High-frequency circuit designers have a number of different circuit types from which to provide solutions from radio frequency (RF) through millimeter-wave frequencies and coplanar waveguide (CPW) might be an approach to consider as an option to popular microstrip techniques. Traditional CPW circuitry consists of a conductor separated by a pair of ground planes, on the same plane on top of a dielectric material. A variation on that circuit approach is grounded coplanar waveguide (GCPW), also known as conductor-backed coplanar waveguide (CBCPW). It adds a ground plane to the bottom of the basic CPW circuit structure, with plated through holes (PTHs) connecting the top and bottom ground planes.
The absence of PTH ground connections in CPW results in less radiation at discontinuities than GCPW, although both circuit types feature superb isolation of adjacent signal channels, resulting in low crosstalk for densely packed circuits. The placement of PTHs in GCPW circuits can be critical for achieving impedance and loss goals, but the use of these grounding PTHs can allow the use of a much thicker dielectric material for a given higher frequency than when using microstrip circuits. The added ground plane in GCPW circuits provides additional mechanical stability compared to CPW circuits, with improved thermal management for higher-power circuits and devices. Microstrip circuits typically suffer increased surface-wave leakage and radiation losses compared to GCPW circuits when using the same circuit materials and at higher microwave and millimeter-wave frequencies.
Both CPW and GCPW circuits are fairly straightforward to fabricate and provide consistent performance through millimeter-wave frequencies. The electromagnetic (EM) energy through the transmission lines, especially for GCPW, remains largely concentrated within the PCB’s dielectric material. Both CPW and GCPW circuits can suffer higher conductor losses than microstrip circuits, but the loss characteristics of CPW and GCPW circuits follow a constant slope with frequency, whereas microstrip undergoes loss transitions at the upper microwave frequencies associated with radiation losses. Such losses can be minimized in CPW and GCPW designs through proper spacing of PTHs and other circuit dimensions.
At higher frequencies, such as millimeter-wave frequencies, circuit dimensions become smaller and more critical as the lengths of circuits begin to approach the dimensions of the wavelengths of the signals propagating through those circuits. Selecting a suitable circuit technology can be critical at higher frequencies, as reflections and radiation losses can increase for microstrip and even stripline circuits at higher frequencies. When treated properly, CPW and especially GCPW can provide excellent results at higher frequencies, and are often even combined with microstrip. In many cases, GCPW provides practical, high-performance interfaces at connector interfaces before making a transition to microstrip transmission lines for the remainder of a high-frequency design.
A CPW circuit includes a conductor fabricated between two ground planes on the top surface of a dielectric layer, in a ground-signal-ground (GSG) configuration on the top of the circuit material. By using a ground conductor that is coplanar with the signal conductor, CPW circuits use the signal line width and gap between the ground and the conductor to control the impedance. The impedance can be kept constant as the width of the signal conductor is tapered to form a connection with a connector pin.
A GCPW circuit adds a ground plane to the bottom of the dielectric layer and a means of interconnecting the ground areas, such as PTHs. In a CPW circuit, the EM energy is mainly concentrated within the dielectric material. Leakage of CPW and GCPW EM energy into the air can be controlled by maintaining a substrate height that is at least twice the conductor width. The characteristic impedance is established essentially by the width of the center conductor and the spacing between the conductor and the ground planes. For CPW, the characteristic impedance can typically be set between 20 and 250 ohms, compared to about 20 to 120 ohms for microstrip circuits and 35 to 250 ohms for stripline circuits.
Coupling For CPW/GCPW
As with any circuit technology, CPW and GCPW have positive and negative aspects, and tradeoffs that must be weighed for different applications, for both analog and digital circuits. The top-layer ground strips in CPW and GCPW circuits can produce even- and odd-mode current flow that cause mode coupling in those circuits. When tight spacing is maintained between the GSG traces of CPW and GCPW circuits, those circuits deliver tight coupling with excellent mode suppression and low radiation losses at higher frequencies. Loss can be minimized when wider spacings are used between the signal trace and the two top-layer ground strips, especially when a wider signal trace (with lower conductor loss and insertion loss) is used. When the signal conductor is widely spaced from the ground traces, in a loosely coupled configuration, the tradeoff for lower conductor and insertion losses is a possible increase in radiation loss and spurious distortion. Finding the right level of coupling in a CPW or GCPW circuit will provide a balance between low loss and good spurious mode suppression.
In general, thinner circuits are better for minimizing radiation loss than thicker circuits. For millimeter-wave circuits of 30 GHz and higher, radiation losses can become a significant part of a circuit’s total loss. Radiation loss also depends on a PCB material’s Dk value, with circuit materials having higher Dk values suffering less radiation loss than circuit materials with lower Dk values. Since tradeoffs must always be considered, those materials with higher Dk values usually exhibit higher conductor losses than circuit materials with lower Dk values since narrower signal conductors are needed for a given impedance on circuit materials with higher Dk values.
Copper surface roughness affects the electric field and current flow for a PCB. The effective dielectric constant increases as the surface roughness of the copper conductor increases. Copper surface roughness can also impact a PCB’s insertion loss, although with less effect on a GCPW circuit than on a microstrip circuit. On the GCPW circuit, the electric field and current are tightly maintained within the GSG configuration of the PCB whereas on the microstrip circuit, the electric field and current move towards the bottom of the conductive metal, towards the roughness of the metal.
The use of CAE software tools, such as EM simulators, can help find the appropriate values for the various CPW and GCPW circuit parameters, such as conductor width, circuit material thickness, and separation between ground planes,which contribute to circuit performance goals. During a technical presentation at the 2009 Automatic RF Techniques Group (ARFTG) by Sonnet Software’s (www.sonnetsoftware.com) father and son team of James and Brian Rautio (“High Accuracy Broadband Measurement of Anisotropic Dielectric Constant Using a Shielded Planar Dual Mode Resonator”), methods were detailed for measuring the Dk values of different circuit materials, even for CPW and GCPW circuits across ranges of different key parameters. The presentation (available for free download from the company’s website) included measurements to 10 GHz on the anisotropic RO3010™ circuit material from Rogers Corp. (www.rogerscorp.com), with the capability to separate even- and odd-mode material resonances numerically. The measurements determined two dielectric constant values for a given material: one for the horizontal electric field (parallel to the substrate surface) and one for the vertical electric field. This report shows how different parameters could be analyzed when considering circuit materials for different frequencies and applications and provides valuable guidance for those specifying circuit materials for CPW and GCPW designs as well as for stripline and microstrip configurations.
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