This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

Much of the buzz on the show floor at the 2017 IMS in Honolulu was about millimeter-wave devices and circuits. At one time, frequencies above 30 GHz were considered “exotic” and only for military or scientific applications. But times have changed, and available spectrum is scarce. Millimeter-wave frequencies are now used in commercial vehicular radars, and big plans are being made for these small wavelengths in Fifth Generation (5G) wireless communications networks, in support of moving massive amounts of data quickly. More and more design engineers are faced with developing practical millimeter-wave circuits to 77 GHz and beyond. But first, they must decide upon the best transmission-line technology for those high frequencies as well as the circuit material that can support those circuits with quality, low-loss signal propagation. Drivers and cell-phone users everywhere will be counting on them!

At microwave frequencies, microstrip is by far the most popular transmission-line technology, compared to stripline and coplanar waveguide (CPW). It has a signal plane on the top copper layer and bottom ground plane. It is relatively simple and cost-effective, and allows surface mounting of components for ease of construction.

Unfortunately, as signal frequencies move into the millimeter-wave range, microstrip circuits can behave like antennas, radiating electromagnetic (EM) energy away from a desired signal propagation path and resulting in much higher radiation losses than at lower frequencies. Microstrip radiation losses are also dependent upon the thickness and dielectric constant (Dk) of the circuit substrate material. Thinner substrates suffer less radiation loss than thicker substrates. Also, circuit materials with higher Dk values have less radiation loss than circuit materials with lower Dk values.

In microstrip, the effective Dk is a combination of the Dk of the substrate material and air, since EM waves in a microstrip transmission line propagate in part through the dielectric and in part through the air above it. In contrast to microstrip, stripline is like a flattened coaxial cable. It consists of a conductor surrounded by top and bottom dielectric layers which in turn are covered by ground planes. The Dk of stripline is the same as that of the dielectric material, since air is not involved in the propagation process.

CPW circuits are fabricated with a number of variations, including as standard, grounded coplanar waveguide (GCPW), and conductor-backed coplanar waveguide. Standard CPW metallizes parallel conductors (in the form of a flat waveguide) on the top of a dielectric layer, with ground metal areas just beyond the conductors. GCPW adds a bottom ground-plane layer but requires plated-through-hole (PTH) viaholes through the dielectric substrate material to connect the top and bottom ground planes. The extra ground planes on the top copper layer helps GCPW achieve high isolation between signal lines and can be designed to minimize spurious wave propagating modes. Placement of the PTH viaholes is critical, and can impact transmission-line impedance and loss.

Like microstrip, GCPW has an effective dielectric constant as the result of EM waves propagating through the dielectric material as well as through the air around the conductors. GCPW, like microstrip, also allows surface-mounting of components for ease of fabrication, in contrast to stripline where PTH vias need to connect the components on the outer circuit layers to the inner signal layer. In terms of millimeter-wave frequencies, GCPW has lower dispersion than microstrip, with less radiation loss, and is capable of supporting higher-frequency propagation than microstrip circuits. GCPW also achieves more effective suppression of spurious propagation than microstrip, and is more amenable to practical signal-launch configurations (such as from waveguide, cables, and connectors) at millimeter-wave frequencies than microstrip.

Finding the Right Circuit Material

If GCPW is the optimum transmission line for millimeter-wave circuits, it should then be fabricated on a circuit material with optimum characteristics for millimeter-wave frequencies. Since signal power tends to decrease with increasing frequency, an optimum circuit material for millimeter-wave circuits should have low loss at those high frequencies. The insertion loss of millimeter-wave transmission lines is due mainly to the aforementioned radiation losses, conductor losses, and dielectric losses. Radiation losses tend to be design-specific, whereas conductor and dielectric losses will depend upon the choice of circuit material.

Dielectric losses are a function of the type of dielectric material, and usually well defined by a material’s dissipation factor (Df), with lower values indicating lower dielectric losses. A circuit material capable of consistent performance at millimeter-wave frequencies will also exhibit minimal variations in Dk, so that dielectric losses do not change dramatically with frequency.

