Theory of Compact Narrow-Band Directional Couplers and Implementation in Silicon IPD Technology

Typical directional couplers are based on quarter-wavelength transmission line segments. Such designs are physically too large for most integrated system-in-package applications. A lumped-element circuit for electrically short, narrow-band directional couplers is described and analyzed. This circuit is much smaller and is better suited for integrated assemblies. Its directivity is shown to derive from the correct balancing of magnetic and electric coupling. Such circuits can be integrated into the designs of other passive networks, like harmonic filters and baluns, to achieve further space and cost savings. Examples and experimental results for such designs are presented. Evaluation and analysis of these devices suggests some improvements to the basic coupler circuit. These changes are presented along with examples of improved circuit designs.