This post authored by John Coonrod originally appeared on the Rog-Blog hosted by Microwave Journal

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.

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2 Responses to Controlling Conductor Losses In Coplanar Transmission Lines

  1. thanks bro. Really helpful.. Really thankful for people like you who are making this kind of information be readily available on net. I am really looking for information like this. I know this is a good one. I like how you introduce it as ” Power Factor is a measure of how efficiently electrical power is consumed”. Seems to me that it is full of knowledge,… Thank you again!!!

  2. Carlos Donado says:

    Good read, thanks for sharing. I was wondering though, besides controlling return loss by controlling the pin to signal interconnect, what other techniques are used to control losses in CPW lines?

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