In considering circuit materials for millimeter-wave circuit applications, the thermal coefficient of dielectric constant (TCDk) parameter provides reliable insight into the stability of a material’s Dk with temperature. The TCDk parameter provides an understanding of what to expect of a particular circuit material’s performance at millimeter-wave frequencies, with lower TCDk values indicating less change of Dk with temperature and less variations in frequency phase response resulting from variations in Dk with temperature.

Conductor losses can be traced to a number of variables at millimeter-wave frequencies, including the surface roughness of the copper conductors and the choice of plated finish for the conductors. Copper is an excellent conductor, but increasing surface roughness results in increasing conductor loss and greater propagation phase delays. The main area of concern for copper surface roughness is at the copper-substrate interface, with conductor loss due to the copper surface roughness increasing as a function of increasing frequency. The small wavelengths of millimeter-wave signals result in less skin depth in the circuit material as part of EM propagation, and circuit materials with greater copper surface roughness will more severely impact the insertion loss and phase response at millimeter-wave frequencies. The effect of copper surface roughness on insertion loss is also dependent upon the thickness of the circuit material, with thinner circuits affected more by copper surface roughness than thicker circuits.

At millimeter-wave frequencies, circuit materials with excessive copper surface roughness will have more impact on the conductor loss of microstrip circuits than on the conductor loss of GCPW circuits. Switching to a circuit material with smoother copper finish will bring less of an improvement in conductor-loss performance for a GCPW circuit than for a microstrip circuit, especially at millimeter-wave frequencies. In particular, tightly coupled GCPW circuits, which feature closely spaced conductors and ground areas, are less subject to the effects of copper surface roughness than loosely coupled GCPW circuits (with greater spacing between conductors and ground).

An optimum circuit material for millimeter-wave circuits should cause minimal phase angle variations, since such behavior can be critical to many millimeter-wave applications, such as 77-GHz vehicular radar systems. By minimizing variations in certain material-based attributes, such as copper thickness, Dk, conductor width, and substrate thickness, variations in phase angles can be minimized at millimeter-wave frequencies.

Additional details on finding the right combination of circuit material and transmission-line technology are available as part of the Microwave Journal webinar, “Design Considerations and Tradeoffs for Microstrip, Coplanar and Stripline Structures at Millimeter-wave Frequencies,” presented by John Coonrod. 

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This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

Growing demand for mobile wireless communications services has quickly eclipsed the capabilities of Fourth Generation (4G) Long Term Evolution (LTE) wireless networks and created a need for a next-generation mobile wireless network solution. Fifth Generation (5G) wireless networks promise more capacity and capability than 4G LTE systems, using wider channel bandwidths, new antenna and modulation technologies, and higher carrier frequencies even through millimeter-wave frequencies. But before 5G wireless networks can become a reality, systems and circuits will be needed for higher frequencies than current 2.6-GHz 4G LTE wireless networks.

Standards are still being formulated for 5G wireless networks, with goals of achieving data rates of 10 Gb/s and beyond with low latency, using higher frequencies than in traditional wireless communications systems. In the United States, for example, last year the Federal Communications Commission (FCC) approved the use of frequency bands at 28, 37, and 39 GHz for 5G.

PCB Materials for Millimeter Waves

For circuit designers, one challenge will be in knowing where to start, which means, for millimeter-wave frequencies, knowing what types of printed-circuit-board (PCB) material characteristics are the most important at higher frequencies. Millimeter-wave frequencies (above 30 GHz) were once used almost exclusively by the military and for research experiments, but 5G represents an opportunity to “popularize” millimeter-wave frequencies and make them part of everyday life, not just for exotic electronic devices in the limited quantities used in research and by the military, but for potentially billions of electronic devices for people and things, as in how Internet of Things (IoT) devices will use 5G networks for Internet access.

Designing circuits at millimeter-wave frequencies starts with the right PCB material, and knowing how different PCB characteristics affect circuit performance at millimeter-wave frequencies. Variations in certain circuit material parameters, such as dielectric constant (Dk), can have greater impact on performance as the operating frequency increases. For example, signal power is a valuable commodity at millimeter-wave frequencies, requiring circuit designers to minimize loss in their circuits as much as possible. This begins with the choice of PCB material, since a PCB material not meant for use at millimeter-wave frequencies can result in excessive signal losses when operated beyond its intended operating frequency range.

