Join Microwave Journal and Rogers Corp. for a free one-hour webinar on March 26, 2015:

Measurement of PIM Distortion in Microstrip Transmission Lines: Effect of Laminate Properties and Measurement Repeatability 

Date: March 26, 2015, 8am PT / 11am ET / 3pm UTC

Presenter: Allen F. Horn III, Ph.D., Rogers Corp.

Screen shot 2015-03-17 at 5.13.04 PMUnderstanding the factors contributing to the generation of passive intermodulation (PIM) distortion in multi-frequency communication systems is a subject of interest to antenna designers. As telecommunications antennas are becoming more complicated, microstrip circuits made on copper clad laminates are replacing bent metal designs. PIM distortion is a circuit or system property, not a basic material property, and depends on the overall design, connectors, and local power densities, among many other factors. However, there are basic laminate properties that can contribute to PIM. In this seminar, we will discuss the measurement of PIM in microstrip transmission lines and the pertinent laminate properties, most notably conductor profile. We also discuss experiments on the effects of circuit processing, laminate thickness, and power level. Understanding small contributions to PIM is complicated by the very low power levels involved. We discuss the special attention that must be paid to the statistics of the measurement method.



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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Digital circuits continue to conquer higher speeds, with components such as microprocessors and signal converters routinely performing billions of operations per second. True, high-speed digital circuits can be flawed by such things as impedance discontinuities in transmission lines and poor plated-through-hole (PTH) interconnections between layers on multilayer circuit boards. But they can also be hurt by less-than-ideal choices of printed-circuit-board (PCB) materials for those high-speed-digital circuits. Which leads to the question: “What are the key parameters to consider when selecting a PCB material for a high-speed-digital circuit application?”

PrintAnalog circuit designers have learned to judge PCB materials by a number of important material parameters related to performance, such as dielectric constant (Dk) and dissipation factor (Df). These material parameters can also serve as yardsticks when comparing different circuit materials for high-speed digital circuit applications. In fact, it can be helpful to understand how high-speed digital signals are related to high-frequency analog signals when considering different PCB materials for those digital signals.

As digital applications have continued to gain in speed, some of the general-purpose PCB materials typically selected for fabricating those circuits, such as FR-4, fall short in performance for various reasons. In many ways, the demands placed on a circuit material by high-speed digital circuits and their signals are similar to what is needed from those PCB materials by analog microwave and millimeter-wave signals.

For example, a high-speed 10-Gb/s digital signal is a square-wave signal that can be viewed as a combination of different, but related, sine waves. A high-speed 10-Gb/s digital signal is comprised of different-frequency signal components, including a fundamental-frequency tone at 5 GHz, a third-harmonic signal at 15 GHz, a fifth-harmonic signal at 25 GHz, and a seventh-harmonic signal at 35 GHz (and, typically, harmonic signal components even higher than that).

Maintaining the signal integrity of a digital signal, and the sharpness of its rise and full times, is the equivalent of transferring millimeter-wave signals (the harmonics) with low loss and distortion. A PCB material capable of maintaining the signal integrity of high-speed digital signals at 10 Gb/s should also be capable of handling analog millimeter-wave signals through about 35 to 40 GHz with low loss and distortion. PCB material parameters that are critical to analog millimeter-wave circuit performance will also be important as guidelines for choosing PCB materials for high-speed digital circuits.

The PCB parameters that can be used for guidelines when choosing circuit materials for high-speed digital applications include Dk, dissipation, loss, and even dielectric thickness. The dielectric constant, Dk, of a PCB material has long been a guiding parameter for both analog and digital circuits since it is so closely related to the impedance of the circuits that will be fabricated on that material. Changes in a PCB material’s Dk, whether as a function of frequency, as a function of temperature, or for other reasons, can adversely affect the performance of broadband high-frequency analog circuits as well as high-speed digital circuits because it will change the impedances of transmission lines in unexpected ways. In particular, these unwanted changes in Dk and impedance result in distortion to the higher-order harmonics making up a high-speed digital signal, with loss of digital signal integrity. In general, PCB materials with low and stable Dk values with frequency and temperature will support high-speed digital circuits with low distortion of the higher-order harmonic signal components, as revealed by measurements with clean and clear eye diagrams for those high-speed digital circuits.

