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

Spurious modes can occur in printed circuit boards (PCBs) in spite of the best-laid plans. These modes support extra, unwanted signals, in addition to the intended signals, that can wreak havoc on a PCB and its application, causing interference and degradation of the intended signals. Although minimizing spurious modes in PCBs is largely a result of careful design practices, the choice of PCB material can have some bearing on the final spurious mode behavior, especially at higher frequencies. Understanding how these spurious modes originate can help in keeping them under control, especially on PCBs operating at millimeter-wave frequencies.

PrintAt RF, microwave, and millimeter-wave frequencies, numerous transmission-line technologies are fabricated on PCB materials, stripline and microstrip are two popular transmission-line methods at higher frequencies. The transmission-line structures propagate electromagnetic (EM) waves in different ways, with stripline supporting transverse-electromagnetic (TEM) wave propagation while microstrip supports quasi-TEM propagation. Quite simply, the mechanical structures of these transmission lines are different, with stripline employing a metallic conductor surrounded by dielectric material while microstrip fabricated the conductor on the top of a dielectric layer with a ground plane on the bottom of the dielectric layer. Coaxial cables, where the conductor is also surrounded by dielectric material, also operate in a TEM propagation mode like stripline.

Spurious waves can be surface waves that propagate through a high-frequency PCB or they can be produced by resonant effects within circuits fabricated on a PCB. Microstrip transmission lines offer very little design freedom for minimizing spurious mode propagation. In terms of physical changes to the PCB, using a thinner microstrip PCB material can diminish the amount of spurious mode propagation in a high-frequency circuit, and this is one of the reasons that thinner circuit materials are used at higher-frequencies.

Of course, many of the PCBs designed with microstrip transmission lines must also make a transition to coaxial cables at a launch point, and this represents a transition from the TEM mode of the cable to the quasi-TEM mode of the microstrip transmission lines. But simply because a PCB has been fabricated with microstrip transmission lines and circuitry does not mean that other modes cannot propagate on that PCB; spurious signals represent one of these other propagation modes. These unwanted spurious or “parasitic-mode” signals can interfere with the desired quasi-TEM-mode signals of the microstrip transmission lines and circuitry.

The quality of the signal launch to a microstrip PCB can affect the amount of spurious mode suppression. For example, EM waves propagating from a coaxial connector to a microstrip PCB will not only make a transition from the TEM mode of the connector to the quasi-TEM mode of the microstrip, but the EM waves from the connector to the microstrip will also make a transition from the polar orientation of the cable and connector to the planar orientation of the microstrip. Even the most ideal coaxial-connector-to-microstrip PCB can suffer stray electrical reactances as a result of the transition of the propagating EM waves across an interface that will have some mechanical variations. Even minor impedance mismatches at the connector-microstrip transition can result in signal reflections and radiation at the transition. In addition, variations between the signal path and the ground return path in the transition area can lead to EM wave skew and additional “interruptions” in the intended propagation path and additional sources for spurious mode propagation.

A grounded coplanar-waveguide (GCPW) launch, which is also known as conductor-backed coplanar waveguide (CBCPW), is capable of a fairly smooth transition to a microstrip transmission line, with minimal spurious signal generation. When even more spurious mode suppression is required, for example at millimeter-wave frequencies, GCPW or CBCPW transmission lines can be used on the PCB in place of microstrip transmission lines. This provides more design freedom to minimize spurious mode generation, with a tradeoff being in added design complexity.

GCPW circuits are often used at millimeter-wave frequencies rather than microstrip transmission lines for better suppression of spurious modes at those higher frequencies. The physical configuration of these circuits helps suppress the resonances that can lead to spurious signals. In addition, the use of grounding viaholes in GCPW circuits can help suppress the propagation of resonance modes between the signal and ground planes. The pitch of these viaholes is important, and related to the wavelength of the operating frequency. The pitch of the viaholes should be 1/8 wavelength or less of the highest intended operating frequency for the circuit.

For a PCB, particularly based on microstrip transmission lines and at higher frequencies, resonances in a circuit and its transmission lines can lead to unwanted spurious signals. Resonances can develop between the transmission line’s signal conductor and the PCB ground plane, with resonances occurring between opposite edges of the signal conductor and paving the way for spurious signal propagation. Such resonances can generate their own EM waves in a circuit or transmission line, especially in microstrip circuits at higher frequencies.

