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 originally appeared on the ROG Blog hosted by Microwave Journal.

Transmission lines are akin to electronic roadways, routing signals along different paths of a printed circuit board (PCB). At RF/microwave frequencies, circuit designers often create PCBs based on three popular planar transmission line approaches: microstrip, stripline, or coplanar waveguide (CPW). Each uses circuit-board materials in a different way, with different results in terms of insertion-loss performance. By getting a grasp on the insertion-loss mechanisms for these different transmission-line formats, circuit designers can better match the mechanical and electrical characteristics of their circuit substrates to their intended applications and transmission lines when choosing PCB materials.

Achieving low loss in an RF/microwave circuit is more critical for some applications than for others, and many excellent low-loss commercial PCB materials such as RO4350B™ laminates from Rogers Corporation are available to help optimize a circuit’s loss performance. But the choice of transmission line for a design can also impact the insertion-loss performance of that circuit. The insertion loss of a PCB’s transmission lines is actually the sum of a number of contributing losses, such as losses attributed to the conductors, to the dielectric material, and due to radiation from the PCB. Microwave transmission lines can also suffer leakage losses, although these tend to be associated more with semiconductors than with PCB materials.

Conductor losses are related to the type of metal (and possible finish on the conductor metal) in the PCB’s conductor layer as well as the operating frequency. Signal propagation at higher frequencies tends to use less of the conductor’s metal as the frequencies increase, with signal “skin depth” becoming very shallow at the highest operating frequencies and only the outer surface of the conductor used for signal propagation at the highest frequencies.

An ideal electrical conductor would exhibit minimal resistance and high conductivity for signals of interest. Of course, real conductors do exhibit loss and have imperfections, including surface roughness, which can contribute significantly to a conductor loss. At RF/microwave frequencies, a rough conductor surface represents a longer propagation path than a smoother conductor surface, with higher loss. A PCB’s dielectric loss is related to the material properties of the circuit substrate, in particular its dissipation factor (Df). Selecting circuit materials with low Df can help minimize this component of transmission-line insertion loss.

Radiation loss is due to energy passed by a PCB’s transmission lines into the surrounding environment. This insertion-loss component can be affected by a number of factors, including the choice of transmission-line topology, the PCB’s dielectric constant, the operating frequency, even the circuit-board thickness. It tends to decrease with thinner PCB materials and for circuit materials with higher dielectric constants. Radiation losses are most noticeable at junctions in a circuit, including impedance transitions and signal launch areas, such as the transition from a transmission line to a coaxial connector’s center pin. Of the three popular RF/microwave transmission-line formats, microstrip is particularly susceptible to radiation loss.

Each of the transmission-line technologies suffers some insertion loss, no matter how good the PCB material. Understanding how loss occurs for the different transmission-line approaches can help guide a circuit designer when choosing a PCB material for a given loss budget. As mentioned, microstrip can suffer more from radiation loss than stripline or CPW, requiring additional shielding for some microstrip circuits. But microstrip is the most popular of the three transmission-line formats, since it is the simplest and least expensive to fabricate. It is basically a metal conductor on the top of a dielectric layer with a metal ground plane on the bottom of the dielectric layer. Factors that can influence performance include the type and weight of the metal for the conductor and ground plane, the width of the conductor lines, the relative permittivity or dielectric constant of the dielectric material, and the thickness of the dielectric layer.

In contrast, stripline transmission lines are sandwiched between top and bottom dielectric layers, which in turn have metal ground planes on the top and bottom of the dielectric materials. Plated through holes (PTHs) are machined through the metal and dielectric layers to electrically connect the top and bottom ground planes. Stripline presents difficulties in adding discrete circuit elements and active devices, which require viaholes to connect components on the outside of the circuit to the internal circuitry and transmission lines. This is in contrast to the simplicity of top-mounting components on a microstrip board. CPW circuits offer the simplicity of top-mounting components, since these circuits are formed with top-layer conductors surrounded by a top-layer ground plane, and with an additional bottom-layer ground plane separated by a dielectric layer. As with stripline, the top and bottom ground planes are electrically linked by PTHs machined through the substrate material. The additional ground planes help improve electrical performance but also add size, complexity, and cost to the stripline and CPW circuits compared to microstrip circuits, which are among the tradeoffs that circuit designers must weigh when choosing a transmission-line format for a particular circuit application.

How does the choice of PCB material impact the insertion loss of one of these high-frequency circuits? The loss characteristics of a microstrip circuit, for example, will change for different thicknesses of the same PCB material. A free personal computer (PC) software tool, MWI-2010, available for download from the Technology Support Hub on the Rogers Corp. web site, can show the influence of a circuit material on transmission-line loss. MWI-2010 contains models of different circuit board materials, permitting designers to explore the impact of different material parameters on performance.

The software was used to analyze the impact of substrate thickness on microstrip transmission-line loss, modeling simple 50-Ω microstrip transmission-line circuits on three different thicknesses (6.6, 10, and 20 mils) of RO4350B circuit material. The material has a process dielectric constant of 3.48 at 10 GHz and low dielectric loss, with Df of 0.0037 at 10 GHz. For microstrip transmission lines, the software shows that the insertion loss is the least for the thickest circuit board, with conductor and dielectric losses that were relatively low and similar in value. The thinnest circuit board had the highest insertion loss, with conductor loss the dominant of the three loss components. Conductor loss can be somewhat diminished by choosing a PCB material with smooth conductor metal, such as RO4000® LoPro™ circuit material from Rogers Corp. The dielectric loss changed little with the three thicknesses of RO4350B laminate, indicating it is an electrically stable PCB substrate.

When loss is critical for a circuit, a low-loss circuit material can help achieve design goals by minimizing dielectric losses. And conductor and radiation losses can be controlled through choice of transmission-line technology, although that choice will also depend on a number of other factors, such as required circuit size, complexity, and cost.

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.