PCB materials can degrade signal power in three ways: radiation losses, dielectric losses, and conductor losses. Losses through radiation of EM energy largely depend on the circuit architecture, so even the lowest-loss PCB material may not save a circuit configuration that has a tendency to radiate energy.

A thoughtful choice of PCB material can help minimize dielectric and conductor losses at millimeter-wave frequencies. A circuit material’s dielectric loss is closely related to its dissipation factor (Df) or loss tangent, which increases with frequency. The Df is also related to a material’s dielectric constant (Dk), with materials that have higher values of Dk often have higher Df loss, although there are exceptions. Attempts to minimize dielectric losses for millimeter-wave circuits can be aided by considering circuit materials with low Df values.

Controlling Conductor Loss

Finding a material with low conductor losses at millimeter-wave frequencies is not as straightforward, since conductor losses are determined by a number of variables, including the surface roughness and the type of finish. As the name suggests, millimeter-wave signals have extremely small wavelengths, mechanical variations in a circuit-board material can have significant effects on small-wavelength signals. Increased copper surface roughness will increase the loss of a conductor, such as a microstrip transmission line, and slow the phase velocity of signals propagating through it. In microstrip, signals propagate along the conductor, through the dielectric material, and through the air around the circuit material, so the roughness of the conductor at the interface with the dielectric material will contribute to the conductor loss. The amount of loss depends on frequency: the loss is greatest when the skin depth of the propagating signal is less than the copper surface roughness. Such a condition also degrades the phase response of the propagating signal.

The impact of copper surface roughness on conductor loss depends on the thickness of the PCB material: thinner circuits are more affected than thicker circuits. The effects of copper surface roughness on loss become apparent at millimeter-wave frequencies. For example, two circuits based on 5-mil-thick RT/duroid® 6002 circuit material from Rogers Corp. but with two different types of copper conductor and surface roughnesses were tested at 77 GHz. The circuit with rolled copper and root mean square (RMS) conductor surface roughness of 0.3 μm exhibited considerably lower conductor loss than the same circuit material with electrodeposited (ED) copper conductor having 1.8-μm surface roughness.

Propagation of the small wavelengths at millimeter-wave frequencies can also be affected by the type of finish used on a PCB’s conductors. Most plated finishes have lower conductivity than copper, and their addition to a copper conductor will increase the loss of the conductor, with loss increasing as the frequency increases. Electroless nickel immersion gold (ENIG) is a popular finish for copper conductors; unfortunately, nickel has about one-third the conductivity of copper. As a result, ENIG plating will increase the loss of a copper conductor, with the amount of loss increasing as a function of increasing frequency.

Environmental Effects

Environmental conditions can also impact the amount of loss exhibited by a PCB material, especially at millimeter-wave frequencies. Many network scenarios for 5G predict the need for many smaller wireless base stations than used in earlier wireless network generations, in part because of an increased number of expected users and the use of millimeter-wave frequencies and their shorter propagation distances than lower-frequency carriers. Where 5G base stations cannot be maintained in climate-controlled environments, circuits may be subject to changing environmental conditions, such as high relative humidity (RH). Water absorption can dramatically increase the loss of a PCB material, and the loss of circuit materials with high moisture absorption will be greatly affected under high RH conditions.

Testing on 5-mil-thick RO3003™ circuit material from Rogers Corp. for two different operating environments showed how loss at millimeter-wave frequencies can increase with RH. One circuit was maintained at room temperature and the other was subjected to +85ºC and 85% RH for 72 hours. At 79 GHz, the room temperature material had about 0.1 dB/in. less loss than the material subjected to higher humidity and temperature. When testing was performed on a third, thermoset circuit material from a different supplier, the increase in circuit loss at 79 GHz was even more dramatic.

For those interested in learning more about the nuances of selecting PCB materials and designing circuits for 5G, in particular at millimeter-wave frequencies, Rogers has created a number of tutorial videos in the “The Road to 5G” series. The videos guide viewers on what different circuit material parameters mean at millimeter-wave frequencies, and which material characteristics make the most difference at those higher frequencies. The videos offer quick and easy ways to learn how to specify PCB materials for 5G, and to get ready for this next revolution in wireless communications.

ROG Mobile App

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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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.

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

Screen Shot 2015-11-23 at 10.05.21 AMAs 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.

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