Metal-pipe Rectangular Waveguides Research Articles

3-D Printed Rectangular Waveguide 123–129 GHz Packaging for Commercial CMOS RFICs

This work demonstrates the hybrid integration of a complementary metal–oxide–semiconductor (CMOS) radio frequency integrated circuit (RFIC) into a host 3-D printed metal-pipe rectangular waveguide (MPRWG). On-chip Vivaldi antennas are used for TE10-to-thin-film microstrip (TFMS) mode conversion. Our packaging solution has a combined measured insertion loss of only 1 dB/transition at 126 GHz. This unique packaging and interconnect solution opens up new opportunities for implementing low-cost subterahertz (THz) multichip modules.

Polymer-Based 3-D Printed 140 to 220 GHz Metal Waveguide Thru Lines, Twist and Filters

This paper demonstrates the current state-of-the-art in low-cost, low loss ruggedized polymer-based 3-D printed G-band (140 to 220 GHz) metal-pipe rectangular waveguide (MPRWG) components. From a unique and exhaustive up-to-date literature review, the main limitations for G-band split-block MPRWGs are identified as electromagnetic (EM) radiation leakage, assembly part alignment and manufacturing accuracy. To mitigate against leakage and misalignment, we investigate a ‘trough-and-lid’ split-block solution. This approach is successfully employed in proof-of-concept thru lines, and in the first polymer-based 3-D printed 90° twist and symmetrical diaphragm inductive iris-coupled bandpass filters (BPFs) operating above 110 GHz. An inexpensive desktop masked stereolithography apparatus 3-D printer and a commercial copper electroplating service are used. Surface roughness losses are calculated and applied to EM (re-)simulations, using two modifications of the Hemispherical model. The 7.4 mm thru line exhibits a measured average dissipative attenuation of only 12.7 dB/m, with rectangular-to-trapezoidal cross-sectional distortion being the main contributor to loss. The 90° twist exhibits commensurate measured performance to its commercial counterpart, despite the much lower manufacturing costs. A detailed time-domain reflectometry analysis of flange quality for the thru lines and 90° twists has also been included. Finally, a new systematic iris corner rounding compensation technique, to correct passband frequency down-shifting is applied to two BPFs. Here, the 175 GHz exemplar exhibits only 0.5% center frequency up-shifting. The trough-and-lid assembly is now a viable solution for new upper-mm-wave MPRWG components. With this technology becoming less expensive and more accurate, higher frequencies and/or more demanding specifications can be implemented.

3-D Printed Plug and Play Prototyping for Low-Cost Sub-THz Subsystems

Polymer-based additive manufacturing using 3-D printing for upper-millimeter-wave ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ca.</i> 100 to 300 GHz) frequency applications is now emerging. Building on our previous work, with metal-pipe rectangular waveguides and free-space quasi-optical components, this paper brings the two media together at G-band (140 to 220 GHz), by demonstrating a compact multi-channel front-end subsystem. Here, the proof-of-concept demonstrator integrates eight different types of 3-D printed components (30 individual components in total). In addition, the housing for two test platforms and the subsystem are all 3-D printed as single pieces, to support plug and play development; offering effortless component assembly and alignment. We introduce bespoke free-space TRM calibration and measurement schemes with our quasi-optical test platforms. Equal power splitting plays a critical role in our multi-channel application. Here, we introduce a broadband 3-D printed quasi-optical beamsplitter for upper-millimeter-wave applications. Our quantitative and/or qualitative performance evaluations for individual components and the complete integrated subsystem, demonstrate the potential for using consumer-level desktop 3-D printing technologies at such high frequencies. This work opens-up new opportunities for low-cost, rapid prototyping and small-batch production of complete millimeter-wave front-end subsystems.

3-D Printing Quantization Predistortion Applied to Sub-THz Chained-Function Filters

