5G. It was going to be a long road to rollouts in 2020. But now it’s looking like 2016 will be the first field implementations. That’s good news as our mobile data traffic is growing rapidly. According to Gartner, “Global mobile data traffic is set to reach 52 million terabytes (TB) in 2015, an increase of 59% from 2014. The rapid growth is set to continue through 2018, when mobile data levels are estimated to reach 173 million TB.”
Most of what we hear about 5G is speed…massive increases that will be 30 to 50 times faster than 4G/LTE. But 5G is also about low latency. Today, latencies run about 75 to 100 ms. 5G is aiming for a mere 1ms. This not only improves the online gaming experience, but allows for remote surgery and mission-critical Industrial IOT (IIoT) applications. 5G will also bring lower power consumption, traffic prioritization, and the real possibility of cutting all those cables running to our front doors.
The Next Generation Mobile Networks Alliance recently published a paper summarizing the core goals of 5G:
- Provide far greater throughput, lower latency, and higher connection density;
- Cope with a wide range of use cases and business models, a high degree of flexibility, and scalability by design;
- Leverage foundational shifts in cost and energy efficiency;
- Offer the end user a consistent customer experience achieved across time and service footprint; and
- Provide a truly global 5G ecosystem, free of fragmentation and open for innovations.
In addition, the group released technical documentation on “Recommendations For Small Cell Development and Deployment” and “Backhaul Provisioning for LTE-Advanced & Small Cells.”
But designing, building, and testing 5G wireless prototypes is a complex engineering feat. According to Kevin Linehan, VP and CTO of antenna systems at CommScope:
If 5G is really going to deliver speeds that are up to 1,000 times faster than the 4G we use today, it needs to utilize the spectrum it will travel over more effectively. Like the journeys to 3G and 4G, the RF path will be critical to the arrival at ‘Destination 5G,’ as will be the need for a high signal to noise ratio (SNR) to ensure a robust data service. This ratio has become increasingly important as the demands for high-speed data increase.
New multi-antenna technologies, such as Massive MIMO systems, are considered the most likely candidates to significantly improve spectral efficiency in 5G networks. Implementing MIMO with large-scale antenna arrays, typically with 64 or more transceiver elements, is expected to increase the capacity of a cell well beyond what is achievable today. Large-scale antenna systems become more practical in terms of size at higher frequencies, where the wavelengths become shorter. These antennas are likely to be an important technology in spectrum bands above 2GHz and in TDD spectrum where handset feedback is not needed.
5G will require adding more spectrum while continuing to support previous air-interface technologies and managing multiple frequency bands. Ever more sophisticated RF beamforming and interference mitigation technologies will need to be utilized.
The Challenge of High Frequency PCB Materials
As users demand smarter, lighter, higher performance devices on ever faster networks, designers of mobile devices and antennas need to balance weight, size, radiation characteristics (such as gain, beamwidth, side-lobe levels, polarization) and cost. The PCB substrate material has a major impact on circuit performance. Low dielectric constant (Dk)/low dissipation factor (Df) materials are desired to maximize radiation efficiencies of antennas while keeping overall losses to a minimum.
Rogers Corporation Advanced Connectivity Solutions provides a broad selection of high frequency circuit materials designed with these considerations in mind. Designers needing best in class performance or commercial high frequency materials can find solutions in Rogers’ extensive product portfolio and through collaboration with Rogers R&D teams to develop unique solutions for this new technology. 5G is shaping-up to impact every aspect of our lives. Finding the right balance of material performance and cost is a challenge for a technology that is yet to be fully defined.
Now you can access Rogers’ PCB materials resources with the ROG Mobile App. Quick and easy access to calculators, literature, technical papers. You can even request samples on your smartphone or tablet
- The app has tools and technical information to assist you with Rogers printed circuit board materials.
- The Microwave Impedance Calculator assists with microwave circuit design in predicting the impedance of a circuit made with Rogers High Frequency circuit materials and also provides capabilities for predicting transmission line losses.
- The ROG Calculators assist RF engineers with thermal and mechanical simulations for microwave PCB designs.
- Data sheets and fabrication guides can be downloaded and material samples can be ordered.
