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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 (www.rogerscorp.com) 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. website, www.rogerscorp.com/acm/technology/index.aspx, 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.
Note: This Blog is based on a paper being presented by the author, John Coonrod, at the 2013 IPC APEX EXPO® Conference & Exhibition (www.ipcapexexpo.org, February 19-21, 2013, San Diego, CA) as part of a “High Frequency” session. The full paper, “Insertion Loss Comparisons of Common High Frequency PCB Constructions,” provides greater details on the three transmission-line technologies and their loss mechanisms.
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.
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