Performance enhancement of microwave circuits using parallel-strip line

Nowadays, demands for compactness, light weight, and broadband have a great impact on design issues at both component and system levels in wireless communications. Multilayered structures fabricated with low-temperature co-fired ceramic (LTCC) technologies have been developed recently in response to these demands. With this trend, double-sided printed circuits using conventional fabrication techniques with lower cost have been receiving plenty of attention. The parallel-strip line, belongs to the family of balanced transmission lines. The conventional printed circuit board technology is able to realize it easily. It is a simple structure comprised of a piece of dielectric substrate sandwiched by two conducting traces. Signals flowing on the upper and lower conductors are always equal in magnitude and 180° out of phase; therefore, parallel-strip line has an inherent advantage of easy realization of a balanced configuration. Balanced or double balanced microwave circuits, such as amplifier, mixer, and antenna, are frequently employed in high-performance wireless communications systems, and they can be directly fed by parallel-strip lines. In contrast, microstrip line is an unbalanced transmission line; therefore, extra effort of designing balanced to unbalanced structures (balun) is required for using microstrip transmission lines.