ROG Mobile for iPhone and iPad devices:
Available for the iPhone and iPad in the Apple App Store
ROG Mobile for Android devices:
Available for Andoid devices in Google Play
This ROG Blog series on printed-circuit-board (PCB) materials has reached the half-century mark, already covering a wide range of topics on circuit materials with this, the 50th ROG Blog. For example, this series has recommended materials for amplifiers, for antennas, for filters, and for different types of transmission lines. It has even detailed the effects of different PCB material thicknesses on circuit performance, and described the influence of conductor roughness on circuit performance.
While it would be difficult to pick out the top 10 Blogs from the first 49 Blogs appearing since August 2010, at least 10 of these ROG Blogs deserve mention for how they have attempted to help readers with their different uses of PCB materials.
From the very first ROG Blog, in August 2010, which compared low-cost FR-4 circuit substrates with higher-frequency PCB materials such as the Rogers substrates, to the latest ROG Blogs, which examine circuit material requirements for emerging millimeter-wave wireless applications through 300 GHz and higher, the ROG Blogs have attempted to provide clear and honest information on the use of circuit materials. The next 50 ROG Blogs will pursue the same ambitious goals, in hopes of providing readers with greater benefits for their uses of high-performance circuit materials.
While it would be difficult to name the “Top Ten” ROG Blogs from the series so far (see the list below), it is not surprising to find that one of the most popular (in terms of viewers/readers) would be one that also refers to something for free: the January 2011 ROG Blog on Rogers’ free transmission-line modeling tool, the MWI-2010 Microwave Impedance Calculator. This easy-to-use modeling tool, which has also been reviewed in many of the leading RF/microwave trade publications, calculates key parameters for most common microwave transmission lines, including microstrip, stripline, and coplanar-waveguide transmission lines. The executable (.exe) file is available for free download from the Rogers’ website and runs on Windows-based personal computers (PCs), including those with Windows XP, Windows Vista, and Windows 7 operating systems. The free software is even backed by a 22-page operator’s manual in PDF file format, also available for free from the Rogers website.
In many ways, the ROG Blog series is like a book on circuit materials, unfolding online before its readers, with each Blog adding a new chapter to the book. Each chapter shares what Rogers’ engineers have learned over the years about making and using circuit materials, and this first set of 50 Blogs has covered some areas of interest to a large number of readers. In line with the ROG Blog on free software, the ROG Blog “Comparing RF Circuit Material Processing Costs & Performance” also offers advice meant to help readers save money without sacrificing their performance goals. Although first appearing on “April Fool’s Day” (April 1) in 2011, this ROG Blog takes a serious look at the total costs of circuit materials, and how some circuit materials may have lower material costs than other materials, but pay for it later with higher processing costs and lower yields. It also explains how some performance parameters, such as passive intermodulation (PIM) in wireless circuits and signal integrity in digital circuits, require a careful consideration of tradeoffs in material and processing costs when choosing a circuit material.
These first 50 ROG Blogs have drawn readers for familiar themes as well as for some not-so-familiar topics. For example, the ROG Blog appearing on November 19, 2010, “What Is Outgassing and When Does It Matter,” addresses a subject that may be unknown to some readers but quite significant to others. Outgassing, which refers to the release of gas inside a solid such as a circuit material, especially when it is placed in a vacuum, can greatly impact the performance of circuits used in satellite-communications systems in space, or in medical electronics systems. This ROG Blog introduced many readers to a material term known as total mass loss (TML), and how the parameter could be used to help guide the selection of a circuit material for space-based or other applications where outgassing was a critical concern.
On the other hand, some of the more popular ROG Blogs covered the roles that circuit materials play in the design of some basic RF/microwave components, such as amplifiers, couplers, and filters, and how the choice of a circuit material can affect transmission-line losses in high-frequency circuits. One of the more popular ROG Blogs, “When Digital Signals Reach Microwave Frequencies,” covered an area of interest to many microwave circuit designers, how to deal with digital circuits operating at microwave frequencies. This ROG Blog, appearing on February 23, 2011, reviews some of the important concerns for selecting a circuit material when circuits cross over from the digital area into the microwave realm. These high-speed digital signals will behave much like analog microwave signals, affected by PCB loss and even conductor surface roughness. To guide those in need of circuit materials for high-speed digital designs or even multilayer circuits that may combine fast digital and microwave circuits, this ROG Blog points out how different circuit material characteristics, such as dielectric constant and even coefficient of thermal expansion (CTE), can impact high-speed digital circuit performance.