This paper investigates physical dimension limits associated with the low-cost, polymer-based masked stereolithography apparatus (MSLA) 3-D printer, with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50 ~\mu \text{m}$ </tex-math></inline-formula> pixels defining the minimum print feature size. Based on the discretization properties of our MSLA 3-D printer, multi-step quantization predistortion is introduced to correct for registration errors between the CAD drawing and slicing software. This methodology is applied to G-band <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5^{th}$ </tex-math></inline-formula> order metal-pipe rectangular waveguide filters, where the pixel pitch has an equivalent electrical length of 8.5° at center frequency. When compared to the reference Chebyshev filter, our chained-function filter exhibits superior S-parameter measurements, with a low insertion loss of only 0.6 dB at its center frequency of 182 GHz, having a 0.9% frequency shift, and an acceptable worst-case passband return loss of 13 dB. Moreover, with measured dimensions after the 3-D printed parts have been commercially electroplated with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50 ~\mu \text{m}$ </tex-math></inline-formula> thick layer of copper, the re-simulations are in good agreement with the S-parameter measurements. For the first time, systematic (quantization) errors associated with a pixel-based 3-D printer have been characterized and our robust predistortion methodology has been successfully demonstrated with an upper-millimeter-wave circuit. Indeed, we report the first polymer-based 3-D printed filters that operate above W-band. As pixel sizes continue to shrink, more resilient (sub-)THz filters with ever-higher frequencies of operation and more demanding specifications can be 3-D printed. Moreover, our work opens-up new opportunities for any pixel-based technology, which exhibits registration errors, with its application critically dependent on its minimum feature size.

Parasitic High Q-Factor Open-Box Modes With 3-D Printed Dielectric-Filled Metal Waveguides

High Q-factor open-box mode resonances have been found in the microwave measurements of several 3-D printed dielectric-filled metal-pipe rectangular waveguides (MPRWGs). These parasitic Fabry-Perot eigenmodes are confined by the conductive walls in the transverse plane of the MPRWG and partially confined by the air-dielectric and dielectric-air boundaries in the longitudinal direction. The excitation of open-box modes was previously speculated to be due to the inhomogeneous and/or anisotropic nature of the 3-D printed dielectric-fillers. This has now been confirmed, by representing the inhomogeneous and anisotropic nature of the woodpile-like dielectric structure (physical realm), with an anisotropic dielectric constant tensor (simulation realm). Analytical and numerical eigenmode solvers, previously used by the authors with MPRWGs, are applied here to parallel-plate waveguides (PPWGs) and circular waveguides (CWGs); identifying all the individual parasitic open-box modes. With the former, its TM11 mode exhibits an ultra-high Q-factor of approximately 2,300 at X-band, which is considerably higher than those found with other modes and in other waveguide structures. Finally, a numerical full-wave frequency-domain simulator that employs the dielectric constant tensor is introduced in this paper. This new modeling technique independently confirms that open-box modes are excited in 3-D printed dielectric-filled MPRWG, PPWG and CWG structures. This paper provides the foundations for accurately modeling parasitic resonances associated with inhomogeneities and anisotropy in 3-D printed microwave components; not just the metal-walled waveguide structures considered here, but the methodology could also be extended to generic 3-D printed dielectric waveguides and substrate-based transmission lines.

3-D printed primary standards for calibration of microwave network analysers

This paper describes the design, fabrication and testing of 3-D printed primary standards for use with the calibration of microwave vector network analysers. The standards are a short-circuit and a quarter wavelength section of line that are designed for use with the Thru-Reflect-Line calibration technique. The standards are realised in metal-pipe rectangular waveguide, covering the frequency range from 12 GHz to 18 GHz (i.e., Ku-band). The standards are polymer-based 3-D printed, which is subsequently metal plated to provide the required electrical conductivity. The performance of the standards is compared with conventionally machined standards that are used as part of the UK’s primary national measurement system for microwave scattering parameters. The authors believe that this is the first time that 3-D printing techniques have been used to produce such calibration standards, and, that this could lead to a new approach to providing metrological traceability for these types of measurement.

Microwave Characterization of Low-Loss FDM 3-D Printed ABS With Dielectric-Filled Metal-Pipe Rectangular Waveguide Spectroscopy

Over time, the accuracy and speed by which a material can be characterized should improve. Today, the Nicolson–Ross–Weir (NRW) methodology represents a well-established method for extracting complex dielectric properties at microwave frequencies, with the use of a modern vector network analyzer. However, as will be seen, this approach suffers from three fundamental limitations to accuracy. Challenging NRW methods requires a methodical and robust investigation. To this end, using a dielectric-filled metal-pipe rectangular waveguide, five independent approaches are employed to accurately characterize the sample at the Fabry–Perot resonance frequency (non-frequency dispersive modeling). In addition, manual Graphical and automated Renormalization spectroscopic approaches are introduced for the first time in the waveguide. The results from these various modeling strategies are then compared and contrasted to NRW approaches. As a timely exemplar, 3-D printed acrylonitrile–butadiene–styrene (ABS) samples are characterized and the results are compared with existing data available in the open literature. It is found that the various Fabry–Perot resonance model results all agree with one another and validate the two new spectroscopic approaches; in doing so, exposing three limitations of the NRW methods. It is also shown that extracted dielectric properties for ABS differ from previously reported results and reasons for this are discussed. From measurement noise resilience analysis, a methodology is presented for determining the upper bound signal-to-noise ratio for the vector network analyzer (not normally associated with such instrumentation). Finally, fused deposition modeling (FDM) 3-D printing can result in a non-homogeneous sample that excites open-box mode resonances. This phenomenon is investigated for the first time analytically and with various modeling strategies.

