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

Circuit designers must often select a circuit technology, such as microstrip or grounded coplanar waveguide (GCPW) circuitry, with a particular design and circuit material to achieve optimum performance. As often detailed in this blog series, circuit materials can be compared by different electrical and thermal characteristics, such as dielectric constant, loss, power-handling capabilities, even bandwidth limits at higher frequencies. Circuit technologies, such as microstrip and GCPW, each have their strengths and weaknesses, and it may help to take a closer look at these two circuit technologies in particular to see how they stack up.

Microstrip circuits are formed by means of thin transmission lines on one side of a high-frequency dielectric printed-circuit-board (PCB) and a conductive metal ground plane on the other side of the PCB. The performance of a microstrip circuit is affected by a number of material-related variables, including the thickness of the dielectric material, the thickness of the conductive metal, even the surface roughness or smoothness of the conductive metal at the copper-substrate interface.

GCPW circuits, also known as conductor-backed coplanar waveguide (CBCPW) circuits, increase the amount of ground around a circuit compared to microstrip by placing ground planes on the bottom of the dielectric material and on the top, on the same plane and on either side of a signal transmission line. GCPW circuit structures achieve electrical stability by literally surrounding a signal line with ground planes.

Both technologies operate by means of a dominant quasi-transverse-electromagnetic (quasi-TEM) propagation mode. GCPW circuits, with their enhanced ground structures, are somewhat more mechanically complex to fabricate. But GCPW circuits also feature low dispersion compared to microstrip circuits, with lower radiation loss than microstrip circuits especially at frequencies extending into the millimeter-wave range.

With their enhanced ground structures, GCPW circuits are capable of wider effective bandwidths than microstrip circuits and wider impedance ranges than microstrip circuits. However, microstrip circuits are relatively robust and easier to fabricate than GCPW circuits, with their straightforward “ground plane on the bottom” circuit structure. In addition, microstrip performance is not as sensitive to circuit fabrication issues as GCPW circuits, with microstrip designs suffering minimal performance variations due to normal etching variation of the conductor/space and conductor thickness.

For a fair comparison of the two high-frequency circuit technologies, several circuits were fabricated on RO4000® series circuit materials from Rogers Corp., including 10-mil-thick RO4350B™ laminate with standard 0.5-oz. electrodeposited (ED) copper. The physical differences in the circuit technologies result in significant differences in the electromagnetic (EM) field patterns around each technology’s transmission lines. In microstrip, most of the EM fields lie between the top signal plane and the bottom ground plane, with high field concentration at the edges of the signal conductors. In GCPW, strong EM fields exist between the ground-signal-ground (GSG) areas on the coplanar circuit layer, with weaker fields lying between the signal plane and the bottom ground plane than for the top and bottom circuit planes of microstrip. The transmission lines in GCPW circuits suffer greater conductor losses than microstrip, but reduced radiation loss compared to microstrip. In addition to low radiation loss, the neighboring ground planes for the GCPW can significantly benefit in the suppression of spurious modes.

Microstrip circuits can provide consistent results even with some inconsistencies in circuit fabrication processes. GCPW circuits are capable of operating at higher frequencies with lower losses than microstrip circuits but are less likely to be unaffected by variations in manufacturing processes. For example, for a microstrip circuit, a thicker copper conductor will result in a slight decrease in a PCB’s effective dielectric constant, with a small improvement in insertion-loss performance. For GCPW, a thicker copper conductor can have a much greater impact, resulting in an increase in the EM fields between the ground-signal-ground structure on the PCB, a decrease in the effective dielectric constant, and decreased conductor loss for that PCB

A great deal of a GCPW circuit’s EM field energy propagates through the air around the circuitry, with its low dielectric constant of 1, rather than through a conductive metal or dielectric material with much higher dielectric constant. The end result is a low net dielectric constant for that GCPW circuit board. Wider conductors used on these GCPW circuit boards can help reduce conductor losses. In addition, thick copper conductors in GCPW circuits can lead to taller conductor walls for those circuits, with a significant portion of EM propagation taking place in the air around the copper conductors and reducing the effective dielectric constant and loss for those circuits.

