3.1 Introduction

This chapter discusses the way in which capacitance is formed on PWBs. Capacitance is one of the fundamental circuit elements forming a transmission line, and capacitive coupling between PWB etch contributes to noise voltages.

The relationship between capacitance and charge is reviewed in Section 3.2, which naturally leads to the introduction of the dielectric constant. This is used in the parallel plate capacitor discussion appearing in Section 3.3, which focuses on the limitation of the parallel plate capacitor model for computing etch capacitance. As discussed, these formulas are not suited for computing microstrip and stripline capacitance. Capacitance and impedance formulas specifically suited for etched conductors are presented in Chapter 9.

Mutual capacitance contributes to crosstalk, jitter, and ISI, and is discussed in Section 3.5. Section 3.5 shows how to interpret the capacitance matrix presented by many field solvers, and it shows how to obtain the mutual and self capacitance of an etch. This background is a fundamental introduction for the crosstalk and differential impedance material presented in Chapter 8.

Dielectric losses contribute to signal degradation at very high signaling rates. The loss tangent is introduced in Section 3.6 and represents lossy dielectrics as a shunt conductance. This is an important prelude to the lossy transmission line model discussed in Chapter 5.

The dielectric characteristics of some laminates are presented in tabular form later in the chapter, and Section 3.7 discusses the effects temperature and humidity has on FR4 type epoxy resins. This material is useful when developing transmission...