However, the flexibility of the design does not restrict the use of the proposed structure that can be used for CPS to CPW transition. The presented work focuses on the slow-wave structures due to their attractive features against conventional TLs. ![]() To this end, this Letter presents a compact S-CPW to S-CPS transition integrated in a 55 nm BiCMOS technology of STMicroelectronics. Furthermore, any solution tackling this problematic needs to be validated over broadband mm-wave frequencies to accompany the current trends in RF applications. In practice, the air-bridge topology cannot be utilised in CMOS process since the metal layers are submerged within dioxide dielectric. Though the mentioned solutions were all fabricated on MMIC process, it is pertinent to address the issue of CPW (or slow-wave CPW – S-CPW) to CPS (or S-CPS) transition in the context of CMOS process. The same concept of air-bridge was utilised in but implemented on different processes. The measured back-to-back revealed an insertion loss smaller to 1.1 dB and a return loss higher than 15 dB from 0.5 to 110 GHz. ![]() In , a transition was designed by tying the two ground planes of CPW at their ends through a strip, then the centre of this latter is connected to the ground strip of CPS by another connection while the CPW and CPS signal strips were connected through an air-bridge to avoid interaction with ground planes connection. Although, broadband impedance matching and small insertion loss up to 65 GHz were measured, this technique cannot be generalised since it considers only CPW and CPS with the same gap. For instance, in CPW to CPS transition are reported where the authors have connected directly the CPW signal strip to one of the CPS strips and one of the CPW ground to the second CPS strip, while connecting the second CPW ground through an air-bridge. Unfortunately, very limited works have been reported in the literature to address this issue. For this reason, a network that enables reversible transition between coplanar waveguide (CPW) pads to CPS (or S-CPS) becomes inescapable in order to carry out standalone characterisation. Indeed, without standalone measurements of these structures, their characterisation cannot be performed accurately. In fact, this measurement constraint has limited the generalisation of the CPS and S-CPS use in the RF circuitry. However, the coplanar probing of such structures has always been an issue because the almost deployed probers are equipped with GSG probes. Based on CPS structure, the slow-wave CPS (S-CPS) is seen as an even more efficient solution that adds to the features of the conventional CPS the advantages of the slow-wave effect, which helps in dimensions shrink as well as in Q improvement . The coplanar stripline (CPS) structure is advertised as an alternative to the conventional TLs like microstrip lines, thanks to its balanced structure, which offers reduced cross-talk, high common mode rejection ratio and wide range of synthesisable. Various solutions were reported in the literature to mitigate these technological limitations. ![]() This has led to higher losses in the integrated passive devices, in particular, the transmission lines (TLs), which suffer from poor quality factor ( Q) and a limited range of synthesisable characteristic impedances ( ). However, at the same time, new design constraints such as stringent metal density requirements along with thinner dielectric layers above bulky substrates appeared. Fortunately, the continuing growing of CMOS/BiCMOS technologies has accompanied these requirements. Consequently, circuits design has become more challenging, demanding transistors owning sufficient, and high frequencies.
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