A GCPW circuit board can be designed and fabricated with loosely and/or tightly coupled circuits, with significant differences in performance between the two coupling approaches. For example, loosely and tightly coupled GCPW circuits will respond differently to the use of conductors with and without a finish, such as electroless nickel immersion gold (ENIG) finish. Because nickel is less conductive than copper, a circuit with an ENIG finish will suffer higher conductor losses than a circuit with bare copper conductors. Furthermore, a tightly coupled GCPW circuit with an ENIG finish will suffer greater conductor loss than a loosely coupled GCPW circuit with the same ENIG finish, especially when those GCPW circuits may involve signal propagation through multiple circuit layers each with copper conductors having its own ENIG finish. Differences in circuit technologies exist due to variations within the same circuit technology, with loss and performance due to such factors as finish, loose or tight coupling, and the width of conductors.

Microstrip and GCPW are both proven high-frequency circuit methodologies and both can provide excellent performance through microwave frequencies and beyond. They offer different approaches to laying out the circuit and, also, the choice of PCB material for a circuit design can have an impact on the final performance possible with each circuit technology. In general, at microwave frequencies, microstrip circuits will suffer less loss than GCPW circuits, especially due to manufacturing variations. But when an application calls for higher, millimeter-wave frequencies, GCPW circuits will suffer less dispersion and radiation losses than microstrip circuits. Also at millimeter-wave frequencies, GCPW circuit approaches provide better bandwidth than microstrip circuits. In addition, when necessary, mode suppression can be achieved more readily with GCPW circuits than with microstrip circuit approaches.

Note: This ROG blog is based on a MicroApps presentation made at the recent IEEE 2015 International Microwave Symposium, May 17-22, 2015, Phoenix, AZ, “Microwave PCB Structure Considerations: Microstrip vs. Grounded Coplanar Waveguide.”

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Download the ROG Mobile app to 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.

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Introducing the newest version of the Microwave Impedance Calculator from Rogers’ Advanced Connectivity Solutions. The new version includes:

  • Additional dielectric materials
  • Thermal model capabilities
  • Bug fixes

This software is intended to assist with microwave circuit design in predicting the impedance of a circuit made with Rogers High Frequency circuit materials. The software also has some capabilities for predicting transmission line losses as well. The user will select the circuit materials and the circuit construction, after which the software will determine the predicted impedance and other electrical information.

The calculator uses well known closed form equations to determine impedance and loss of a given circuit model. The loss calculation is divided into conductor loss and dielectric loss. With specific circuit designs, the calculator also predicts other properties such as wavelength in the circuit, skin depth and thermal rise above ambient.

Use the Online Microwave Impedance Calculator now.

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

Microstrip edge-coupled bandpass filters (BPFs) can help clean the spectrum around a desired center frequency. Fabricated on printed-circuit-board (PCB) materials, these compact filters can be integrated with other circuit functions to provide dependable filtering of communications bands and high-frequency signals for a wide range of applications. Microstrip edge-coupled BPFs can be quickly designed for a bandwidth of interest with the aid of modern computer-aided-design (CAD) circuit-design software tools, and reliably fabricated on RF/microwave PCB materials to achieve impressive performance levels. Unfortunately, even the most carefully designed edge-coupled BPFs can suffer the effects of spurious harmonic responses, resulting from even-order harmonics of the filter’s center frequency. But there is a way to control these harmonic responses, and it relies on combining a PCB laminate and a PCB bondply material to form a simple composite material structure with dramatically improved harmonic performance compared to single-PCB-material designs.

The composite PCB material is formed by combining RT/duroid® 6010.2LM laminate with the versatile 2929 bondply material, both from Rogers Corp. The dielectric layers are clad with copper metallization which is used to form the transmission lines for filters and other microstrip circuits. RT/duroid 6010.2LM circuit material has a high relative dielectric constant of 10.7 in the z-axis (thickness) of the material when measured at 10 GHz. This high-performance PCB building-block material is “fine-tuned” in terms of harmonic filter performance by adding the “glue,” 2929 bonply material. This is an unreinforced thin-film adhesive material nominally developed for bonding together the layers of multilayer PCBs. However, as found with experiments on edge-coupled BPFs, this is more than just a multilayer adhesive system: it is extremely useful in tailoring the harmonic responses of high-frequency filter circuits. The 2929 bondply material exhibits a much lower relative dielectric constant than the RT/duroid 6010.2LM circuit material, at 2.94 in the z-axis measured at 10 GHz. It features low loss, with an extremely low dissipation factor of 0.003 in the z-axis at 10 GHz. The 2929 bondply material is available in different sheet thicknesses and layers can be stacked for applications requiring increased thicknesses.

