Miniaturization in electronic design is a driving force behind increasing use of multilayered printed-circuit boards (PCBs). Circuits with layers occupy more space in the vertical dimension than the horizontal dimension and make it possible to stack designs into compact housings. The number of layers usually refers to the conductor layers, which are also separated by dielectric material layers. With such a large current choice in dielectric materials and wide ranges of material properties and thicknesses, circuit developers can achieve performance goals in tight places with multilayer circuit design approaches. Some niche RF design concepts can ensure that performance levels measured in multilayer circuit prototypes meet or exceed those levels in production.

Multilayer PCBs with RF/microwave circuitry often employ multiple high frequency circuit technologies on the outer layers, including microstrip, stripline, and grounded coplanar waveguide (GCPW) circuits. These circuit technologies can be implemented with defected ground structures (DGSs). DGSs, originally developed for microstrip resonators, are intentional flaws created in the ground plane of a PCB, usually as etched-out patterns. Properly applied, they can help with current flow, effective wavelength reduction, and signal isolation.

Designing and fabricating circuits with DGSs has historically been a concern due to their susceptibility to electromagnetic (EM) radiation. DGS models within circuit and EM simulation software tools have traditionally been limited. But, as computers and circuit simulators have grown, models for DGS have also improved to where the behavior of most circuits even with custom DGS can be simulated accurately within reasonable computer processing times.

Eliminating Emissions

DGSs can be employed in multilayer circuits without the concerns for unwanted EM emissions when used in simpler circuits. For example, in a circuit with the top copper layer serving as microstrip signal conductor, dielectric material, then copper layer beneath acting as the microstrip ground plane, the DGS can be realized as a void in the layer 2 ground plane. It will be realized and placed with the dimensions needed to achieve a notch in the filtered frequency response and rejection at a desired frequency or band of frequencies. For a circuit with only the two conductive layers—the filter circuitry and ground plane—the void in the ground plane might be a potential source of EM radiation. For a multilayer circuit, however, a third copper layer can suppress radiation emitted by the DGS. In addition, closely placed grounding viaholes around the DGS can help isolate any EM radiation between copper layers 2 and 3 in a three-layer PCB.

DGSs can be beneficial for the performance of many types of RF/microwave circuits without adding to the challenges of a circuit fabricator even when applied to multilayer circuit structures. In one such case, a stepped-impedance lowpass filter was realized in a three-layer PCB structure.[1] The filter achieves areas of high impedance with narrow conductors and areas of low impedance with very wide conductors (see figure). Filter performance, such as rejection, can be enhanced by increasing the impedance difference between the low- and high-impedance areas. The high-impedance value is typically limited to the characteristics of the dielectric material and the copper etching resolution of the fabrication process.

Using DGS forms in the high-impedance areas, however, the impedance can be increased significantly without risking etching problems at the circuit fabricator. The increase in impedance helps minimize unwanted harmonics while improving the filter’s stopband response. To illustrate, the figure shows the top copper signal of a two-layer lowpass PCB filter in dark orange and the bottom copper layer or ground plane in light orange, with DGS voids in the ground plane as white areas. In a multilayer lowpass PCB filter, those white areas representing the DGS voids would appear on the second copper layer, giving access to the third copper layer as the ground plane in the DGS areas.

Fig 1. This lowpass filter incorporates narrow conductors at high impedances and wider conductors at lower impedances, with performance impacted by the difference between the impedances. 

This design flexibility afforded by DGS for RF lowpass filters can also be used at millimeter-wave (mmWave) frequencies. The use of DGS can be especially beneficial for microstrip series-fed patch antenna arrays commonly used in mmWave radars. DGS can help adjust the mutual coupling between the array’s radiating patches, reduce unwanted frequency responses, and suppress sidelobes. Due to its impact on impedance, integration of DGS permits the use of wider conductors between radiating patches for a given impedance. Wider conductors can be fabricated with higher yields when attempting to produce PCBs such as patch antenna arrays in high-volume manufacturing processes.

An alternative approach to DGS in microstrip patch antenna arrays is the use of stripline conductors to feed the array’s radiating patches. Stripline is a low-loss conductor at high frequencies with minimal radiation loss and high isolation, but it requires an etched opening through the PCB to connect the stripline feeding structure to the microstrip radiating patches fabricated on the conductor layer above the stripline layer. Fortunately, these apertures through the PCB can be fashioned to provide excellent coupling to the radiating patches and outstanding antenna performance at mmWave frequencies.

The use of dissimilar circuit materials can provide excellent RF/microwave performance in miniature, multilayer circuits. As an example, coupled stripline circuits can be formed with four-layer PCBs, with the top (layer 1) and bottom (layer 4) copper layers serving as the ground planes and the two inner copper layers (layers 2 and 3) working as coupled signal conductors. Since coupling increases with circuit material Dk, if the two inner copper conductor layers are fabricated on a circuit material with high Dk, the coupling coefficient will be high and the coupled RF energy between the two layers will be high.

As with coupling in the microstrip patch antenna array, any use of dissimilar circuit materials for high frequency coupling in stripline circuits must be carefully modeled because of the different even- and odd-mode phase velocities resulting from the different Dk values of the dielectric materials. Increased phase velocity differences can result in increased levels of unwanted responses.

The use of circuit materials with low and high Dk values can be beneficial to a multilayer microstrip edge-coupled structure.[2] As an example, in a multilayer PCB with three copper layers, a high-Dk dielectric material separates the top copper layer (layer 1) and layer 2 beneath it while a low-Dk dielectric material separates copper layer 2 from the copper layer (layer 3) beneath it. In certain areas, copper is removed so microstrip conductors on copper layers 1 and 3 are isolated by a sandwich of dielectric materials with two different Dk values.

Microstrip edge-coupled filters yield unwanted resonances due to differences in their even- and odd-mode phase velocities. This is because coupling fields travel through different effective Dk media: through air and dielectric material in the odd mode but only through the dielectric material in the even mode. By controlling the ratio of the high-Dk to low-Dk material thicknesses, the odd-mode coupling fields will still travel in part through air and will also travel through the high-Dk dielectric material but not the low-Dk material. By optimizing the ratio of the high-Dk to low-Dk material thicknesses, the phase velocities of the two modes can be made as possible to equal, reducing unwanted harmonics and improving filter stopband performance.

These examples represent just a handful of the many opportunities to use a multilayered PCB structure for enhanced RF performance. To learn more about the benefits of multilayered PCBs, please contact your local Rogers’ representative or visit our Rogers Technology Support hub.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

References

[1] John Coonrod, “Multilayer PCB Technology Supports Microstrip DGS Without Radiation Loss,” Microwave Journal, Vol. 61, No. 2, February 2018.

[2] John Coonrod, “Harmonic Suppression of Edge Coupled Filters Using Composite Substrates,” Microwave Journal, Vol. 55, No. 9, September 2012.