As signal speeds increase with new communication protocols and technologies, board structures must provide the necessary level of support for signal integrity and performance (both of which tend to decrease with higher frequencies). While many transmission options are available during fabrication beyond the standard microstrip and stripline varieties, selection will depend on the frequency (or frequency band) and secondary concerns, such as how the fabricated line fits into the board’s design constraints. On the higher end of the frequency spectrum sits the coplanar waveguide: while the theory of coplanar waveguide design is not a new topic, its usage is only now becoming more prominent due to its compatibility with millimeter-wave technology. Optimizing coplanar waveguide performance depends on a solid understanding of electromagnetism and the various fabrication limitations for the transmission line.
Controlled Impedance Trace Structures
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Coplanar Waveguide Design Can Improve High-Speed Performance
Coplanar waveguides (CPWs) are center-strip conductors with ground return strips on either side, separated by a constant air gap. Since the return path is in the same plane as the signal trace (most traces have a return path to a ground plane above or below the signal layer, forming a current loop), it travels in a loop coplanar to the signal. Theoretically, the return strips need to extend semi-infinitely relative to the signal-carrying trace, but practically, a large enough in-plane width suffices. Like standard traces, a CPW can combine with a ground plane above/below the signal layer for increased isolation.
It’s worth noting that despite the express designs of the CPW, some portion of the electric and magnetic fields associated with the driving signal will exist out-of-plane – given that the mediums on either side of an outer layer signal trace are different (i.e., air and substrate), the dielectric constant will differ, resulting in some of the electromagnetic fields associated with the driving signal projecting out of the plane. In transmission theory terminology, CPWs are a quasi-transverse electromagnetic mode (TEM). Unlike a true TEM, CPWs have a portion of the E/M field projecting out of the plane, which is small enough relative to the in-plane field that it is negligible. The quasi-TEM quality provides CPWs with low dispersion (keeping the response mostly constant across the frequency band) and minimal dielectric loss for high signal integrity.
There are multiple benefits to incorporating a CPW into a board over a standard trace, should the design call for it:
- Design – The design of microwave circuits using CPW is much more dense than similar circuits, allowing significant scalability (including up to millimeter-wave and microwave IC technology). At the system level, CPW design can balance compactness and integrability with automated placement and soldering equipment. A few ECAD tools feature significant options for simulating CPW design to account for shorts, opens, and lumped circuit elements while accounting for complex CPW variants like multi-port junctions.
- Manufacturing – A CPW simplifies fabrication by reducing the need for other signal integrity structures, like via holes and wraparound edge plating. The characteristic impedance of the CPW is much simpler than that of a microstrip, being only a ratio of the signal trace thickness over the total air gap distance (including signal trace).
- Performance – The planar ground surrounding the signal lines diminishes signal radiation greatly, even for high-speed lines, making it an effective method for separating extremely high-speed signals from other signals (and vice versa). A CPW shows much higher gain capabilities than an equivalent signal trace employing via techniques by reducing the parasitic source inductance since the ground “plane” is closer to the signal.
Transitions to and from coplanar waveguides deserve a special mention. Depending on the circuit integration, there are two methods for signal transfer to a different transmission line medium at the boundary of the waveguide: electromagnetic and direct coupling. Except for coupling between ground conductors, a secondary substrate is necessary to transition from one transmission line to another. The effect on the signal is equally important as the physical construction, as filter behavior such as low-pass or band-pass often accompanies these transitions; designers must include this behavior in any models or simulations to minimize the practical filter effects during production.
Design Appropriateness for Coplanar Waveguides
The primary use of CPWs is to create printed antennas, as high speeds can wreak havoc on standard microstrips or purely component-built antennas. The signal line of the CPW terminates in a square or rectangular patch to produce an antenna that directs the signal in a direction normal to the board. A traditional microstrip antenna patch creates a higher amount of cross-polarized radiation – that is, field misalignment orthogonal to the intended polarization direction – that reduces the power output, resulting in greater inefficiency and thermal dissipative effects that can cause premature aging of the board.
However, CPWs can be design-limiting: the dielectric constant seen by the signal is approximately half of the substrate plus one-half (the average of the substrate and air’s dielectric constant), which means worse heat dissipation as the potential energy stored in the electric field dissipates. The width of the signal line can vary, but the width incurs greater transmission line losses as it shrinks. This factor, combined with the need for a thick substrate, reduces the miniaturization of the circuit compared to microstrip implementations. At the same time, the thicker substrate can save on per-board costs because backside processing is unnecessary for CPW designs.
Your Contract Manufacturer Is On The Same Frequency
Coplanar waveguide design requires knowledge of what was once an esoteric transmission line (though recent developments have brought it to the forefront) and a PCBA manufacturer that understands fabrication processes. While coplanar waveguides have some unique requirements, general layout best practices still apply to ensure minimal interference to or from the high-speed line. Here at VSE, our team of engineers is committed to building electronics for our customers, and that includes a robust review of the circuitry and layout to reduce any apparent points of poor signal integrity before production, reducing the number of revisions and time spent before production finishes. We’ve built life-saving and life-changing boards alongside our manufacturing partners for over forty years.
