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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.

Learn To Apply Design Dk

Dielectric constant (Dk) is one of the most essential of printed-circuit-board (PCB) material parameters. Circuit designers rely on it for determining such things as impedances and the physical dimensions of microstrip circuits. Yet, it is not unusual to see a laminate data sheet with different values of Dk for the same material, such as a process Dk and a specification Dk. A material supplier may even recommend an additional value of Dk, to be used in computer-aided-engineering (CAE) software simulators. Why all the different numbers and is there one value of Dk that is the one to trust when designing a circuit?
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Test Dielectric Constant With Microstrip Circuits

Circuit designers select laminates for printed-circuit boards (PCBs) by merit of relative dielectric constant (Dk), among other parameters. Suppliers of laminates furnish Dk values on their data sheets and web sites, but designers often prefer the reassurance of knowing the Dk value as it relates to their specific application. The last blog explored the way that materials manufacturers typically use four techniques to evaluate the Dk of a dielectric material in its “raw” form, meaning without circuits. This blog will explore some common methods that materials users employ for determining a laminate’s Dk value and focus on a practical method.
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Detecting The Dk Of “Raw” Circuit Boards

RF/microwave designers have a wealth of circuit-board materials from which to choose, which can be good and bad. Having so many options can make the selection process difficult, so that many designers start with relative dielectric constant--Dk for short--as a key sorting parameter. As was pointed out in the last blog, the value of Dk can depend as much on the material composition as the type of test used to measure it. Tests for Dk can be performed either on “raw” laminate material, without circuits formed on it, or by making use of test circuits that have been fabricated on the laminate and measuring the electrical responses of those circuits. This blog will address the four most widely used Dk tests in the first group; the next blog will examine four popular Dk tests in the second group.
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Measuring Performance Of Microwave Substrates

Circuit-board material parameters are printed on every laminate data sheet. They describe the electrical and mechanical characteristics of a PCB material, including such parameters as relative dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), and thermal conductivity. Design engineers count on these values to be accurate, since their circuits depend on them. But the accuracy often depends on the test method used to measure a material parameter. Even when different laboratories perform the same test on the same material, they can obtain different results. This blog will provide a brief overview of the different tests used to evaluate a printed-circuit material’s characteristics; the next several blogs will go into greater details on specific tests, and will explain how various test results impact the way PCB materials are modeled with modern computer-aided-engineering (CAE) software tools.
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Comparing RF Circuit Material Processing Costs & Performance

Performance requirements typically guide the selection of a PCB material. Some applications may also be cost-sensitive, and require evaluation of the total costs of choosing a circuit material. This includes the cost of the material as well as costs associated with processing the material. For example, FR-4 is a low-cost material with minimal processing costs. However, its performance is also low relative to some higher-costing materials, such as PTFE- or hydrocarbon-based circuit materials, although these materials can have considerably different processing requirements and associated costs. By considering the costs of the material as well as its processing requirements, it’s possible to determine if “you get what you pay for” truly applies to circuit materials.
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When Digital Signals Reach Microwave Frequencies

Digital circuit design once had less demands. When clock speeds were 100 MHz or less, signal loss wasn’t an issue. Digital circuits, in fact, have long been designed to be more tolerant of signal level variations than analog circuits. But with digital circuits continuing to increase in speed, they are assuming more of the characteristics of analog microwave signals, and requiring more attention to design detail and even choice of PCB material as in the case of high-frequency analog circuits.
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Transmission-Line Modeling Tool: Free Downloadable Software

The old expression, “you get what you pay for,” usually holds true. Except in the case of a handy little design program called the MWI-2010 Microwave Impedance Calculator, available for free download from the Rogers Corporation website. Visitors to the DesignCon® 2011 exhibition (Santa Clara Convention Center, Santa Clara, CA, February 1-2, 2011) can learn more about this powerful transmission-line modeling tool by visiting Rogers at Booth 711.
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Substrate Anisotropy Affects Filter Designs

Isn’t designing a microwave filter as simple as loading parameters into a computer-aided-engineering (CAE) program? In truth, many modern CAE software tools are quite good, and can provide accurate predictions of performance when fed sufficient input data. However, most do not account for all variables influencing a high frequency filter, including the effects of anisotropic printed-circuit-board (PCB) materials. When designing RF and microwave filters, it helps to choose your PCB material wisely.
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Microstrip Versus Stripline: How To Make The Choice

Microstrip or stripline? That choice has been faced by high frequency designers for decades. Both transmission-line technologies are widely used in both active and passive microwave circuits, with excellent results. Is one approach better than the other? Before tackling such a question, it might help to know how each transmission-line technology works and what kind of demands each place on a printed circuit board (PCB) material.
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