOS Wireless Handset Receivers

The presented work deals with system and circuit design aspects for Complementary Metal Oxide Semiconductor (CMOS) implementations of wireless handset receivers. First, an overview, from a historic perspective, on the use of CMOS in cellular applications is provided. Based on this the tremendous developments in CMOS technology are considered and the short-comings from an analog design perspective are evaluated. The lack of high quality passive devices, inductors in particular, is found to be one of the major obstacles in achieving a fully integrated RF design based on CMOS. Following this, an overview of different receiver architectures is given and a discussion of some fundamental problems in relation to CMOS integration is addressed. Based on the standards provided for Universal Mobile Telephone System (UMTS) a set of requirements is derived for a UTRA/FDD (UMTS Terrestial Radio Access - Frequency Division Duplex) direct-conversion receiver (DCR). The wideband nature of the UMTS signal opens up for simple DC-offset cancellation schemes. In line of this the use of highpass filtering as a means to reduce the DC-offset is pursued using link simulations. To simplify receiver planning it is common practice to employ a full separation of different distortion mechanisms. While this approach is very useful when an implementation performance surplus is available it is not an option when a low-cost silicon technology is the target. To manage this, a simple approach that allows all interfering components to be considered simultaneously is presented. Concerning the practical implementation of the DCR, LO leakage represents a significant source to performance degradation. Due to its antenna-like characteristics and typically large areas the inductor is especially prone to crosstalk and therefore a potential contributor to LO leakage. To minimize the coupling to and from inductors the traditional approach is to use guard-ring structures. While guard-rings improve isolation they also form a trade-off between device area and performance. The relation between guard-ring area and inductor performance is evaluated and it is shown that, depending on the size of the guard-ring, the Q-value reduction is found to be significantly reduced at RF frequencies. In continuation of this, various coupling effects for CMOS on-chip co-planar spiral inductors are presented. Simple guard-rings are shown to improve isolation between closely spaced adjacent inductors by approximately 10-15dB. At larger distances the gain of having a guard-ring reduces and eventually the gain reduces to zero dB. For modeling purposes an extended lumped element model is proposed and found to fit very well with crosstalk measurements.