The resonances occur according to the dimensions of the transmission-line conductor and the wavelength of the frequency of interest for the circuit. For example, if the physical width of a microstrip conductor is equal to ½ or ¼ the wavelength of the circuit’s operating frequency, resonances will occur. These resonances can lead to EM waves that can interfere with the intended quasi-TEM waves that are meant to propagate through a microstrip circuit. As with the pitch of the grounding viaholes in the GCPW circuits, a design goal that can help avoid the generation of circuit-based resonances (and their accompanying spurious modes) in microstrip circuits is to make certain that no transmission line or circuit features are greater than 1/8 wavelength of the intended operating frequency.

What does the choice of PCB material or PCB material characteristics have to do with spurious mode rejection? The quest for increased spurious mode rejection typically becomes more difficult at higher frequencies, notably at millimeter-wave frequencies, and is not highly dependent on the choice of PCB material, although the dielectric constant (Dk) of a circuit material is one parameter that can have an impact on spurious mode rejection. When a circuit material with higher Dk value is selected, it results in shorter wavelengths for a given operating frequency, which in turn can affect the target size of the microstrip transmission lines when trying to ensure that these transmission lines and circuit features are no greater than 1/8 wavelength of the intended operating frequency.

Screen shot 2014-08-08 at 1.33.54 PMAlthough the thickness of a PCB material can be a concern at higher frequencies, such as millimeter-wave frequencies, the particular conductor width (as noted earlier) is more of a concern at these higher frequencies (with their smaller wavelengths). Still, thinner circuit laminates can help minimize spurious modes at millimeter-wave frequencies, and thinner laminates are also beneficial for reducing radiation losses in higher-frequency circuits. A tradeoff in selecting thinner PCB materials is that they tend to have higher losses than thicker circuit materials. Fortunately, advances in modern circuit materials, such as the lower insertion loss exhibited by RO4000® LoPro™ laminates from Rogers Corp., make it possible to achieve good spurious mode suppression at higher frequencies without necessarily compromising circuit loss performance.

Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

Make your reservations now for the 2014 International Printed Circuit & APEX South China Fair at the Shenzhen Convention & Exhibition Center, December 3-5, 2014. Stop by and see us in booth #2F31.

We’ll also be presenting as part of the technical program. Sharon Young, Market Development Manger – Asia, will present on December 3, 2014, 13:30-14:30pm, Hall 2:

“How to Choose PCB Material When Facing High Power and Temperature. How Lamination Selection Contributes to Thermal Management of Microwave Circuit Performance.”

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Formula Group T is a project run by Belgian masters students in engineering from GROUP T. They design and build small electric vehicles and compete with other universities worldwide in the Formula Student competitions during the summer.


Design 1: Areion

Areion was their first car and was built in 2011-2012. One of the key technologies Formula Group T implemented in Areion was additive manufacturing techniques, which can be found in the production of the uprights of the suspension system. Stereolithography techniques were used in the production and development of the body enclosure.

Design 2: Eve

In 2013, Formula Group T finished their second car, Eve. Like her predecessor, Eve is a pioneer, a platform for companies to demonstrate existing core technologies or to test new technologies. For instance, Eve’s electrical drivetrain assembly starts with a 360 Lithium-ion cell accumulator at a voltage of 444V. The accumulator has a carbon fibre fire resistant enclosure and is divided into 10 modules. The accumulator empowers the self assembled motor drive. This lightweight drive controls our 2 motors and weighs less than 15 kg

Eve_busbarsWith the help of Rogers Corporation, Formula Group T was able to make the battery pack by connecting the cells and cell stacks through the use of busbars. Also, the circuit to switch the motor between Δ and Y was made with RO-LINX® busbar technology. Finally, to make fireproof barriers between cell stacks and to damp for vibrations, BISCO® silicone was used.

In addition, Group T used power modules from one of the Rogers PES Division’s key customers, which contain curamik® substrates and BISCO® silicones.

A lot of Rogers in a small car! PES presented Eve at the 2013 PCIM Europe and caught the attention of many attendees.

Eve: Overall specifications

  • 0-100kph: 3.2 s
  • Topspeed: 140km/h
  • Weight: 260 kg
  • Length: 2400 mm
  • Wheelbase: 1550 mm
  • Track width: 1220 mm (front) 1170 mm(rear)

June1Eve: Energy

  • Accumulator: Lithium ion
  • Energy: 5.3 kWh
  • Power: 85 kW
  • Transmission: Gearbox
  • Motorcontroller: Self made

Eve: Mechanics

  • Chassis: Steel tubular space frame
  • Suspension: Carbon double-A arm with titanium uprights
  • Transmission: Internal gear
  • Motor: 2x100kW peak axial flux motor
  • Seat: Fully integrated in firewall
  • Body: Full composite body shell

Design 3: June

Formula Group T has finished and raced their 3rd car, June. June features a monocoque design, a sandwich structure made out of carbon fibers and a foam core. The goal of a monocoque is to transmit the forces through the skins, this gives a better force distribution and provides a lighter chassis.