Dispersion is a PCB material characteristic closely related to Dk. All PCB materials exhibit some amount of dispersion, which refers to the change in Dk as a function of frequency. A circuit material with minimal change of Dk with frequency will exhibit minimal dispersion, a good characteristic for high-speed digital circuits. Dispersion can be caused by a number of different circuit material traits, including the polarity of the dielectric material, the loss of the material, and even how the surface roughness of the copper conductor affects the PCB material loss at higher frequencies. If a PCB material exhibits different Dk values for the different harmonic signal components comprising a high-speed digital signal, it will cause losses and even shifts in frequency for those harmonics, resulting in degradation of the high-speed digital signals.

PCB signal losses at increasing frequencies, especially at the higher frequencies needed by a high-speed digital circuit’s higher-order harmonic signal components, can suffer excessive losses to the amplitudes of those higher harmonic signals, resulting in distortion to those high-speed digital signals. As noted in many earlier blogs, losses in a PCB can come from a number of different causes, including the dielectric material and the copper conductors.

The length of a high-speed digital circuit on a PCB material can also have a great deal to do with maintaining the integrity of those high-speed digital signals. Circuit losses for any PCB material are a function of frequency and will increase with increasing frequencies. A PCB material with acceptable losses within a bandwidth closer to the fundamental-frequency tone of a high-speed digital circuit, such as 5 GHz as in the earlier example, and perhaps even with low loss at the third-harmonic signal component, such as 15 GHz, may have excessive loss at the fifth- and seventh-harmonic signal components of that high-speed digital signal. In addition, signal losses are additive with length: a signal experiencing a loss of, for example, 0.5 dB per inch at 5 GHz for the first inch of a 10-inch-long high-speed digital circuit, will suffer loss of 5 dB at 5 GHz across the length of the circuit.

Depending upon the circuit’s dielectric losses and copper conductor losses, the total loss across the length of the circuit can be considerably higher for the high-speed digital signal’s higher-order harmonic signal components than for the lower-order harmonic tones. For some circuit materials, the loss for a 10-in.-long circuit may be 10 dB or more at the fifth- and seventh-harmonic signal components of a high-speed digital signal, resulting in considerable distortion to the high-speed digital signal transferred across that PCB material.

As noted, changes in a PCB’s transmission-line impedance from changes in Dk can cause distortion in high-speed digital signals. But when working with PCBs for high-speed digital circuits, attention should be paid to physical details as well. Such things as right-angle bends in transmission lines can affect performance. A right-angle bend represents a change in the effective width of the transmission line, resulting in an impedance discontinuity, and an increase in the capacitance at that portion of the transmission line. The use of mitered 45-deg. bends can minimize the impedance discontinuity and minimize the reflections of the signal passing through that junction.

The choice of PCB material for high-speed digital circuits can be guided by the speed of those digital circuits, with such material characteristics as loss and dissipation factor (Df) targeted for lower values at higher frequencies. Circuit materials with medium to low loss are suitable for digital circuits to 10 Gb/s, while lower-loss circuit materials are usable for digital circuits to about 25 Gb/s, and circuit materials considered to exhibit extremely low loss are well suited for the fastest digital circuits, such as operating at 50 Gb/s and faster. In terms of circuit material Df, typical values might be 0.010 to 0.005 for applications to about 10 Gb/s, 0.005 to 0.003 for applications to about 25 Gb/s, and 0.0015 or less for circuit applications to 50 Gb/s and faster.

Screen shot 2014-08-08 at 1.33.54 PMAs an example, RO4003™PCB material from Rogers Corp. is a ceramic filled hydrocarbon laminate with woven glass reinforcement and a Dk of 3.38 at 10 GHz through the thickness (z axis) of the material. It offers impressive Dk consistency over frequency, and is rated for Dk variations of only ±0.05. The Df is only 0.0027 through the z axis at 10 GHz. With its low and consistent Dk value, the material has been developed for broadband analog applications through millimeter-wave frequencies and low-distortion, high-speed digital applications through 25 Gb/s. In support of those digital applications, the material features extremely tight dielectric thickness tolerance and is compatible with multilayer PCB applications.

