Miniaturization of electronic designs is a driving force behind the widespread use of multilayer printed circuit boards. Multilayer circuits occupy more vertical space than horizontal space, allowing designs to be stacked in a compact space. The number of layers in a circuit refers to the number of conductor layers in the circuit, usually separated by dielectric layers. Due to the wide range of dielectric materials to choose from, with a variety of material properties and thicknesses, circuit developers can achieve performance goals in a very small space by adopting a multilayer circuit design approach. Certain unique RF design concepts can ensure that the measured performance levels of multilayer circuit prototypes meet or exceed the performance levels after production.

Multilayer printed circuit boards for RF/microwave circuits usually use a variety of different high-frequency circuit technologies on the outer layers, such as microstrip, stripline, and grounded coplanar waveguide circuits. These circuit technologies can be designed as defective ground structures to optimize and improve RF performance. Defective ground structures were originally developed for microstrip resonators, and they are etched patterns that purposefully create copper hollowing areas on the ground plane of the circuit board. When properly applied, defective ground structures help improve current flow, effectively reduce wavelengths, and achieve signal isolation.

The design and fabrication of circuits that use defective ground structures have always been a concern because they are susceptible to electromagnetic radiation. Historically, models of defective ground structures within circuits and software tools for electromagnetic simulation have been limited. However, as computers and circuit simulators have advanced, defective ground structure models have improved, and even circuits that use custom defective ground structures can accurately simulate the behavior of most circuits within reasonable computer processing time.

Suppressing radiation

When the circuit is relatively simple, there may be no need to worry about the generation of unnecessary electromagnetic radiation from the defective ground structure in the multi-layer circuit. For example, in some multi-layer structures, the top layer is the signal layer, followed by the dielectric material, the second copper layer, the dielectric material, and the third copper layer. At this time, the defective ground structure can be designed in the second layer. If a filter needs to be designed, the required dimensions can be designed in the second layer to form a suppression depression at the required frequency or frequency band to achieve the required frequency response of the filter. For circuits with only two conductive layers (i.e., the filter structure circuit and the ground layer), defects in the ground layer may be a potential source of electromagnetic radiation. However, for multi-layer circuits, the third layer can suppress the radiation emitted by the defective ground structure. In addition, the dense ground vias around the defective ground structure help isolate any electromagnetic radiation between the second and third copper layers in the three layers.

Defective ground structures can help improve the performance of many RF/microwave circuits, and even when applied to multi-layer circuit structures, they do not pose additional challenges to circuit manufacturers. For example, a step-impedance low-pass filter is implemented in a three-layer circuit structure. ^[1] This step-impedance filter uses narrow conductors to achieve high impedance and wide conductors to achieve low impedance (as shown in the figure). By increasing the impedance difference between high and low impedance, the filter performance (e.g., stopband region) can be enhanced. However, the realization of high impedance values ​​is usually limited by the properties of the dielectric material and the etching accuracy of the manufacturing process.

However, using defective ground structures in high impedance areas can significantly increase impedance while avoiding the risk of etching narrower lines for circuit manufacturers. The increase in impedance helps to reduce unwanted harmonics and improve the filter’s stopband response. For ease of explanation, the following two-layer low-pass filter is used as an example. The dark orange shows the top signal of the circuit, the light orange shows the bottom or ground layer, and the defective ground structure in the ground layer is represented by the white area. If a multi-layer low-pass filter is used, the white area representing the defective ground is located on the second layer, and the third layer below is the complete ground layer of the filter.

This design flexibility of defective ground can also be used in some circuit designs at millimeter wave frequencies. The use of defective ground structure is very beneficial for microstrip line series powered patch antenna arrays commonly used in millimeter wave radars. This structure helps to adjust the mutual coupling between the array radiating patches, reduce useless frequency response, and suppress side lobes. With the defective ground structure, wider conductors can be used in the feed lines between the radiating patches to meet specific impedance requirements. Wider conductor feed lines are conducive to further improving production yields in the mass production process of patch array antennas.

Another way to use defective ground structures in microstrip patch antenna arrays is to use stripline conductors to feed the array’s radiating patches. Stripline is a high-frequency, low-loss transmission line structure that achieves minimal radiation loss and high isolation. Through openings etched into the circuit board, the stripline feed structure can be connected to the microstrip radiating patch located on the conductor layer above the stripline. Fortunately, this etched opening allows good coupling to the radiating patch for feeding and provides good antenna performance at millimeter-wave frequencies.

Using different circuit materials can enable miniaturized multilayer circuits with superior RF/microwave performance. For example, coupled stripline can be realized with a 4-layer circuit structure, where the top layer (layer 1) and the bottom layer (layer 4) serve as ground planes, and the two inner layers (layers 2 and 3) serve as coupled signal conductor layers. Since coupling strength increases with the dielectric constant (Dk) of the circuit material, a higher coupling coefficient and interlayer coupling can be achieved if the two inner conductor layers are made on a circuit material with a higher dielectric constant.

It is important to note that high-frequency coupling using different circuit materials in stripline circuits must be carefully modeled because the difference in dielectric constant of the materials will result in different even-mode and odd-mode phase velocities. When the phase velocity difference is large, some unwanted responses will increase, which may affect the circuit performance.

The use of circuit materials with different dielectric constants is beneficial for multilayer microstrip edge coupling structures. ^[2] For example, in a multilayer board containing three copper layers, a material with a high dielectric constant is used between layers 1 and 2, and a material with a low dielectric constant is used between layers 2 and 3. Thus, in some areas of layer 2 where there is no copper foil, two dielectric materials with different dielectric constants separate the first and third layers.

Microstrip edge-coupled filters produce unwanted resonances due to the difference in phase velocities between the even and odd modes. This is because the coupling field passes through media with different effective dielectric constants: through air and dielectric material in odd mode, but only through dielectric material in even mode. By controlling the thickness ratio of high and low dielectric constant materials, the odd-mode coupling field will still partially pass through air and will also pass through the material with a higher dielectric constant, but will not pass through the material with a lower dielectric constant. By optimizing the thickness ratio of high and low dielectric constant materials, the phase velocities in the two modes can be made as equal as possible, thereby reducing unwanted harmonics and improving the filter stopband performance.

These are just a few examples of the many applications where multi-layer printed circuit board construction is used to enhance RF performance. To learn more about the benefits of multi-layer circuits, please contact your local Rogers representative or visit one of our Rogers Technical Support Centers.