Impedance Control of Conductors Acting as Transmission Lines in PrintedBoards for High‐frequency Digital Applications

High‐frequency performance of printed boards (PBs) — a term which encompasses printed wiring boards (PWBs) and printed circuit boards (PCBs) — is becoming increasingly important in digital circuits and knowledge of the electrical characteristics associated with conductors acting as transmission lines is essential. This paper is an introduction to the electrical theory associated with transmission lines and develops the fundamental principles into formulae of use to PB users, designers and manufacturers. Established equations for calculating propagation delay and characteristic impedance based on physical parameters of conductor configurations are then summarised. The concept of ‘critical conductor length’ is proposed as a method of ascertaining whether control of characteristic impedance is required for a particular transmission line. The paper uses the surface microstrip configuration as a vehicle for mathematical analysis but the analysis is equally applicable to other transmission line configurations such as embedded microstrip, symmetrical stripline and dual stripline for which the associated physical parameter equations are included.