Controlling Conductor Losses In Coplanar Transmission Lines
Mar 14, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm
Microwave circuit designers often have to choose, not only among different layouts and substrate materials, but among transmission-line technologies. Stripline has its advantages for certain components and circuits, and microstrip is popular for both active and passive microwave circuits. But when does a coplanar transmission-line technology make sense?
How do coplanar transmission-line technologies compare with other high-frequency printed-circuit-board (PCB) transmission-line approaches, such as stripline and microstrip? Stripline is often described as a “flattened coaxial cable,” with conductors on the outside of the dielectric material and the dielectric surrounding an internal conductor. Microstrip is even simpler, with a signal conductor on the top of a dielectric substrate and a ground plane on the bottom of the dielectric substrate. It is the most popular microwave transmission technology due to its simplicity and ease of fabrication.
In both stripline and microstrip, the choice of dielectric layer thickness directly impacts how the transmission lines are structured. In both cases, the impedance is determined by the separation distance between the ground layer and the signal conductor, with the two being isolated by the dielectric layer. This can be inconvenient, however, when the dimensions of a circuit are not compatible with the dimensions of a coaxial connector or device pin, where the conductor’s trace width may be too wide to fit between device pins. Some designers will build a taper into a conductor’s trace width in order to make the mechanical connection from conductor to pin or connector, but it is extremely difficult to maintain constant impedance with this approach.
Coplanar transmission-line technologies provide a means of moving signals from a PCB to a connector or device pin without unwanted variations in impedance that can cause signal reflections at higher frequencies. As the name suggests, a coplanar transmission line features a ground conductor that is coplanar with the signal conductor. The impedance is controlled by the line width of the signal conductor and the ground gap. As a result, the impedance can be maintained at some constant value, such as 50 Ω, even as the signal conductor’s width is tapered to a smaller size to meet a pin, and the impedance is maintained without changing the thickness of the dielectric substrate. Coplanar transmission lines come in a variety of configurations, including traditional coplanar waveguide (CPW) and coplanar waveguide with ground (CPWG). Because they can readily make signal transitions from wider transmission lines to relatively narrow terminations without altering the thickness of the dielectric material, they are widely used in CPW test probes for on-wafer testing of discrete and integrated circuits.
(Cross-sectional view of a grounded coplanar transmission line with electric field lines shown)
In terms of modeling, a CPW transmission line is often treated as a metal conductive strip separated by two narrow slots from a ground plane at some distance. The metal strip has a width dimension (W) and the slots have a width dimension (s). The CPW transmission line is symmetrical along a vertical plane. The conductor is separated by the ground plane by the dielectric material. If the dielectric is considered to have infinite thickness relative to the much smaller thickness of the conductor, the CPW structure can be modeled like a parallel plate capacitor that is filled with dielectric material.
Of course, a CPW conductor does have some finite thickness and CPW-based circuits will suffer some losses due to the conductors and the dielectric material. Most of the larger high-frequency design suites, such as the Advanced Design System (ADS) from Agilent Technologies (www.agilent.com) and Microwave Office from AWR (www.awrcorp.com) include CPW transmission-line models. Even some specialized analysis programs, such as Simulink from The MathWorks (www.mathworks.com), and electromagnetic (EM) simulators such as the Sonnet Suites from Sonnet Software (www.sonnetsoftware.com), are effective tools for modeling circuits based on CPW.
CPW-based circuits may appear to offer benefits over other transmission-line technologies, especially when tapered conductors are needed for transitions. CPW supports a large range of possible impedance values, making it possible to fabricate many different circuit functions, such as filters, couplers, and attenuators, with a single PCB laminate material. Microstrip approaches, in contrast, may require the use of hybrid PCB structures formed of laminates with different dielectric constants, to achieve the same range of impedances as CPW.
Laminate materials for CPW-based circuits should be characterized by tightly controlled material thickness and dielectric constant across a board, to ensure consistency of impedance in fabricated CPW circuits. For example, Rogers RO4000® Series high-frequency circuit materials, such as RO4003C™ and RO4350B™ laminates, provide the stable mechanical and electrical characteristics that make them ideal for CPW-based circuits. The former features a z-axis dielectric constant of 3.38 at 10 GHz, while the latter has a z-axis dielectric constant of 3.48 at 10 GHz, both controlled to a tolerance of ±0.05 across the board.
Both materials are hydrocarbon ceramic laminates that can be processed with the low-cost fabrication techniques used for FR-4 materials, except these are substrates engineered for higher frequencies. In addition to supporting extremely stable impedance by their controlled dielectric constant, they also feature a coefficient of thermal expansion (CTE) that is tightly matched to that of the copper conductor metal, so that both conductor and dielectric expand and contract together with changes in temperature. This results in excellent mechanical stability and high reliability when plated through holes (PTHs) are needed to connect different layers in a multilayer assembly.
Multilayer circuits are becoming more commonplace at higher frequencies, and these circuit constructions often consist of multiple transmission-line technologies, such as CPW and microstrip. As important as maintaining constant dielectric constant for controlled impedance lines, the tight tolerance in the laminate’s dielectric constant is critical for fabricating controlled-impedance transitions between microstrip and CPW, or CPW and stripline circuits. Both RO4003C and RO4350B laminates provide the levels of electrical and mechanical performance that makes them well suited for use in CPW designs.
As an aid to designers using these and other laminate materials for CPW circuits, Rogers offers its MWI-2010 Microwave Impedance Calculator software tool, which can be downloaded free of charge from the website at www.rogerscorp.com/acm. The software computes transmission-line parameters based on the use of Rogers’ high-frequency circuit materials, and can calculate parameters for microstrip, stripline, and CPW transmission lines. In addition to discussing the best use of their materials for designers working with high-frequency CPW circuits, representatives from Rogers Corp. will be on hand this month at Satellite 2011, a leading trade show and conference for designers of commercial and military satellite communications (satcom) equipment and systems. The event is scheduled for the Walter E. Washington Convention Center (Washington, DC), March 15-17, 2011. For more information, visit the link at: www.satellitetoday.com/satellite2011.