At times, readers of the ROG Blog series shared their areas of interest and applications for circuit materials, and these applications are many and diverse, from lower-frequency analog and power circuits to high-speed digital and even microwave/millimeter-wave circuits. The ROG Blog series is written to serve its readers with new information on circuit materials as that information is needed, much like new chapters to an on-going, online book about circuit materials. Do you have a suggestion for future ROG blogs? We’d love to get your input. Let us know what you are interested in reading about.
Top 10 Popular ROG Blogs (based on reader feedback)
- “Transmission-Line Modeling Tool: Free Downloadable Software” (1/27/11)
- “What Is Outgassing And When Does It Matter” (11/19/10)
- “Comparing RF Circuit Material Processing Costs & Performance” (4/1/11)
- “Controlling Conductor Losses In Coplanar Transmission Lines” (3/14/11)
- “When Digital Signals Reach Microwave Frequencies” (2/23/11)
- “Do You Have An Award Winning Application?” (11/11/11)
- “The Role of PCB Materials In Impedance Matching” (12/3/12)
- “Choose Circuit Materials For Bandpass Filters” (1/16/13)
- “Make Waveguide In Planar PCB Form” (10/18/12)
- “Celebrating ROG Award Contest Winners at IMS 2012” (7/17/12)
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.
Millimeter-wave frequencies (about 30 to 300 GHz) were once associated with at least two things: circuits for these frequencies are extremely difficult to fabricate, and they will probably be used for some military-electronics application. However, the United States’ Federal Communications Commission (FCC), among other organizations around the world, is doing its part to free wide portions of bandwidth for unlicensed radio use at millimeter-wave frequencies.
The FCC is treating wide millimeter-wave bandwidths, such as the 7 GHz span centered at 60 GHz (57 to 64 GHz) as Industrial-Scientific-Medical (ISM) band frequencies so that they can be used for commercial and other unlicensed applications by the general public. Because these frequencies are available for use without licenses, a growing number of circuit designers are considering different applications at these higher frequencies and, of course, choosing the right printed-circuit-board (PCB) material is an important part of any practical efforts to realize millimeter-wave circuits.
Millimeter-Wave ISM Bands
Organizations such as the FCC have set aside a number of different millimeter-wave bands for unlicensed use in addition to 60 GHz, such as 94, 140, and 220 GHz. Receivers and transmitters at these frequencies are currently being produced in the form of integrated circuits (ICs) based on gallium arsenide (GaAs) and even silicon semiconductor processes, such as silicon CMOS and silicon germanium (SiGe) technologies. As the speeds of computers increases, and the demand for faster Internet access grows, the fast (better than 1-Gb/s) data rates available in these unlicensed millimeter-wave ISM bands makes the use of millimeter-wave links attractive for a variety of short-range communications links.
For example, fixed-frequency millimeter-wave wireless links offer the bandwidth possible with fiber-optic links, but with a fraction of the time and cost required to install a high-speed fiber-optic communications link. For this reason, millimeter-wave links are popular solutions for providing radio backhaul for cellular-communications base stations.
Of course, while government agencies around the world may be freeing millimeter-wave frequency bands for unlicensed use, the task of designing and fabricating circuitry at these elevated frequencies has not become any easier. This blog series has already taken a look at some of the circuit-material characteristics that can impact the performance of millimeter-wave circuits. For years, military-electronic systems have employed phased-array radar systems at millimeter-wave frequencies. And millimeter-wave frequencies have been used extensively in high-end automotive electronic systems, including for long-range adaptive cruise control at 77 GHz and anti-collision systems at 79 GHz. At higher frequencies, millimeter-wave circuits have been part of airport security and imaging systems at 94 GHz. But with increasing opportunities for millimeter-wave circuit applications, especially for communications at ISM bands, it may help to review some of the key circuit material considerations when working at millimeter-wave frequencies.
Circuit Material Considerations at Millimeter-Wave Frequencies
Wavelengths for signals from 30 to 300 GHz are extremely small, from about 1 cm to 1 mm. Although this translates into reduced circuit dimensions, it also makes possible the use of modest-sized antennas with focused beamwidths. As an example of the reduction in size that is possible at these higher frequencies, an antenna with 1-deg. beamwidth for a line-of-sight communications link at 3.5 GHz has a nominal diameter of 12 ft. But for a line-of-sight link at 60 GHz, an antenna with a 1-deg. beamwidth is a mere 8 in. in diameter.