Edge-coupled BPFs provide effective high-frequency filtering functions in microstrip circuits but may exhibit spurious harmonic responses, which may interfere with an application’s desired performance. Numerous methods have been developed to suppress unwanted harmonic responses, most of which are based on equalizing the phase velocities of even- and odd-mode propagation characteristics of a PCB. Various corrective approaches have been applied, including the use of suspended microstrip circuit patterns, adding capacitors, or adding circuit patterns to make phase velocity adjustments, but all of these approaches add to the complexity of the initial filter circuit. The combination of RT/duroid® 6010.2LM  circuit material and 2929 bondply materialmakes use of a fairly straightforward suspended microstrip technique and is inexpensive and does not sacrifice reliability.

Prior to designing different filter circuit patterns for fabrication on the composite and on standard circuit material for comparison, the materials were studied to better understand the effects of combining the dielectric materials. Commercial electromagnetic (EM) simulation software from Sonnet Software was employed to model the even- and odd-mode effective dielectric constant of each of the materials used for fabricating the edge-coupled BPFs. This is a three-dimensional (3D) planar EM simulator for predicting the behavior of predominantly planar circuits, such as microstrip, stripline, and coplanar-waveguide (CPW) circuits, and the Professional version of the software, revision 14.52, was used in the filter studies. Following design and simulation, different filters were fabricated on the different material options, and fabricated filters were characterized with the help of a model E8364C programmable vector network analyzer (VNA) from Keysight Technologies. The analyzer was prepared for the measurements by means of a two-port, 12-term short-open-load-through (SOLT) calibration.

To better understand the effects of using a composite material on the harmonic responses of an edge-coupled microstrip BPF, four filters were modeled on the Sonnet software and then fabricated for analysis on the two different circuit materials, the standard material and the combination of RT/duroid® 6010.2LM  and 2929 bondply material. Two different filter circuits were used, based on a Chebyshev transfer function with center frequency at 2.5 GHz and bandwidth of either 180 or 440 MHz.

The measured results for the different filters were remarkably close to the responses predicted by the Sonnet Software simulations, with significant suppression of second-harmonic responses achieved for both filters fabricated on the composite circuit material compared to the standard circuit material.

In all cases, the fabricated filters were characterized with the model E8364C analyzer across a measurement range from 10 MHz to 6 GHz for full analysis of the different filters’ upper and lower sidebands. For both filter bandwidths, significant spurious responses appearing in the upper sidebands of the BPFs fabricated on the standard circuit material were essentially eliminated when those same filter circuits were fabricated on the composite material.

These filter circuits demonstrate one benefit of using composite circuit materials to fine-tune a circuit response. Composite circuit materials such as the combination of RT/duroid® 6010.2LM and 2929 bondply materials also exhibit differences in the coupling behavior of resonators, transmission lines, and other microstrip circuit structures, making it possible to fine-tune the harmonic responses of different circuit designs by exploring the effects of composite materials with a circuit simulation program such as Sonnet Software. In addition, many modern circuit designs employ multilayer circuit constructions, with general-purpose materials, such as FR-4, used for many standard analog and digital functions, and higher-performance materials based on polytetrafluoroethylene (PTFE) for RF/microwave and higher-speed digital circuits. In addition to providing a reliable bond between circuit layers, a versatile circuit adhesive such as 2929 bondply material can be included in the circuit modeling stage to fine-tune circuit responses, such as second-harmonic responses, as needed.

Screen shot 2014-08-08 at 1.33.54 PMAuthor’s Note: This ROG blog is based on several presentations at the 45th European Microwave (EuMW) Conference at the Palais des Congres, Paris, France, September 6-11, 2015. The presentations include a company-sponsored MicroApps session, “Composite Circuit Materials Used to Suppress Harmonic Modes in Microstrip Edge Coupled Filters,” and the EuMW Conference presentation, “Applied Methodology for Harmonic Suppression of Microstrip Edge Coupled Bandpass Filters using Composite Circuit Materials.”

ROG Mobile App

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.

Ask an Engineer

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.


Rogers Corporation (NYSE:ROG) will be a major participant in European Microwave Week 2015 (EuMW 2015), Europe’s largest trade show devoted to RF/microwave technology and applications. The conference and exhibition run September 6-11, 2015 at the Palais des Congres, Paris, France. EuMW 2015 includes the 45th European Microwave Conference, the 10th European Microwave Integrated Circuits Conference (EuMIC 2015), the 12th European Radar Conference (EuRAD 2015), and a large exhibition floor.