Hybrid 3-D-Printing Technology for Tunable THz Applications

In recent years, additive manufacturing has experienced rapid growth, due to its inherent capabilities for creating arbitrary 3-D structures, accessibility, and associated low manufacturing costs. This paper first reviews the state of the art in 3-D printing for terahertz (THz) applications and identifies the critical features required for such applications. The future potential for this technology is demonstrated experimentally with the first 3-D-printed, optically controlled THz IQ vector modulator. Here, miniature high-resistivity silicon implants are integrated into metal-pipe rectangular waveguides. The 3-D-printed split-block assembly also houses two packaged infrared laser diodes and a heat sink. The measured performance of a proof-of-principle 4-quaternary amplitude modulation (4-QAM) vector modulator that operates up to 500 GHz is reported. This new hybrid 3-D printing THz technology, which combines semiconductor devices with potentially low-cost, high-performance passive guided-wave structures represents a paradigm shift and may prove to be an ideal solution for implementing affordable transceivers in future ubiquitous THz applications.

Printing: the future of THz

Researchers from Imperial College London, the UK's National Physical Laboratory (NPL), and the University of Tokyo, have demonstrated a 3D-printed metal-pipe rectangular waveguide operating at a record-breaking frequency of 1.1 THz. Stepan Lucyszyn and William Otter, two of the researchers behind the record-breaking waveguides. 3D printing has found uses in a huge number of industries and applications, and its strengths over traditional manufacturing techniques were quickly exploited by the communications community. William Otter and Stepan Lucyszyn, two of the researchers on the THz project, told us that at Imperial College London, ‘the initial research into 3D printing of radio frequency metal-pipe rectangular waveguides was for the microwave frequency range of 8 to 12 GHz (X-band), and the success of this work gave our team the confidence to move up into the millimetre-wave frequency band, of 75–110 GHz (W-band), where the small dimensions require a tighter manufacturing tolerance.’ 3D printing, also known as additive manufacturing, is the layer-by-layer addition of material to create a structure. It dramatically reduces the time and the cost of producing components, while allowing more design freedom. In comparison with traditional manufacturing, ever more complex geometries can be realised with 3-D printing as fewer mechanical constraints are imposed on the process. With large complex structures, this means reducing the number of individual components needed to create a subsystem, thereby reducing the number of tests required during the manufacturing process. This design freedom helped produce excellent initial results in the W-band, comparable with off-the-shelf commercial waveguides. However, as Otter and Lucyszyn explained, they did not stop there, ‘This high level of performance was also demonstrated at W-band with a 6th-order Chebyshev filter, which are notoriously sensitive to manufacturing tolerances.’ The next challenge was to demonstrate a mechanically tuneable component that was fully 3D-printed. This was achieved using an X-band dielectric-flap phase shifter. After these early successes the team kept increasing the frequencies of their components. ‘We were pushing the mechanical limits of all 3D printing processes, but, we set our sights specifically on breaking the record for metal-pipe rectangular waveguides,’ said Otter and Lucyszyn, ‘and this was achieved with an I-Q vector modulator operating up to 500 GHz.’ Overcoming the limitations of 3D printing, specifically in regards to THz technology, to achieve record frequencies is where the collaboration between the UK and Japanese began: ‘The main limitation for THz waveguides is the ability to 3D print with relatively large structures, having small feature sizes and high-quality surface finishes.’ For this reason, the prototype for a new class of 3D-printers called RECILS, and developed at the University of Tokyo was used. The new RECILS technology, through its innovative method of controlling the photocurable resin level and detachment of cured resin, provides the required finish without further post-processing.’ Now that the team have achieved inexpensive THz technology, what doors do they hope to have opened? The work is expected to find future applications in high-performance imaging systems for security monitoring, satellite payloads and medical diagnostic equipment. Thanks to the team's techniques, according to Otter and Lucyszyn, ‘these systems can be developed in a shorter time and at a significantly lower cost, and, as such, [the method] represents a paradigm shift in the manufacture of precision components using mass marketable 3D printers.’ From left to right: X-Band phase shifter, W-Band thru line, W-Band 6th-order Chebyshev filter, 500 GHz I-Q vector modulator. In the background is an X-band twist and the foreground shows the 1.1 THz waveguide. To move ever closer to such a reality, the group have been developing minimal-part subsystems that break conventional design rules by exploiting the multiple degrees-of-freedom offered by additive manufacturing. And this is, as Otter and Lucyszyn explained, the really exciting thing about the technology. ‘Within a decade, ultra-high resolution low-cost 3D printing will be ubiquitous. It will become the norm to replace parts by downloading CAD files and printing and spray coating metal onsite.’ This is particularly important for remote and inaccessible locations. Moreover, a printer offering multiple materials, perhaps even having exotic properties, will be capable of printing whole systems, the only limit being our imaginations.’