In terms of millimeter-wave circuitry, it is important to remember the impact of various circuit parameters on performance at millimeter-wave frequencies. Circuit designers typically work with a material that is familiar to them based on such characteristics as dielectric constant and dissipation factor, using those parameters where possible in a computer simulation program to project the performance of a particular circuit configuration. Because the physical size of a high-frequency circuit transmission line is dependent on the dielectric constant of the PCB material, the value of the circuit material’s dielectric constant is particularly critical at millimeter-wave frequencies, where circuit dimensions can be so small.
For this reason, PCB materials with the lowest possible dielectric-constant values are to be preferred for millimeter-wave circuit applications. Circuit dimensions shrink with higher values of dielectric constant, but reducing the size of necessarily small circuit dimensions can make those circuits difficult to fabricate with consistency.
The consistency of the dielectric constant across a circuit board can also be an important concern at millimeter-wave frequencies since, at those frequencies, variations in the dielectric constant can introduce variations in the signal phase. For a given consistency of dielectric constant, the phase variations will increase with increasing frequency, and will hinder the performance of circuits that depend on reliable phase behavior, such as in phase-modulated communications systems and phased-array radar systems.
Millimeter-wave phase performance can also be affected by the composition of the PCB material. For example, circuit materials that are reinforced using a glass weave can exhibit phase-based problems when the glass weave is not consistent throughout the material. In a manner somewhat akin to an inconsistent dielectric constant, this can lead to perturbations in a circuit’s signal propagation velocity, which cause signal integrity issues, including uneven phase performance. The unwanted results can be distortions in phase modulation and errors between phase-matched channels in radar systems.
Minimizing Atmospheric Losses
Millimeter-wave communications links typically support high data rates at line-of-sight distances to about 1 km, but they are subject to atmospheric losses even for such short links. To minimize losses at those higher frequencies, PCB materials with the lowest possible dissipation factors should be used for millimeter-wave circuits. The quality of a PCB’s conductor surface can also play a role in loss performance at millimeter-wave frequencies. A rough copper surface will yield higher conductor losses at higher frequencies, so that selecting a PCB material with smooth copper conductor surface can help minimize loss at millimeter-wave frequencies.
The thickness of a PCB’s conductor layer can also be a concern at millimeter-wave frequencies because of a parameter known as skin depth. Skin depth refers to the thickness into the conductor material at which a propagating electric field has decreased by about 37%. Skin depth decreases rapidly with increasing frequency, at about 6.6 μm at 100 MHz, about 0.66 μm at 10 GHz, and about 0.2 μm at 100 GHz. With the small skin depth at millimeter-wave frequencies, it is easy to see the impact that copper conductor roughness can have on loss performance.
The thickness of the PCB material is also a consideration at millimeter-wave frequencies, since moding effects and unwanted resonances can result from the use of a thick circuit material with such small wavelengths. Circuit materials used for millimeter-wave circuits are typically in the thickness range of 2 to 10 mils. As they do with increasing dielectric constants, circuit line widths also shrink with thinner PCB materials. Since circuit loss decreases with the increasing thickness of circuit dielectric materials, selecting a PCB material for millimeter-wave applications is something of a tradeoff between creating stable, practical, and producible circuits and achieving low loss for those circuits.
Given this challenging set of requirements, what types of real-world materials are suitable for millimeter-wave circuits? Two examples are RT/duroid® 5880 and RT/duroid 5870 laminates from Rogers Corp. Both are PTFE-based composite materials with low dielectric constants, good consistency of dielectric constant, and low loss. RT/duroid 5880 laminate has a dielectric constant of 2.20 in the z direction at 10 GHz with dissipation factor of a low 0.0009 at 10 GHz. RT/duroid 5870 laminate has a dielectric constant of 2.33 in the z direction at 10 GHz with dissipation factor of 0.0012 at 10 GHz. These materials can be supplied in sheets as thin as 3.5 mils for excellent performance at millimeter-wave frequencies.
In summary, when working at millimeter-wave frequencies, circuit materials should ideally be electrically homogeneous, as thin as possible, with low dielectric constant, low dissipation factor, and with a smooth conductor surface.
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.