Artificial-Synthesized Edge-Coupled Transmission Lines for Compact CMOS Directional Coupler Designs

This paper presents artificial-synthesized edge- coupled transmission lines (TLs) based on complementary metal– oxide semiconductor (CMOS) technology. The artificial coupled- line (CL), the so-called the complementary-conducting-strip coupled line (CCS CL), consists of the unit cell and is meandered in the 2-D plane. The unit cell, whose dimensions are much smaller than the guiding wavelength at the operating frequency, provides five structural parameters for CL syntheses. The synthesized CLs, which are fully characterized by using theoretical and experimental procedures, reveal the following unique guiding properties. First, the coupling coefficient ($K$ ) can be 7.7% higher than that of the meandered conventional edged-coupled TLs. Second, the even- and odd-mode propagation constants can be synthesized as identical at the same time. Two backward-wave directional couplers for achieving the tight coupling and high directivity are designed by incorporating artificial CCS CL. Two prototypes are fabricated by the standard 0.18-$\mu$m CMOS technology, occupying the chip areas of 120.0 $\mu {\hbox {m}}\,\, \times\,$ 240.0 $\mu$m. The measured results show that the isolation is higher than 25.0 dB from 10.0 to 40.0 GHz with a coupling of 14.4 dB. On the tight coupler design, the through and coupling are 4.65 $ \pm$0.15 dB from 30.0 to 40.0 GHz with an amplitude imbalance of less than 0.3 dB.