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Design and Characterization of a W-Band Micromachined Cavity Filter Including a Novel Integrated Transition From CPW Feeding Lines
Yuan Li, Student Member, IEEE, Bo Pan, Student Member, IEEE, Cesar Lugo, Member, IEEE, Manos Tentzeris, Senior Member, IEEE, and John Papapolymerou, Senior Member, IEEE
Sep 29, 2008:
Abstract—This paper demonstrates a novel coplanar waveguide (CPW) to rectangular waveguide (RWG)transition and its application as an integration enabling structure for filters at W-band for the first time to the best of the authors’ knowledge. In the proposed wideband transition, the CPW and RWG are integrated on one side of the substrate, while the coupling probe is patterned directly on the substrate instead of being made separately. The transition is fabricated using silicon micromachining and thick-film surface micromachining techniques with high precision, which is suitable for millimeter-wave subsystems and are easily extendable to terahertz applications compared to the existing approaches. In the proposed filter design, the metallized probes are used to couple signals from the CPW to waveguide resonant cavities, therefore, the need for waveguide input/output ports is eliminated. This significantly reduces the size of the filter by approximately 30%. The transition and filter components are optimized using Ansoft’s High Frequency Structure Simulator 10 (HFSS 10). The measured response of the filter has the center frequency at 96.6 GHz, 2.9% bandwidth, and a 4.14-dB insertion loss in the passband.
Index Terms—Cavity filter, coplanar waveguide (CPW), micromachining, transition, W-band.
I. Introduction
Recently, communication and satellite systems are seeking a fully integrated solution where waveguide components can be accessed through planar circuit boards such as printed microstrip and coplanar waveguide (CPW) lines. Some intrinsic obstacles in this integration effort can be tackled through the reconciliation of the many fundamental differences that exist between planar and waveguide transmission.
The rectangular waveguide (RWG) has the advantages of low loss and high power capacity. However, the traditional RWG at low frequencies is bulky, making their integration into system-on-chip geometries almost impossible. Nevertheless, the size of RWG shrinks dramatically at or above the millimeter-wave frequency range or even in terahertz frequencies [1]–[3]. Meanwhile, the losses of other compact transmission lines, such as microstrips, keep increasing when the frequency goes up. The integration of subsystems into a packaged solution with a combination of planar and waveguide components provides advantages in terms of high performance with reduced size. For this reason, it is crucial to build an understanding of planar to waveguide transition structures and their application in microwave component designs at millimeter-wave frequencies.
In the existing approaches, the transitions [4]–[6] with conventional machining techniques are not easily adopted by micromachining, which is feasible for the integration of sub-millimeter-wave circuits. The transitions from planar waveguides to substrate integrated rectangular waveguides (SIRWs) are reported [7]–[10]. These transitions have the advantages of using a low-cost printed circuit board (PCB) process. However, they suffer the reduced Q factor due to the dielectric filling, as well as relatively large minimum slot and linewidth. In the meantime, several transitions are demonstrated using the low temperature co-fired ceramic (LTCC) technique not only from the planar waveguide to RWG [11], [12], but also from laminated waveguide to air-filled waveguide [13]. These transitions implemented with the LTCC technique provide good responses, although they are limited by using metal vias to form the cavity. In [14] and [15], the transitions from the CPW line to RWG on a silicon substrate are reported with wet and dry etching techniques. The coupling probe is fabricated separately and has to be assembled on the substrate.
In our previous research [16], a novel transition is proposed. It has the advantage of high precision and easier mass production by using silicon micromachining and thick-film surface micromachining techniques. The CPW, probes, and RWG are patterned and integrated on the same side of the substrate. In [16], the back-to-back transition is designed. The measured -parameters are quite encouraging and the preliminary transition design is discussed.
In this paper, the comprehensive design analysis of the transition is given, as well as the parameters study; secondly, a waveguide filter that takes full advantage of this transition is demonstrated for the first time. The fabrication, experimental results, nd the discussion of the two-pole filter are finally presented.