Best of luck in with the 4th generation car, which will launch shortly!




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See our Metatarsal Guards in action when this work boot faces the realities of work…

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A message from Bruce Hoechner, CEO, Rogers Corporation:

Read the corporate financials news release: Rogers Corporation Reports 2014 Third Quarter Results.

Rogers had another terrific quarter: sales, margins and earnings all exceeded guidance. In Q3, we achieved an all-time quarterly sales record of $163.1M, up 14.2% over the prior year, out-performing our target of 10% sales growth. In addition, our commitment to improving our operational efficiency led to an all-time quarterly record in gross margin of 39.6%, an improvement of 380 basis points.

Screen shot 2013-11-01 at 10.53.55 AMWe also achieved a record operating margin of 17.4%, a 410 basis points improvement compared to the prior year measure and a 300 basis point improvement over the prior year on a non-GAAP basis. This operating margin performance surpassed our goal of 15%.

Overall, we had an outstanding Q3, delivering Earnings per Diluted Share of $1.09, the highest in the company’s 182-year history on a comparable non-GAAP basis. The third quarter  of 2014 also marks our seventh consecutive quarter of year-over-year sales growth.

5-Year Strategy

Before we turn to our Business Segment highlights, I want step back to review our company’s progress to this point. In 2012, we began what we called our transformational journey. Our goal: to enable Rogers to deliver consistently strong revenue and profit growth. 2013 was a watershed year.

We established our five-year strategy – defining our long-term goals of 10%-plus organic sales growth, operating margin of greater than 15%, improving our operational proficiency and actively pursuing synergistic M&A opportunities. Also in 2013, we embarked on specific cost-reduction programs and operational excellence efforts, as well as value in use pricing strategies.

In the second half of 2013, our streamlining,  operational excellence efforts and market-driven focus began to pay off. We began 2014 building on that momentum, delivering improvements across many key performance metrics. I believe our focus on operational excellence, our participation in select, higher growth global markets and our innovative market-driven solutions, will drive continued strong performance into the future.

Q3 2014

For this quarter, all three business segments contributed to our growth, led by Printed Circuit Materials (or PCM). Screen shot 2014-10-29 at 2.11.21 PM

Our PCM business segment provides our customers with innovative, unique Printed Circuit Board laminate materials with exceptional electrical performance and tailored properties. This was another record quarter for PCM with a 34.5% increase in sales over the year prior. PCM is having an incredible year.

This quarter, our business results were driven  by significant demand for 4G/LTE wireless infrastructure power amps (up 39%), wireless antenna (up 75%), as well as automotive safety sensor applications (up 34%) and applications in portable electronics for improved internet connectivity (up 17%).  As we look forward, we see strong growth drivers for the 4G/LTE market area. The recent Chinese government approval to expand the 4G/LTE mixed domain trial to 24 new cities in addition to the original 16 cities, heightens expectations for the 4G/LTE base station build out in 2015 by China Unicom and China Telecom. In addition, China Mobile will continue to add to their 4G/LTE network. We are starting to see 4G/LTE demand growing in markets beyond China, such as India and Europe.

In automotive safety systems, we are at the early stages of this growth market with only about 5% worldwide of new car production equipped with this capability. Clearly the strong consumer demand for this feature will accelerate broader adoption of this technology.

In the Power Electronic Solutions (or PES) segment, we offer high efficiency power module substrates and highly engineered busbars to meet the ongoing trend of greater electrification and automation. Our exceptional engineering design skills are used extensively by our customers to to support their complex design needs. The third quarter sales increase for PES was led by strong demand in EV/HEV vehicles (up 10.8%), rail propulsion (up 32.1%), energy-efficient motor control applications (up 10.3%) and vehicle electrification (x-by-wire) applications (up 10.2%). These increases more than offset lower demand in laser diode (down 33.4%) and certain renewable energy applications (down 13.0%).  As we look forward, we see continued opportunities for unique solutions in clean energy, specifically in the HEV/EV, motor control and automotive electrification applications as well as rail.

