Millimeter-wave circuits were once considered exotic and only used for specialized applications, typically in the military space. For one thing, frequencies with such small wavelengths, from about 30 to 300 GHz, required special components and circuits scaled to those diminutive wavelengths. But lower-frequency bands are being consumed by a growing number of wireless applications, and millimeter-wave frequency bands are looking more and more attractive for communications systems of the future. Such high frequencies have even been proposed as part of an emerging fifth-generation (5G) wireless standard that will be challenged to connect billions of global Internet of Things (IoT) devices by means of available wireless bandwidths. Millimeter-wave bandwidths have long been employed for military radar systems and are increasingly being used in commercial automotive collision-avoidance radar systems. Achieving millimeter-wave circuit designs on reliable printed-circuit-board (PCB) materials in a practical manner will be the challenge in making these higher frequencies affordable. Substrate-integrated-waveguide (SIW) circuit technology may just be the solution.
As noted in an earlier blog (“Make Waveguide in Planar PCB Form”), SIW structures are essentially waveguide in planar form, with the capability to support millimeter-wave signals with relatively low loss even at those higher frequencies. SIW technology offers improved performance at millimeter-wave frequencies compared to traditional transmission-line technologies, such as microstrip, stripline, and even grounded coplanar-waveguide (GCPW) approaches, with limitations at millimeter-wave frequencies.
SIW has often been described as a form of transition between microstrip and dielectric filled waveguide (DFW). SIW can be fabricated with many of the same methods as microstrip. At millimeter-wave frequencies, however, microstrip circuits require small features and extremely tight machined tolerances to support the transmission of such high frequencies. In addition, at millimeter-wave frequencies, SIW circuits do not exhibit radiation losses suffered by microstrip. In fact, SIW circuits in general do not have the potential problems with electromagnetic interference (EMI) of the other transmission-line formats. SIW technology provides the means to realize extremely compact components; it is suitable for passive components, such as filters, but has also been used as active components, such as oscillators, at microwave through millimeter-wave frequencies. Commercial EM simulation software is most often used to aid in the design, simulation, and optimization of SIW circuitry, and such software programs can effectively model the effects of the dielectric substrates used as the foundations for SIW circuitry.
In forming SIW transmission lines, a rectangular waveguide is created within a substrate, usually on circuit-board material such as RO4350B™ LoPro® laminates from Rogers Corp. (www.rogerscorp.com) which has a low relative dielectric constant of 3.48 in the z-axis (through the thickness) measured at 10 GHz. This low-loss circuit material, which is widely used as the foundation for wireless base-station power amplifiers, features properties well suited to SIW circuits. It can be fabricated with the methods used for FR-4 circuit materials, to maintain low production costs.
SIW circuits and their dielectric-filled waveguide transmission lines are formed on a circuit material such as RO4350B LoPro laminate by adding a top metal plane over a laminate with a ground plane, then fabricating rows of conductive plated viaholes on both sides along the length of the substrate material. These plated-through-hole (PTH) viaholes are used to make the sidewalls of the rectangular waveguide structure formed on the PCB material. In forming the SIW embedded waveguide structure, more conductive metal is actually used than in stripline or microstrip transmission lines for similar wavelengths, resulting in less conduction loss at microwave and millimeter-wave frequencies.
What is critical in the fabrication of SIW circuits is the formation and spacing of the viaholes. Close spacing yields less conduction loss through the use of more conductive metal to form the waveguide structures, but also results in longer and more complex production times in fabricating the SIW PCBs. Wider spacing can save production time, but can also raise conduction losses and can result in higher EM leakage losses because of the wide spacing. SIW circuits will also suffer dielectric losses (as will all high-frequency circuit formats), which are dependent upon the choice of circuit laminate material. For SIW circuits, whether at microwave or millimeter-wave frequencies, and really with all high-frequency circuits, PCB materials should be chosen wisely for optimum balance between performance and cost.
One PCB material parameter that is critical for SIW reliability is coefficient of thermal expansion (CTE) which gauges the expansion of a circuit material with elevated temperatures. The SIW viaholes are plated through holes (PTHs) through the dielectric PCB material, and high values of CTE, which denote excessive expansion with temperature, will result in undue stress on the sidewalls of the PTHs. Circuit materials, such as RO4350B LoPro noted previously, and RO4835 LoPro material from Rogers Corp., with stable CTE characteristics, are ideal candidates for high-reliability SIW circuits. RO4835 LoPro circuit materials have been used for years in the fabrication of multilayer circuits with high layer counts, relying on PTHs for interconnection of those many layers. As an added benefit for creating cost-effective SIW circuits, both RO4835B LoPro and RO4835 LoPro materials can be fabricated with standard FR-4 epoxy/glass processes to help minimize production costs.
At millimeter-wave frequencies, SIW circuits exhibit low loss similar to their larger mechanical waveguide descendants, and considerably less than the other, more conventional transmission-line formats, such as microstrip, stripline, and GCPW. But SIW circuits also share other traits of larger mechanical waveguide, including a lower-frequency cutoff point. As with mechanical waveguide, SIW circuits are designed for particular operating frequencies and bandwidths depending upon the circuit dimensions, and designers must be aware that they will be working with a lower-frequency (and upper-frequency) cutoff point and a target low-loss passband. But SIW circuits can also work quite well with traditional transmission-line formats, with microstrip and GCPW transmission lines serving as fairly simple and effective feedlines from other parts of a circuit to the SIW circuitry.
With the expected growth in the demand for millimeter-wave circuits, especially with the expansion of IoT and higher-frequency automotive applications, SIW technology appears quite attractive as an effective and practical solution for designing and fabricating affordable millimeter-wave circuits. In particular, SIW technology provides the means to realize miniaturized planar antennas as millimeter-wave frequencies, perhaps in volumes reaching millions of components as needed for these many emerging wireless applications.
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