3D printed 1.1 THz waveguides

For the first time, 3D printed metal-pipe rectangular waveguides (MPRWGs) have been demonstrated in the WM-380 (500-750 GHz) and WM-250 (750 GHz-1.1 THz) waveguide bands. The ultra-high spatial resolution offered by the new RECILS additive manufacturing technology enables the precision fabrication of these prototype MPRWGs at such high frequencies. This enabling technology avoids the need for access to expensive microfabrication resources and, thus, opens up the terahertz spectrum to the low-cost manufacture of passive components.

3-D Printed Variable Phase Shifter

This letter presents the first fully 3-D printed microwave variable phase shifter. The design methodology for a 3-D printable dielectric flap metal-pipe rectangular waveguide variable phase shifter is described. The ABS building material was independently characterized, revealing a dielectric constant of 2.34 and loss tangent of only 0.0015 at 10 GHz. The predicted and measured performance is given, demonstrating a maximum relative phase shift of 142° at 10 GHz, with a near uniform relative phase shift across the whole of X-band and a low variation in differential-phase group delay.

3-D Printed Metal-Pipe Rectangular Waveguides

This paper first reviews manufacturing technologies for realizing air-filled metal-pipe rectangular waveguides (MPRWGs) and 3-D printing for microwave and millimeter-wave applications. Then, 3-D printed MPRWGs are investigated in detail. Two very different 3-D printing technologies have been considered: low-cost lower-resolution fused deposition modeling for microwave applications and higher-cost high-resolution stereolithography for millimeter-wave applications. Measurements against traceable standards in MPRWGs were performed by the U.K.’s National Physical Laboratory. It was found that the performance of the 3-D printed MPRWGs were comparable with those of standard waveguides. For example, across X-band (8–12 GHz), the dissipative attenuation ranges between 0.2 and 0.6 dB/m, with a worst case return loss of 32 dB; at W-band (75–110 GHz), the dissipative attenuation was 11 dB/m at the band edges, with a worst case return loss of 19 dB. Finally, a high-performance W-band sixth-order inductive iris bandpass filter, having a center frequency of 107.2 GHz and a 6.8-GHz bandwidth, was demonstrated. The measured insertion loss of the complete structure (filter, feed sections, and flanges) was only 0.95 dB at center frequency, giving an unloaded quality factor of 152—clearly demonstrating the potential of this low-cost manufacturing technology, offering the advantages of lightweight rapid prototyping/manufacturing and relatively very low cost when compared with traditional (micro)machining.

Rectangular waveguide enabling technology using holey surfaces and wire media metamaterials

A new enabling technology for implementing tunable rectangular waveguide components and circuits is reported for the first time with the use of 2D and 3D metamaterials; a holey metal surface and wire media, respectively. Traditional solid metal irises are replaced by a wire medium metamaterial. These media are well known and used to emulate plasma behavior and, therefore, can be used to replace solid metal. As proof of concepts, results for tunable rectangular waveguide filters are presented with the use of pin block inductive irises and capacitive posts. Furthermore, by adapting the traditional metal-pipe rectangular waveguide for tunability, regions of the solid metal walls are replaced by holey metasurfaces that enable adjustments in the position and spacing of the pin blocks. Prototype tunable structures were measured for verification and good agreement is achieved between full-wave simulations and measurements. The results clearly demonstrate the potential for this tunable/reconfigurable rectangular waveguide enabling technology. Potentially new applications for this permeable enabling technology include lightweight and forced-air/cryogenically cooled subsystems, gas/vapor/humidity/pressure/light sensors, optoelectronic and even real-time tunable/reconfigurable components, circuits and subsystems.