Our High Performance Foams (or HPF) segment provides specially engineered impact cushioning and sealing materials for high-reliability applications where unique performance characteristics are required. In Q3, HPF sales increased 5.1% primarily due to higher demand in general industrial (up 12%), battery applications for hybrid and all electric vehicles (up 40%), and consumer comfort and impact protection applications (up 22%).  Growth in these areas offset lower demand in portable electronics applications (down 8%).  We see additional opportunities for growth in our HPF product lines through geographic expansion in Asia and Europe as well as in the rapidly growing consumer application areas.

The diversity of Rogers’ three core business segments helps to moderate the impact of unfavorable business performance in any one market sector. We saw this play out over the past 18 months, as sales in HPF were negatively impacted by design modifications in certain portable electronics applications. As a company, we were able to counter-balance this sales decline by sales gains made by PCM as well as through our strong sales growth in other HPF segments such as consumer comfort and impact protection and general industrial applications.


For the third quarter, 61% of Rogers’ sales were in our strategic megatrend categories, again reinforcing our belief that we are focused on the right markets that show strong sustainable global demand. We saw strong growth in both Internet Connectivity and Mass Transit categories, while we remained essentially flat in Clean Energy.

In Internet Connectivity, we continue to see significant demand for high frequency circuit materials to support wireless base station and antenna applications in connection with the previously mentioned global 4G/LTE infrastructure buildout. In addition, we experienced continued demand for high frequency circuit materials in applications that improve wireless connectivity in portable electronics.

In Mass Transit, we experienced a 20% increase overall, based primarily on demand in rail propulsion applications, specifically in Europe and China.

Our flat performance in the Clean Energy category was due in part to the very strong comparator quarter  of Q3 2013, as well as lower demand for laser diode and certain renewable energy applications during Q3 2014. This lower demand was offset by strong growth in hybrid electric and all-electric vehicles, mass transit, energy efficient motor control applications and vehicle electrification (x-by-wire) applications.

Beyond our strategic megatrend categories, we continue to build momentum in radar-based automotive safety systems, where we had a 34% sales increase over Q3 2013. In addition, we continue to see solid growth in the consumer comfort and impact protection category, which was up 22%. We believe our Megatrend Design Opportunity Pipeline is a helpful indicator of future sales growth prospects. And in Q3, it continued to strengthen. We had 859 opportunities under evaluation in Q3 2014, up from 671 in Q3 2013 and up from 834 in Q2 2014. We also moved 45 projects into production this quarter.


Rogers’ core strength is our ability to provide engineered materials solutions for the most demanding applications and highly specialized designs. We marry this strength with a focus on applications in higher growth markets, which are  driven by the megatrends of Internet Connectivity, Clean Energy and Mass Transit. There are four critical elements to our strategy:

We take a market-driven approach across the company. Our employees are actively engaged in understanding customer and market needs and aligning our actions to deliver differentiated value for our customers. Our scientists, engineers, marketers, manufacturing, supply chain and commercial teams regularly collaborate with key customers and industry contacts in order to fully understand current requirements, as well as future needs. Our goal it to ensure Rogers is providing highly valued products and services with the right performance at the right time.

As our investment in the Rogers Innovation Center at Northeastern University demonstrates, we are creatively and cost-effectively developing innovative technologies to meet market demand. We continue to increase our investment in R&D, as well as enhancing our stage gate process to enable greater speed to market on products that we believe will have a significant impact.

Our third element is synergistic M&A, where we are targeting companies that are aligned with one or more of our current businesses. Our objective is to enhance our market reach and technology platform capabilities.

The final element of our strategy is our operational excellence, where we are implementing continuous improvement approaches through Lean, Six Sigma, Kaizen and other leading methodologies. These efforts have already led to improved manufacturing yields and greater efficiency and effectiveness of our supply and demand planning capabilities in manufacturing.

We are starting efforts to gain further efficiencies in our work processes, particularly in our finance organization, through our IT systems improvements. And we are working to standardize those processes across divisions and locations in order to more efficiently and cost-effectively scale the company as we grow.

As previously mentioned, in Q3 2014, we outperformed our 10% sales growth goal, as well as our 15% operating margin goal. We don’t report quarterly ROIC data, but we will review our annual ROIC results for 2014 when we report our annual earnings in early 2015.

We believe we have a winning, sustainable approach that – as we have seen – is already yielding strong results.

Read the full transcript here.

View the accompanying presentation here.

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