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Our Advanced Circuit Materials group is pleased to announce the availability of ULTRALAM® 3850/3850HT PCB laminates. These adhesiveless circuit materials use temperature resistant liquid crystalline polymer (LCP) as the dielectric film. They are designed for single layer and multilayer substrate constructions, and are well suited for high speed and high frequency applications, including:

  • Telecommunication network equipment, such as high speed routers
  • High-speed computer data links

The ULTRALAM 3850/3850HT laminates feature a high temperature LCP material to facilitate more robust, high temperature, multi-layer designs. The higher melt temperature provides improved dimensional stability.

Find out more about the ULTRALAM® 3850/3850HT PCB laminates. Samples available.



“Survival of the fittest” describes a form of natural selection that occurs in the face of some real world challenge. For product designers, the challenge is to create new products that have clear market demand and can be affordably produced. That can be easier said than done. So the question is, can we better prepare students so they hit the ground running after they graduate and join a company?

NU_survival_class“We asked ourselves, how can we involve students in the conceptual stage of product development,” said Shawn Williams, VP R&D at Rogers Corp. “Traditional collaborations between corporations and educational institutions use sponsored research and experiential, co-op opportunities for students. That’s good, but we need to take the next step. We need to find a way to facilitate and reward creative product design within the academic environment and connect it to real, commercially viable products.”

A logical starting point was the Rogers Corporation Innovation Center, located in Northeastern University’s George J. Kostas Research Institute for Homeland Security. Launched in 2014, the Innovation Center is a unique academic-industry partnership that builds closer connections between academic research, industry know-how, and technology commercialization. The Center’s goal is to develop commercially-viable breakthrough innovations in advanced materials to address global challenges for clean energy, internet connectivity, safety and security

A conversation between Williams and Dr. Sara Wadia-Fascetti, Associate Dean for Research and Graduate Studies, led to the creation of a new course at Northeastern – Survival of the Fittest – focused on product development and commercialization. The course brings together interdisciplinary teams of engineering and business students supported by Northeastern faculty and Rogers’ scientists, engineers, designers, and product managers. For the first class this spring, 83 students applied for 16 positions.

The Challenge

The student teams are challenged to select a technical/commercial opportunity and develop it into a workable business plan. Rogers provided a list of product ideas that are of commercial interest to the company, from high level concepts to application specific products. Employees brainstormed across divisions and came up with 50 different ideas for the students, including:

•            Consumer robotics

•            Next generation materials for integrated circuit packaging

•            New materials for home and automotive applications

•            Large format sensors

Student teams choose an idea from the list and then learn about Rogers, it’s technical expertise in high performance materials, and its diverse range of customers and their applications. The teams then define, refine, and validate opportunities through market and technical analyses, modeling, and prototyping.

The Reckoning

At defined points in the course, “product reckonings” occur where teams pitch their ideas, advocating for continued development of their ideas. A panel of industry and academic experts determine whether the idea should receive additional resources for further development (“persist” according to Eric Ries’ lean startup model), whether it should be modified for a better “fit” between the product and market (“pivot”), or whether it should be discontinued (“perish”). Team members from discontinued opportunities join the teams working on surviving opportunities.

The Fittest

At the end of the course, the “fittest opportunity” moves over to Rogers for potential commercialization. Along the way, students learn about project management, team dynamics, building successful teams, intellectual property, cost and financial modeling, technology commercialization, and product development in high technology.

“Students are used to mastering material, but this course teaches them to also rely on teammates for crucial pieces of the business plan,” said Williams. “Each team is a mix of majors and backgrounds. An electrical engineering major on the team turns to the finance major for calculation of Net Present Value, for instance.”

Williams continued, “The beauty is that we know this process works because we use it every day within Rogers. Now we’re developing a way for it to work in the classroom, to better prepare the product designers and business managers of tomorrow.”