Defining Material Parameters in Commercial EM Solvers for Arbitrary Metal-Based THz Structures

Frequency-domain solvers are used extensively for modeling arbitrary metal-based terahertz structures. Four well-known commercially available electromagnetic (EM) modeling software packages include HFSS™, CST Microwave Studio®, EMPro, and RSoft. However, there are a number of operational issues that relate to how they can be used to obtain more meaningful and accurate results. Even experienced users of these and similar software packages may not fully appreciate some of the subtle ambiguities in defining boundaries and material parameters for use in THz applications. To this end, a detailed comparative study has been undertaken, in consultation with all four vendors. First, in order to avoid introducing ambiguities, frequency dispersion in materials has to be clearly defined from first principles; in both intrinsic and effective forms. Different frequency dispersion models are then introduced for `metal-like' materials. To act as benchmark structures, conventional air-filled metal-pipe rectangular waveguides, associated cavity resonators and a spoof surface plasmon waveguide have been simulated, using a raft of different approaches; with a view to illustrating quantifiable weaknesses in commercial software packages for simulating arbitrary metal-based THz structures. This paper highlights intuitive and logical approaches that give incorrect results and, where possible, makes recommendations for the most appropriate solutions that have hitherto not been given in Technical Notes.

THz Applications for the Engineering Approach to Modelling Frequency Dispersion within Normal Metals at Room Temperature

The use of the accurate classical relaxation-effect model for frequency dispersion in normal metals at room temperature with THz structures can be mathematically cumbersome and not insightful. Recent work has demonstrated that it is possible to dramatically simplify otherwise complex analysis and allows for a much deeper insight to be gained into the classical relaxation- effect model. This paper gives applications for this engineering approach. Here, using the concept of Q-factor for a metal, the synthesized equivalent transmission line model is validated. Then, using the concept of complex skin depth, analysis is performed on hollow metal-pipe rectangular waveguides and their associated cavity resonators. This work proves that the mathematical modelling of THz structures can be greatly simplified by taking an electrical engineering approach to these electromagnetic problems.

HFSSTM Modelling Anomalies with THz Metal-Pipe Rectangular Waveguide Structures at Room Temperature

Air-fllled metal-pipe rectangular waveguides (MPRWGs) represent one of the most important forms of guided-wave structure for terahertz applications. Well-known commercial electromagnetic modelling software packages currently employ over-simplifled intrinsic frequency dispersion models for the bulk conductivity of normal metals used in terahertz structures at room temperature. This paper has compared various conductivity modelling strategies for normal metals at room temperature and characterized rectangular waveguides and associated cavity resonators between 0.9 and 12THz. An expression for the geometrical factor of a rectangular cavity resonator has been derived for the general case of a metal characterized with r 6 1 and !? > 0. In addition, a method for determining the corresponding lossless frequency of oscillation has been given for the flrst time for such models. Using these techniques, a quantitative analysis for the application of difierent models used to describe the intrinsic frequency dispersion nature of bulk conductivity at room temperature has been undertaken. When compared to the use of the accurate relaxation-efiect model, it has been found that HFSS TM (Versions 10 and 11) gives a default error in the attenuation constant for MPRWGs of 108% at 12THz and 41% errors in both Q-factor and overall frequency detuning with a 7.3THz cavity resonator. With the former, measured transmission losses will be signiflcantly lower than those predicted using the current version of HFSS TM , which may lead to an underestimate of THz losses attributed to extrinsic efiects. With the latter error, in overall frequency detuning, the measured positions of return loss zeros, within a multi-pole fllter, will not be accurately predicted by the current version of HFSS TM . This paper has highlighted a signiflcant source of errors with the electromagnetic modeling of terahertz structures, operating at room temperatures, which can be rectifled by adopting the classical relaxation-efiect model to describe the frequency dispersive behavior of normal metals.

Photoimageable thick-film millimetre-wave metal-pipe rectangular waveguides

A novel dielectric-filled metal-pipe rectangular waveguide has been fabricated using photoimageable thick-film materials. The waveguides incorporate a new transition from CPW-to-TFMS-to-MPRWG. Standard on-wafer measurement techniques, waveguide STD calibration standards, demonstrated low attenuation across the 60 to 90 GHz frequency range.

Design of compact monolithic dielectric-filled metal-pipe rectangular waveguides for millimetre-wave applications

The designs of two dielectric-filled metal-pipe rectangular waveguide structures are presented which are both compatible with monolithic-integrated circuit technology. The first design uses ultralow-loss materials while the second employs materials that were available at the time. The measured performances of an experimental 105 GHz proof-of-concept π-network clearly demonstrates that the dominant TE10 mode of propagation is supported in this uniplanar technology.