Microstrip Slow-Wave Transmission Line With Low-Complexity Zigzag-Pattern Ground Shield for Area-Efficient Design

초록

This letter presents a microstrip (MS) slow-wave transmission line (SWTL) with a low-complexity zigzag-pattern ground shield for area-efficient design. The proposed ground shield is a combination of diagonal zigzags placed alternately. Unlike the uni-directional straight lines used in the conventional slotted ground shield structure, the zigzag-pattern eliminates the image current equally in both X-and Y-directions. As a result, the proposed MS SWTL can be implemented at 90 degrees angles without degrading circuit performance or requiring additional tuning effort. To validate the concept, SWTLs and passive devices utilizing SWTLs, including slow-wave branchline couplers (SW BLCs) and slow-wave rat-race baluns (SW RRBs), operating at 150 GHz, are also fabricated in a 28 nm CMOS process. Measurements show that conventional and proposed MS SWTLs deliver similar performance, with approximately half the wavelength of a regular transmission line (TL). By integrating the proposed MS SWTLs at junctions, the proposed SW BLC can take full advantage of the size reduction from the slow-wave technique, occupying about 23.1% of the area compared to the regular BLC. In direct comparison, the proposed SW BLC is about 20% smaller than the conventional SW BLC, while other performance metrics, such as insertion loss, amplitude, and phase mismatches, are almost identical. This area efficiency is further enhanced by employing a meander-line design, as demonstrated by the proposed meander-SW BLC (M-SW BLC) and meander-SW RRB (M-SW RRB) consuming <= 10% of the area of their regular TL-based designs.

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