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How Defective Ground Structures Can Be Used in RF PCBs

defective ground structure

RF systems and high-speed digital systems rely on ground to provide noise suppression and isolation against external EMI sources. This has to be balanced against impedance along an interconnect with the goal of hitting a particular target. The use of ground both sets impedance and modifies impedance looking into a component or connector, and one way to modify impedance in an RF system is to use a defective ground structure (DGS).

A DGS is essentially like a ground cutout that can be applied in a certain section of a system such that the impedance in that section is modified. The result is a modification of the input impedance looking into the region with the defective ground structure. In cases where there is an impedance mismatch between two portions of an interconnect, a defective ground structure is one of the tools that can be used in general to provide impedance matching.

Impedance Matching and Filtering With Defective Ground

A DGS can be used on an interconnect as part of filtering or impedance matching. A DGS is not required for impedance matching, and it is not the most common method of applying impedance matching or filtering to an RF interconnect. Typically, you will see the following forms of impedance matching or filtering in an RF interconnect:

  • LC circuit matching, such as a T-filter or Pi-filter
  • Applying a taper between two portions of an interconnect
  • Applying a transmission line stub or group of stubs
  • Using an attenuator or filter IC

A DGS is not any kind of special structure or component in a PCB layout. It is simply a ground cutout with a particular shape below some microstrip line in the PCB. In this way, it modifies the characteristic impedance in that region of the PCB within some frequency band.

By modifying the ground beneath some region of the interconnect, you are removing parasitic capacitance to ground in that region. A common case is removal of ground beneath an impedance matching network with the goal of reducing any capacitance that could modify the circuit behavior. Another instance is to modify the input impedance into a section of transmission line by removing the ground beneath it.

DGS Modifies Input Impedance and Bandwidth

When applied to a pad on a component or a connector that is connected to a transmission line, the DGS reduces the self-capacitance to ground and increases the self-inductance in that region. Therefore, you have effectively increased the characteristic impedance in that region. This then modifies the input impedance looking into the receiving component.

An example with an SMA edge connector with a connected feedline is shown below. For the moment, assume GND is on L2. Because the pad for the connector is somewhat large, it could have a smaller characteristic impedance than the attached transmission line. This would then affect the input impedance. By adding a ground cutout below the connector pad, the region with cutout could have its impedance become closer to the impedance of the attached transmission line.

defective ground structure

So how large should the defective ground structure be? It depends on the impedance mismatch between the target pad and its attached feedline. Typically, this kind of interconnect might also be built with a taper to match the line and connector pad impedances.

DGS for Filtering on a Microstrip Line

Another usage of a DGS, which is in some ways equivalent to impedance matching, is as a filter on a microstrip line. In the image below, a dumbbell-shaped DGS is applied below a microstrip line leading to the connector shown in the above image. The structure of a dumbbell DGS can be tailored to provide any of the standard filtering functions and can be modeled as RLC circuits. An electromagnetic field solver needs to be used to determine the filtering functionality of a DGS placed below a transmission line.

defective ground structure filter

Example dumbbell filter placed below a microstrip line.

Together, these two structures provide filtering and impedance matching as a signal travels to the connector. Other filtering structures can be used to tailor the system’s response to an input signal as it travels to the connector while maintaining impedance matching.

Reduced Capacitance On a Component

There are other instances where a DGS is used that do not necessarily involve radio frequency systems. Instead, you might see these uses arise generally in some precision analog systems. For example, one particular case occurs in precision measurements with ADCs and op-amps. An example is shown below.

defective ground structure

Example DGS placement near the input of an op-amp.

This type of structure would be used in these situations for three reasons:

  1. The input capacitance on the I/O is very low (from 1-10 pF), and the nearby ground creates additional capacitance that increases the response time of the buffer.
  2. The input is not impedance controlled so there is no trace impedance requirement to worry about.
  3. Depending on the feedback loop reactance, the ground capacitance might modify a pole in the feedback loop and shift or gain any oscillation frequency.

For these reasons, you may see a recommendation for a DGS in an op-amp or ADC. The exception above is in RF ADCs, which do have an impedance specification that needs to be hit. In these devices, DGS ground recommendations may vary and should be simulated before finalizing a design.

When you’re ready to simulate a DGS structure in your RF interconnects, make sure you use the complete set of system analysis tools from Cadence to evaluate systems functionality. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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