TABLE OF CONTENTS Planar transmission lines are well suited for use in a multilayer microwave printed circuit board (PCB), which consists of one more layers of metallized dielectric laminated together and interconnected with via holes. Active and passive surface mount components are soldered or epoxied to the top layer. A microwave PCB often carries mixed signals, in that one or more of its layers conduct microwave signals, and the others conduct the analog and digital signals that bias and control the microwave devices. In this section we focus on the passive circuit aspects of circuit board grounding. In Chapter 5, we will discuss the grounding of active devices on circuit boards.
Figure 3.37 depicts a typical four-layer circuit board that carries microwave, DC, and digital signals [14]. The top conductor is the microwave signal layer, and it carries the signals between amplifiers, mixers, filters, and other microwave devices that might be used in a multifunction circuit board. For the optimum microwave performance and impedance control, a low loss material with a nearly frequency-independent permittivity and well-constrained thickness should be chosen as the top layer of dielectric material. The next layer down is the RF ground plane, ideally a solid conductor that carries the microstrip ground current and isolates the RF signals from the digital and DC signals beneath it. As the grounding scheme for a microwave PCB generally is multipoint, one should avoid using the RF ground plane as...
