Achieve High Routing Density with Grounded Coplanar Waveguide Technology

Key Takeaways

• Coplanar waveguides are planar transmission lines consisting of signal conductors in the center and semi-infinite ground conductors placed on both sides.

• For easy implementation of high-frequency RF and microwave circuits, advanced versions of coplanar waveguides have been developed.

• Grounded coplanar waveguides are waveguides in which the coplanar side grounds are connected to the lower ground plane using vias.

Modern communication employs components and systems that are capable of displaying, transmitting, and processing information at an extremely high data rate. The components and systems utilized for high data rates in wireless and satellite communication include superconducting transmission lines, printed circuit antennas, photonic bandgap structures, tunable devices made of ferroelectric materials, etc. Most of these systems and components are embedded on printed circuit boards using coplanar waveguide technology. A variant of coplanar waveguides, called grounded coplanar waveguides, offsets the use of conventional microstrips when working on thick dielectric substrates. In this article, we will discuss the evolution of grounded coplanar waveguides and their advantages.

Coplanar Waveguides

Generally, circuit technologies such as microstrip or coplanar waveguides are selected in high-frequency analog and digital circuits due to their optimum performance. Among these two, coplanar waveguides are most common in printed circuit boards to embed high-frequency analog components or other RF systems.

Coplanar waveguides are planar transmission lines consisting of signal conductors in the center and semi-infinite ground conductors placed on both sides. The signal conductor and ground planes are fabricated on a dielectric substrate. Coplanar waveguides are most commonly employed in RF and microwave integrated circuits and PCBs. Their advantages include:

• Ease of fabrication
• Reduction in radiation loss
• Easy direct integration of passive and active components onto transmission lines
• No impedance variation over frequency
• Wider effective bandwidth than microstrips
• Wider impedance range than microstrips
• Low dispersion effect

However, there were some disadvantages to coplanar waveguides, leading to the development of advanced coplanar waveguide structures and hybrid or mixed transmission line technology. Coplanar waveguide structures generate higher losses than microstrip transmission line circuits. The sensitivity to circuits and structures placed adjacent to coplanar waveguides makes it difficult to miniaturize circuit dimensions. 

Coplanar Waveguide Variants

For easy implementation of high-frequency RF and microwave circuits, advanced versions of coplanar waveguides are used.

Conductor-backed coplanar waveguides (CBCPWs): In CBCPWs, an additional ground plane is introduced at the bottom of the dielectric substrate. The lower ground offers mechanical support, interfacing for heat sink structures, and prevents the field from coupling to the interconnects in the lower layers of the circuit.

Conductor-backed coplanar waveguides with finite side grounds (FB CBCPWs): FB CBCPWs optimize the width of the side grounds. FB CBCPWs also support the dominant coplanar waveguide mode when fabricated with a finite-width substrate. 

Grounded Coplanar Waveguides

In our discussion regarding FB CBCPWs, we saw that the side ground width is an important parameter in exciting higher-order waveguide modes in coplanar waveguides. As the side ground width increases, it excites more higher-order modes. To suppress the higher-order modes, a new structure called grounded coplanar waveguides (GCPWs) can be introduced.

A grounded coplanar waveguide is a modified version of FB CBCPWs, in which the coplanar side grounds are connected to the lower ground planes using vias. Connecting the side grounds to the lower ground using vias suppresses the higher-order coplanar waveguide modes in the grounded coplanar waveguide. The vias in the grounded coplanar waveguide are called mode selectors, as they influence the coplanar waveguide modes. The ground planes in the grounded coplanar waveguide that enclose the signal conductor act as an electromagnetic shield. The ground plane surrounding the grounded coplanar waveguide offers high isolation and decreases the radiation losses associated with it. Enclosing the signal conductor by ground planes helps boost the electrical stability of grounded coplanar waveguides.

The distance between the signal conductor strip and grounds in grounded coplanar waveguides is less than the carrier frequency of the signal and greater than the substrate thickness. This arrangement of the grounded coplanar waveguide helps contain the electromagnetic field between the signal strip and the lower ground plane. It also ensures low impedance and the capability to tune the impedance by adjusting the spacing between the ground layer and the signal conductor. The signal propagation is dependent on the via gap. A via gap of less than half of the carrier frequency enables transverse electromagnetic (TEM) propagation in grounded coplanar waveguides.

The use of grounded coplanar waveguide technology increases the routing density of the printed circuit boards being designed. As the grounded coplanar waveguide allows some control over the number of traces that can be routed through the space between the adjacent vias, the circuit layout is optimized. This optimization reduces the number of layers in the PCB, thereby increasing the routing density and decreasing PCB costs. 

The choice of transmission line technology can influence the performance of high-frequency circuits for RF and microwave applications. Grounded coplanar waveguide technology is a strong candidate for RF designs, particularly for production volume applications at high radio frequencies. Cadence’s software can help you design grounded coplanar waveguide technology-based RF circuits. 

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