I. Introduction
Crosstalk document or coupling between neighboring transmission lines is one of the most critical signal integrity issues, especially in high-density, high-speed/high-frequency design. A large amount of published material on this topic is available in the literature, reflecting the huge interest accorded to the subject. However, the phenomenon still seems confusing for many printed circuit board (PCB) designers, and difficult to grasp. In order to reduce the magnitude and effect of crosstalk between adjacent transmission lines many design rules and techniques have been widely adopted by high-speed PCB designers (e.g., placing ground/guard traces or platted though vias holes between the adjacent lines, reducing the coupling length, maintaining large enough distance on the order of the substrate thickness between the neighboring lines or increasing the separation distance from one to three times the trace width to alleviate crosstalk between them, etc.). Some of these rules are unfortunately not well documented, and the question of whether or not they help reducing high-speed and high-frequency PCB crosstalk is still being asked by many designers [1]. Among these techniques for preventing or minimizing crosstalk between parallel traces, the route to route spacing or 3-W rule is of important interest, and mostly followed by high-speed PCB designers. The 3-W rule [2] is used by many PCB designers, to comply with PCB layout and signal integrity requirements. However, the capability of this rule in reducing crosstalk is still not well established, especially at microwave frequencies. Therefore, it is necessary to experimentally evaluate this design rule at high-frequency. In this paper, the efficiency of the spacing rules for crosstalk reduction in high-frequency PCBs is discussed, based on stripline and microstrip transmission lines.