Waveguide-to-Substrate Transition Based on Unilateral Substrateless Finline Structure: Design, Fabrication, and Characterization

Abstract

We report on a novel waveguide-to-substrate transition with prospective use for broadband mixer design. The transition employs a substrateless finline, i.e., a unilateral finline structure with the substrate removed between the fins. This distinctive feature diminishes the overall insertion loss and facilitates matching with the waveguide. The transition is designed on a thin silicon substrate covered by a superconducting niobium thin layer. An auxiliary Au layer situated on top of the Nb layer provides grounding for the fins and facilitates the mounting process in the split-block waveguide mount. Aiming to compare simulations with measurements, a back-to-back transition arrangement for the 211-373 GHz frequency band was designed, fabricated, and characterized at cryogenic temperatures. The simulation results for the back-to-back structure show an insertion loss of less than 0.6 dB in the whole band, i.e., 0.3 dB per transition. Furthermore, a remarkable fractional bandwidth of 55% with a return loss better than 15 dB is predicted. Experimental verification shows consistent results with simulations.

Ultra-Wideband 90° Waveguide Twist for THz Applications

“…The overall sensitivity of the design was analyzed employing a relative sensitivity coefficient RS S11 [27], [28]. The coefficient is described as follows:…”

Wideband Slotline-to-Microstrip Transition for 210–375 GHz Based on Marchand Baluns

“…As a starting point for our Chebyshev design, we selected 𝜃 equal to 63.711°, which theoretically provides 50 % fractional bandwidth, to calculate the characteristic impedances detailed in Table I. In order to realize the slotline impedances Z 0S , Z 2 , and Z 3 and compute their width and length, the approximations presented in [17] were employed. The length of the transmission lines was established to λ g /4, where λ g correspond to the guided wavelength at the center frequency, i.e.…”

“…The process employed for fabricating the test structures is similar to the one described in [17]. The samples were fabricated from a silicon-on-insulator (SOI) substrate with a 300 μm handle layer, 2 μm barrier layer SiO 2 , and 30 μm device layer.…”

“…Other transmission lines are occasionally used at the THz range, e.g., coplanar waveguides (CPW) and slotlines. In particular, slotlines are an attractive solution for implementation of planar devices [15], antenna feed [16], and finline transitions [17], [18]. Nevertheless, at THz frequencies, the realization of wideband and low loss slotline-to-microstrip…”

“…This problem is especially relevant for low impedances microstrip lines. While thin-film microstrip lines can reach an impedance of few Ω, thin-film slotlines cannot be fabricated with sufficient accuracy for impedances lower than 40 Ω [17], which hinders the application of classical solutions over large bandwidths, i.e., single-section 90° radial stubs [19] and double-Y baluns [20], where the best matching is found when the slotline and microstrip impedances are the same. The bandwidth limitation could be overcome through indirect transitions, where a piece of an auxiliary transmission line is placed between the slot and the microstrip [21].…”