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See Rogers Advanced Connectivity Solutions at Booth 263

Rogers ACS will be exhibiting in Booth 263 Sept. 8-10. Stop by and let us help with advice and design information concerning our high-performance printed-circuit-board (PCB) materials, including RO3003™ laminates. RO3003 laminates have tightly controlled dielectric constant (3.04+/-0.04 at 10GHZ) and very stable dielectric constant performance over temperature (-3 ppm/°C from -50°C to 150°C). The RO3003 PTFE-ceramic composite resin system enables very low dielectric loss (0.0010 at 10GHZ), and laminates can be purchased with rolled copper to further enhance PCB insertion loss performance. Due to its excellent electrical properties, RO3003 laminates are often chosen for millimeter wave applications such as 77-79GHZ automotive radar sensors and 60GHz point to point backhaul applications.

Rogers + Arlon®

Visitors to the exhibition booth can also learn about the recent acquisition of material supplier Arlon, LLC. The Arlon business is well established as a top supplier of high-frequency circuit materials and engineered silicone materials and is an excellent strategic fit with our PCB materials and high-performance elastomers. The acquisition adds to the materials diversification and expertise at Rogers and benefits customers with a significantly expanded choice in high-performance materials.

Rogers’ John Coonrod to Present at Tech Conference & MicroApps

John Coonrod, Technical Marketing Manager at Rogers Advanced Connectivity Solutions & author of the popular ROG Blog series, will deliver four presentations: one during the Microwave Conference and three MicroApps presentations on the exhibition floor.

The conference presentation, scheduled for 8:30-8:50 AM on Wednesday, September 9, will be of interest to designers and users of high-frequency filters and PCB material specifiers: “Applied Methodology for Harmonic Suppression of Microstrip Edge Coupled Bandpass Filters Using Composite Circuit Materials.”

Coonrod will also deliver three different MicroApps presentations during EuMW 2015:

  • Composite Circuit Materials Used to Suppress Harmonic Modes in Microstrip Edge Coupled Filters” (Tuesday, September 8, 13:00)
  • Microwave PCB Structure Selection: Microstrip Versus Grounded Coplanar Waveguide” (Wednesday, September 9, 14:30)
  • PCB Fabrication Influences on Microwave Performance” (Thursday, September 10, 15:30)

Rogers Corporation is a Founding Member of RF Energy Alliance

The RF Energy Alliance (RFEA) is standardizing, promoting, and educating audiences in solid-state RF energy—a clean, highly efficient, and controllable heat and power source. Members envision a fast-growing, innovative marketplace built around this sustainable technology, contributing to quality of life across many applications. As a founding member, Rogers Corp. will host RFEA’s Dr. Klaus Werner as he presents the state of the Alliance at 11:30 on Wednesday, September 9, and at 11:30 on Thursday, September 10 at booth 263.


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

Circuit performance may start with the choice of printed circuit board (PCB) material, but achieving a desired level of circuit performance can also have a great deal to do with how circuits are fabricated on a chosen PCB material. Attendees to the MicroApps sessions at the recentIEEE 2015 International Microwave Symposium (Phoenix, AZ) in particular, the Rogers Corp. MicroApps session, “PCB Fabrication Influences on Microwave Performance,” learned how such factors as circuit-board thickness and choice of transmission-line metal can impact the final performance of both active and passive circuits fabricated on a particular PCB material.

Insertion loss is usually an important parameter for most high-frequency circuits, especially where signal power is limited, and for RF/microwave printed circuits, insertion loss is highly dependent upon the choice of PCB material thickness. To demonstrate, three high-frequency 50 ohm circuits were made and modeled from the same circuit-board material, but at three different thicknesses. The material was RO4835™ circuit laminate from Rogers Corp. and the thicknesses were 6.6, 10.0, and 30.0 mils. RO4835 rigid thermoset laminate material has a dielectric constant of 3.48 at 10 GHz through the z-axis (thickness) of the material, controlled to a tight tolerance of  ±0.05. This circuit material exhibits thermal conductivity of 0.69 W/m/°K and features excellent dimensional stability in the x-y plane. But what some designers may not realize when using this material is that the choice of thickness does matter, especially regarding insertion-loss performance.

All three circuits were modeled, fabricated, and measured, to compare simulated and measured performance levels for the different PCB thicknesses. Modeling was performed with the aid of the MWI-2014 simulation software from Rogers Corp., using Hammerstad and Jensen closed-form equations. Simulations were compared with measurements performed on a wideband vector network analyzer (VNA), a model E8346C from Agilent Technologies (now Keysight Technologies), capable of performing broadband S-parameter measurements from 10 MHz to 50 GHz. The swept-frequency simulated and measured results for each different circuit thickness agree quite closely across a modeled/measured frequency range of DC to 20 GHz. It is the ways in which the total losses for each thickness of PCB material break down in the simulations that are quite different.

