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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Making Microstrip Coupled Features Work

July 21, 2014
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Microstrip transmission lines are widely used throughout the high-frequency industry, for both active and passive circuits. They are building blocks for many components, including couplers, filters, resonators, and power dividers/combiners, along with various coupled features formed from microstrip lines that help transfer energy from one point in the circuit to another. Of course, the printed-circuit-board (PCB) material also plays a major role in how these microstrip transmission lines perform their duties in these RF and microwave circuits, and it can be helpful to understand how certain PCB material characteristics contribute to the ways that microstrip transmission lines and their coupled features perform in these different high-frequency components.

Circuit-board materials are selected by designers for a number of reasons, but usually with dielectric constant (Dk) at the top of the list. Maintaining consistent impedance for microstrip lines depends on consistent Dk for a PCB material since a change in PCB Dk at any point in the material will result in a change of impedance for the microstrip transmission lines at that point in the material. Using microstrip coupled features can complicate the choice of circuit materials since such coupled features typically exhibit different, even- and odd-order, wave modes as a function of the PCB material and circuit design. For electric fields between microstrip coupled features, the even-order modes use mainly the thickness or z-axis of the material, while the odd-order modes of the electric fields are mostly in the planar or length-width dimensions (x and y axes) of the PCB material as well as using some z-axis properties.

Ideally, PCB materials would exhibit tightly consistent Dk values in their x, y, and z dimensions, and modern computer-aided-engineering (CAE) software tools typically assume that they do. But in the real world, circuit materials more typically have differences between the Dk value through the thickness (z axis) of the material and the Dk value across the length and width (x and y axes) of the material. PCB materials are referred to as anisotropic in nature when they have different Dk values in the different axes of the material. In contrast, a PCB material with consistent Dk values in all axes is considered isotropic in nature.

Why the differences in Dk values through the material, and what effects can they have on different circuit designs? Most commercial PCB materials are at least slightly anisotropic in nature, due to the composition of those materials. They are formed with dielectric resin materials and some filler material, such as a glass or a ceramic filler, used for reinforcement and attribute adjustments, but which contribute to Dk variations. The manner in which fillers can orient within a substrate during the laminate manufacturing process accounts for the isotropic or anisotropic behavior for some laminates. Other laminates may have glass weave for reinforcement, which can cause Dk variations; the type of glass weave can impact the anisotropic behavior of the laminate. Combining the effects of these potential filler orientation variations with the effects of the glass weave can cause some laminates to exhibit higher variations in Dk in the different axes of the material, making them more anisotropic.

Designers working on microstrip circuits with coupled features often lean towards the use of PCB materials with higher Dk values, since those materials provide more efficient coupling of electric fields than their lower-Dk counterparts. In addition, since circuit dimensions required for a given impedance shrink on PCB materials with higher Dk values, smaller components can be developed with these higher-Dk materials. Unfortunately, the higher-Dk circuit materials also tend to be more anisotropic in nature than lower-Dk circuit materials, adding to the challenge of designing filters, directional couplers, resonators, and other high-frequency circuits based on microstrip transmission lines with coupled features.

PCB materials with high Dk values, typically 10 or more as measured in the z-axis of the circuit material at 10 GHz, can suffer from serious anisotropic characteristics that can challenge even the best of CAE simulation and design software programs. Circuit materials with a high degree of anisotropy may have, for example, a Dk of 10 through the thickness (z-axis) of the material as measured at room temperature and 10 GHz, but the Dk in the x-y plane of the same material may be different by 10% or 15%. When designing a microstrip circuit with coupled features, such as a filter, and using a CAE program, these variations in Dk values can be accounted for as a form of statistical approximation, but specific differences in Dk values, as might occur at a critical microstrip coupled feature, may not be precisely predicted in the CAE program. The variations in Dk typically result in performance variations in the final circuit, which yield differences between performance parameters predicted by a CAE program and performance parameters measured for a prototype with test-and-measurement equipment.

For designers of microstrip circuits with coupled features, variations in PCB material Dk can be detrimental to achieving expected performance results, fortunately commercial PCB materials with high Dk values are available with relatively isotropic natures. For example, the TMM® 10i circuit materials from Rogers Corp. (www.rogerscorp.com) are quite isotropic compared to other circuit materials with Dk value around 10.0. These are ceramic hydrocarbon thermoset polymer composite materials well suited for both microstrip and stripline high-frequency circuits. The TMM 10i circuit materials exhibit a Dk of 9.80 which remains within +/- 0.245 of 9.80 in all three axes of the material. (The Dk measurements are performed at 10 GHz in the z-axis of the material according to IPC-TM-650, method 2.5.5.5.) For designers in need of a PCB material with even higher Dk value, the TMM 13i circuit material offers a Dk of 12.85 +/- 0.35 as measured at 10 GHz in the z axis using the same test method.

These circuit materials are more isotropic than most, with differences of typically 3% or less between the Dk value in the z-axis of the material and the x-y plane of the material. Compare this to the circuit materials noted earlier with differences of 10% or more. In addition to their isotropic natures, the TMM 10i and TMM 13i materials feature coefficients of thermal expansion closely matched to that of copper, supporting production of such circuit features as plated through holes (PTHs) with high reliability.

Of course, CAE design and simulation software continues to advance, and get better at anticipating such variations as found in anisotropic PCB materials. But for some designs, such as those with microstrip transmission lines and coupled features, even small variations in Dk can be disruptive. For designers working with microstrip coupled features and hoping to avoid surprises, the right choice of PCB material can help.

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.

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