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Make Waveguide In Planar PCB Form
Forming resonant cavities on microwave printed-circuit boards (PCBs) is a good first step to the design of high-frequency oscillators and filters. Another approach is the use of substrate-integrated-waveguide (SIW) technology, which is not only suitable for oscillators and filters, but can be formed into extremely compact antennas, and can support signals well into the millimeter-wave range. SIW technology structures are versatile design elements for integrating active and passive circuits together with radiating elements, such as antennas, onto compact circuits using popular PCB laminate materials. As the last blog showed, creating resonant cavities in different circuit materials can lead to high-performance microwave oscillators and filters. As this blog will reveal, SIW structures can also serve as resonators in planar, multilayer PCBs, helping to create compact, high-performance filters, oscillators, and other resonator-based circuits.
Put simply, certain types of structures in a dielectric material will resonate under the proper conditions, depending upon the dielectric constant of the material and the dimensions and form of the structures. SIW or buried-waveguide structures, as they are sometimes known, combine the principles of hollow metal waveguide and dielectric materials, producing controllable resonances that can be used as building blocks for RF/microwave oscillators, filters, and even antennas. SIW structures are especially attractive for use in affordable millimeter-wave circuits, supporting the cost-effective design of automotive collision-avoidance radars and other millimeter-wave products.
A SIW structure is realized by means of a dielectric substrate with top and bottom conductor layers, usually copper. A SIW circuit is formed from two arrays or rows of metalized viaholes fabricated through the substrate and connecting the two conductor layers, emulating the sidewalls of a standard rectangular waveguide. The resonant frequency of a SIW structure depends on its horizontal and vertical dimensions as well as the permeability and dielectric constant (or permittivity) of the PCB substrate material.
In a SIW structure, the height or thickness of the PCB substrate is the same as what would be the vertical wall of the waveguide. As long as the thickness or vertical dimension of the PCB material is much less than the horizontal dimension, then the cutoff frequency of the SIW is mainly determined by the horizontal dimension of the PCB material and the dimensions of the SIW structure formed on it. The use of SIW structures essentially allows rectangular waveguide to be fabricated in planar form, using standard PCB processing methods. Being able to achieve waveguide-like filter performance, for example, integrated on a PCB with other standard planar components, such as amplifiers and mixers, can be a huge boost to applications, such as satellite communications, that depend heavily on standard waveguide components.
SIW structures exhibit propagation characteristics quite similar to those of standard rectangular waveguide, including their field-pattern and dispersion characteristics. SIW circuits also adopt most of the performance advantages of conventional rectangular waveguide, including high quality factor (Q) and high power-handling capability. SIW technology allows a variety of different component and circuit types to be integrated on the same PCB substrate, including passive components, active elements and even antennas. SIW circuitry supports the fabrication of affordable millimeter-wave components and circuits in planar form, with ready integration with standard microstrip, stripline, and other standard microwave transmission lines.
SIW circuits usually imply the use of somewhat thicker PCB materials than for standard microstrip circuits. At the same time, those thicker substrate materials can lead to moding when they are used for standard microstrip circuits. But in many real-life applications, SIW circuit structures, such as filters or power dividers, will be combined with additional microwave circuitry, such as amplifiers and mixers, and a single PCB will combine a number of different circuit technologies.
Of course, in combining different circuit technologies, such as SIW, microstrip, and coplanar-waveguide (CPW) technologies, transitions are needed from the conventional circuit formats to the SIW structures, such as bandpass filters. As an example, a SIW-to-microstrip transition is needed to combine SIW and microstrip technologies. Such a transition can be accomplished by means of a tapered microstrip line section that connects a 50-Ω microstrip line and the SIW structure. The transition is in effect transforming the quasi transverse-electromagnetic (TEM) mode of the standard microstrip line into the transverse-electric (TE) mode of the waveguide structure. But such a transition can be based on a simple taper, when the microstrip and SIW structure are on the same substrate.
As examples, SIW technology has been applied to the design of high-performance bandpass filters with low passband insertion loss and broadband antennas capable of supporting ultrawideband (UWB) operation. Although SIW structures can be as simple as arrays of plated through holes (PTHs) in multilayer PCB structures, such structures are also subject to electromagnetic (EM) leakage. Because SIW represents a form of periodic guided-wave structure, potential leakage problems (which are usually greater at lower frequencies) exist because of the periodic gaps in the arrays of PTHs. And because EM leakage translates into wave attenuation, it must be minimized by aligning the arrays or PTHs or plated slots in the PCB substrate with the current flow through the SIW structure. This can be analyzed with the help of commercial EM simulation software tools, such as HFSS from Ansoft/Ansys or the Advanced Design System (ADS) software from Agilent Technologies.
Of course, knowing which PCB materials to use for any SIW-based circuit design can also help in achieving optimum performance. Two strong candidates for microwave and millimeter-wave SIW-based circuits are the RO4350B™ laminates and RO4450F™ prepreg materials from Rogers Corp. (www.rogerscorp.com). These materials are very robust in handling the multilayer PCB fabrication process needed to build SIW circuits.
For example, RO4350B laminate has a dielectric constant of 3.48 with dissipation factor of 0.0037 at 10 GHz in the z-axis, with thermal conductivity of 0.69 W/m/K to handle high power levels with minimal generation of heat. For designing multilayer SIW circuits, 0.101-mm-thick RO4450F prepreg has a dielectric constant of 3.52 with 0.004 dissipation factor at 10 GHz in the z-axis. It features a thermal conductivity of 0.65 W/m/K closely matched to RO4350B laminate. Both the laminate and the prepreg form SIW and other multilayer circuit structures using standard FR-4 processing methods.
SIW structures may appear somewhat novel, but they are simply waveguide in planar form. They offer a compact and cost-effective solution for integrating waveguide components with active circuits, passive circuits, and radiating elements, such as antennas, on the same PCB substrate. Along with its integration possibilities, SIW technology can realize circuits with light weight, small size, and high power capacity for a variety of applications.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.