The measured and modeled swept-loss plots showed total insertion loss. The modeled total insertion loss, however, is further broken down and compared in terms of dielectric and conductive circuit losses for each thickness of circuit-board material. Any fabricated circuit can be evaluated in terms of its insertion-loss components, which include the dielectric loss of the circuit material, the conductor loss of the circuit traces, the leakage loss of the PCBs, and the radiation loss of the circuit traces. In this presentation, two of the four insertion-loss components, dielectric loss and conductor loss, were examined for the three different PCB thicknesses to better understand how circuit thickness played a role in this important high-frequency circuit performance parameter.

The dielectric losses for the three thicknesses of PCB material are quite close in value, increasing steadily with frequency and with an overall increase in modeled dielectric loss as a function of frequency. But these differences are very slight—almost negligible when comparing the modeled dielectric losses for the 6.6- and 10.0-mil circuit materials. The largest differences in simulated dielectric losses occurred between the thinnest and thickest circuit materials, with about 0.2 dB/in. dielectric loss at 15 GHz for the 6.6-mil-thick RO4835 material compared to about 0.25 dB/in. dielectric loss at 15 GHz for the 30-mil thickness of the same circuit material.

Simulated conductor losses, on the other hand, were not quite as similar for the three thicknesses of PCB material, with this loss component of PCB insertion loss increasing steadily as thinner circuit laminates are used. For the thickest of the three RO4835 PCB materials, the modeled conductor losses remained under 0.1 dB/in. of transmission line through about 10 GHz and only slightly above 0.1 dB/in. of transmission line through 20 GHz. In comparison, for the thinnest of the three RO4835 PCB materials, the modeled conductor losses were slightly less than 0.4 dB/in. at 10 GHz, climbing to about 0.6 dB/in. at 20 GHz. Conductor losses for transmission lines on the 10-mil-thick RO4835 PCB material were just about midway between the conductor loss values for the thinnest and thickest of the circuit laminates that were modeled.

As a further examination of how fabrication choices can affect PCB performance, microstrip edge-coupled bandpass filter circuits fabricated on RO4835 circuit material were compared for circuit laminates with bare copper transmission lines and for circuit laminates with copper transmission lines having solder mask protection. Solder mask is a polymer layer added to PCB copper traces to protect against the effects of oxidation and to prevent solder bridges from forming between closely spaced circuit traces. Both filter circuits were fabricated on 20-mil-thick RO4835 circuit laminates, identical except for the solder mask. Adding the solder mask provides reliable long-term protection against the deleterious effects of oxidation, but it also results in some tradeoffs, such as additional transmission-line insertion loss. For example, the modeled insertion loss for the filter circuit with bare copper transmission lines was about 0.25 dB/in. at 10 GHz, climbing to about 0.50 dB/in. at 20 GHz. In comparison, the modeled insertion loss for the filter circuit with copper transmission lines covered with solder mask was slightly more than 0.30 dB/in. at 10 GHz, rising to about 0.60 dB/in. at 20 GHz.

Perhaps even more significant, especially in the design process for such a filter, the choice of using or not using solder mask on the PCB material made a difference in the location of the filter center frequency and quality factor (Q), with the center frequency slightly lower for the filter using PCB material with solder mask. S-parameter measurements on the microwave VNA revealed a center frequency of 2.9499 GHz and a Q of 8.7993 for the filter with solder mask, and a center frequency of 3.0144 GHz and a Q of 8.9627 for the filter with bare copper conductors. The bandwidths for the two filters were almost identical, at 335.25 MHz for the filter with solder mask and 336.32 MHz for the filter with bare copper transmission lines

Screen shot 2014-08-08 at 1.33.54 PMOf course, these are just a few examples using RO4835 circuit laminates, but they point to the importance of carefully considering PCB fabrication approaches and material parameters before starting a design. As shown, such parameters as thickness and use of solder mask can affect performance and perhaps make the difference between achieving circuit performance results in just one design iteration versus having to perform multiple design iterations. For those interested in further information on these comparisons, using additional RO4000® series materials, copies of the MicroApps presentation are available for free download from Rogers Corporation’s Technology Support Hub.

ROG Mobile App

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.

Ask an Engineer

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.

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