6.1 Introduction

In Chapter 5, a transmission line was described as a wire "and its return," with little discussion of just what was meant by a return. In fact the return path the route taken by the current back to the source to complete a series circuit with the load fundamentally determines how a signal appears at the load. This is because the physical relationship between a signal conductor and its return is what determines the basic electrical characteristics of the line. For example, the line's capacitance and inductance depend directly on the separation between the two conductors forming the transmission line. And in multiple stripline or microstrip situations, the width of the line (and the width of its return) are factors in setting the loop resistance and in determining the magnitude of mutual resistances.

This chapter discusses signals and the return paths used by them under a variety of conditions. Sections 6.2 through 6.4 discuss proper and improper return paths (such as power supply splits and mote crossings). The focus is on single-ended (ground-referenced) signals because these signals are best at illustrating return path concepts.

Signal integrity when routing signal traces through dense pin fields is discussed in Section 6.5, with a discussion of the trade-offs in determining the pad/antipad sizes.

Power supply integrity is a crucial factor in achieving satisfactory signal integrity and has been extensively researched in the literature. This chapter concludes with a review this work in Section 6.6, with emphasis on power supply decoupling, including...