Circuit designers must often select a circuit technology, such as microstrip or grounded coplanar waveguide (GCPW) circuitry, with a particular design and circuit material to achieve optimum performance. As often detailed in this blog series, circuit materials can be compared by different electrical and thermal characteristics, such as dielectric constant, loss, power-handling capabilities, even bandwidth limits at higher frequencies. Circuit technologies, such as microstrip and GCPW, each have their strengths and weaknesses, and it may help to take a closer look at these two circuit technologies in particular to see how they stack up.
Microstrip circuits are formed by means of thin transmission lines on one side of a high-frequency dielectric printed-circuit-board (PCB) and a conductive metal ground plane on the other side of the PCB. The performance of a microstrip circuit is affected by a number of material-related variables, including the thickness of the dielectric material, the thickness of the conductive metal, even the surface roughness or smoothness of the conductive metal at the copper-substrate interface.
GCPW circuits, also known as conductor-backed coplanar waveguide (CBCPW) circuits, increase the amount of ground around a circuit compared to microstrip by placing ground planes on the bottom of the dielectric material and on the top, on the same plane and on either side of a signal transmission line. GCPW circuit structures achieve electrical stability by literally surrounding a signal line with ground planes.
Both technologies operate by means of a dominant quasi-transverse-electromagnetic (quasi-TEM) propagation mode. GCPW circuits, with their enhanced ground structures, are somewhat more mechanically complex to fabricate. But GCPW circuits also feature low dispersion compared to microstrip circuits, with lower radiation loss than microstrip circuits especially at frequencies extending into the millimeter-wave range.
With their enhanced ground structures, GCPW circuits are capable of wider effective bandwidths than microstrip circuits and wider impedance ranges than microstrip circuits. However, microstrip circuits are relatively robust and easier to fabricate than GCPW circuits, with their straightforward “ground plane on the bottom” circuit structure. In addition, microstrip performance is not as sensitive to circuit fabrication issues as GCPW circuits, with microstrip designs suffering minimal performance variations due to normal etching variation of the conductor/space and conductor thickness.
For a fair comparison of the two high-frequency circuit technologies, several circuits were fabricated on RO4000® series circuit materials from Rogers Corp., including 10-mil-thick RO4350B™ laminate with standard 0.5-oz. electrodeposited (ED) copper. The physical differences in the circuit technologies result in significant differences in the electromagnetic (EM) field patterns around each technology’s transmission lines. In microstrip, most of the EM fields lie between the top signal plane and the bottom ground plane, with high field concentration at the edges of the signal conductors. In GCPW, strong EM fields exist between the ground-signal-ground (GSG) areas on the coplanar circuit layer, with weaker fields lying between the signal plane and the bottom ground plane than for the top and bottom circuit planes of microstrip. The transmission lines in GCPW circuits suffer greater conductor losses than microstrip, but reduced radiation loss compared to microstrip. In addition to low radiation loss, the neighboring ground planes for the GCPW can significantly benefit in the suppression of spurious modes.
Microstrip circuits can provide consistent results even with some inconsistencies in circuit fabrication processes. GCPW circuits are capable of operating at higher frequencies with lower losses than microstrip circuits but are less likely to be unaffected by variations in manufacturing processes. For example, for a microstrip circuit, a thicker copper conductor will result in a slight decrease in a PCB’s effective dielectric constant, with a small improvement in insertion-loss performance. For GCPW, a thicker copper conductor can have a much greater impact, resulting in an increase in the EM fields between the ground-signal-ground structure on the PCB, a decrease in the effective dielectric constant, and decreased conductor loss for that PCB
A great deal of a GCPW circuit’s EM field energy propagates through the air around the circuitry, with its low dielectric constant of 1, rather than through a conductive metal or dielectric material with much higher dielectric constant. The end result is a low net dielectric constant for that GCPW circuit board. Wider conductors used on these GCPW circuit boards can help reduce conductor losses. In addition, thick copper conductors in GCPW circuits can lead to taller conductor walls for those circuits, with a significant portion of EM propagation taking place in the air around the copper conductors and reducing the effective dielectric constant and loss for those circuits.
A GCPW circuit board can be designed and fabricated with loosely and/or tightly coupled circuits, with significant differences in performance between the two coupling approaches. For example, loosely and tightly coupled GCPW circuits will respond differently to the use of conductors with and without a finish, such as electroless nickel immersion gold (ENIG) finish. Because nickel is less conductive than copper, a circuit with an ENIG finish will suffer higher conductor losses than a circuit with bare copper conductors. Furthermore, a tightly coupled GCPW circuit with an ENIG finish will suffer greater conductor loss than a loosely coupled GCPW circuit with the same ENIG finish, especially when those GCPW circuits may involve signal propagation through multiple circuit layers each with copper conductors having its own ENIG finish. Differences in circuit technologies exist due to variations within the same circuit technology, with loss and performance due to such factors as finish, loose or tight coupling, and the width of conductors.
Microstrip and GCPW are both proven high-frequency circuit methodologies and both can provide excellent performance through microwave frequencies and beyond. They offer different approaches to laying out the circuit and, also, the choice of PCB material for a circuit design can have an impact on the final performance possible with each circuit technology. In general, at microwave frequencies, microstrip circuits will suffer less loss than GCPW circuits, especially due to manufacturing variations. But when an application calls for higher, millimeter-wave frequencies, GCPW circuits will suffer less dispersion and radiation losses than microstrip circuits. Also at millimeter-wave frequencies, GCPW circuit approaches provide better bandwidth than microstrip circuits. In addition, when necessary, mode suppression can be achieved more readily with GCPW circuits than with microstrip circuit approaches.
Note: This ROG blog is based on a MicroApps presentation made at the recent IEEE 2015 International Microwave Symposium, May 17-22, 2015, Phoenix, AZ, “Microwave PCB Structure Considerations: Microstrip vs. Grounded Coplanar Waveguide.”
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