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Successful microstrip circuit design requires a great many factors to fall neatly into place, not to mention high consistency and performance from the printed-circuit-board (PCB) material. Maintaining tight microstrip circuit tolerances helps ensure predictable impedances from those transmission lines, assuming a consistent relative dielectric constant across the PCB. With so many challenges in designing and fabricating high-performance microstrip circuits, why would so many RF and microwave circuit designers currently be interested in adding “defects” to their circuits? As strange as it may sound, the use of circuit defects is a growing trend in high-frequency circuit design, especially for passive circuits such as filters. More precisely, the trend is in the increased use of defected ground structures (DGSs) and defected microstrip structures (DMSs) to alter the responses of microstrip circuit designs.
Just what are these DGS and DMS forms, and does incorporating them into a high-frequency circuit change the way the PCB material should be specified? Although both types of structures are referred to as “defects,” they are well planned and calculated, essentially resonant gaps or slots that are placed in a PCB’s ground plane or transmission lines, respectively, to achieve modifications in impedance. By inserting a gap in the ground plane underneath a narrow microstrip transmission line, for example, a much higher impedance can be achieved than with a traditional microstrip transmission line having the same dimensions. Impedance transitions are useful in a number of different RF/microwave circuit designs, including lowpass filters which can be formed with a cascade of high- and low-impedance sections.
A DGS allows a circuit designer to insert a transmission zero or notch anywhere in the transfer function of a microstrip transmission line. This type of added attenuation can sharpen the rolloff of a bandpass filter or deepen the stopband attenuation of a lowpass filter. Designers have employed this phenomenon in both active and passive circuits. In active circuits, DGS circuit elements have helped improve the efficiency of power amplifiers. In passive circuits, they are often used to fine-tune a filter’s response or improve the performance of a patch antenna. Even simple DGS and DMS forms can help extend a filter’s stopband by adding attenuation at the resonant frequencies represented by the physical gaps in the PCB’s ground plane or microstrip transmission lines. In recent years, designers have become quite creative in exploring the possibilities of DGS and DMS forms and shapes in high-frequency circuits, eschewing simple slots or gaps in a ground plane, for example, for dumb-bell shapes and meander lines in an attempt to achieve higher-impedance transitions in smaller PCB areas.
As part of distributed circuit design, DGS and DMS forms are treated as distributed circuit elements, more or less as inductors. They can replace a conventional distributed transmission-line inductor while possibly minimizing circuit loss. In terms of practical PCB fabrication, however, DGS and DMS circuit elements must be treated with care. While they can be used to add selective notches in a circuit’s frequency response, they will not improve passband insertion loss, which will largely be a function of the PCB’s material parameters and the transmission-line characteristics. The best way to achieve low passband insertion loss in a filter is still by choosing a low-loss laminate, such as Rogers RO4000® or RO3000® circuit materials, which are available with a wide range of dielectric constants and dissipation factors as low as 0.0013 at 10 GHz. In addition, by using circuit materials with a high relative dielectric constant, such as RO3010™ or RT/duroid® 6010.2LM laminates, both with dielectric-constant value of 10.2 at 10 GHz, the benefits of DGS and DMS circuit elements can be readily applied to edge-coupled filter designs.
DGS and DMS circuit elements can increase the size of a circuit, especially when these structures are fabricated as more element forms, such as meander lines. A DGS essentially acts like a resonant circuit in parallel with the transmission line above it, and the dimensions of the DGS and its position relative to the transmission line will determine the impact of the DGS on the circuit’s response. The precision with which the DGS or DMS form can be fabricated can also affect the circuit’s response. If the DGS is thought of as an equivalent circuit element, such as an inductor, in a distributed circuit design, then the value of that inductor will change according to the dimensional tolerances of the DGS and its alignment to the transmission line. Even the consistency of the PCB’s dielectric constant can play a role in the inductance of a DGS within a circuit. When applying these types of “defects” to a high-frequency circuit, they should be designed well within microstrip fabrication tolerances and taking into account the consistency of the PCB material’s relative dielectric constant.
If DGS and DMS circuit elements are so finicky, how is it possible to integrate them into practical RF/microwave circuit designs? For the most part, both types of structures are modeled by means of electromagnetic (EM) simulation software, based on a number of different analytical methods, such as the method of moments (MoM) or the finite-element method (FEM). Through EM simulation, it is possible to estimate the effects of DGS and DMS placement, dimensional tolerances, and even how using circuit laminates with different values of permittivity affect expected performance. However, it is only fair to note that DGS elements can act as radiators as well as resonators. A slot in a laminate’s ground plane essentially forms a slot antenna. Fortunately, most of the incident energy of a DGS slot at its resonant frequency is reflected back into the circuit’s transmission lines, although depending upon the circuit topology, some of this energy can be coupled into different circuit features. Such coupling effects are difficult to model in a practical EM simulation.
This blog has only touched upon some of the unconventional structures being used by RF/microwave circuit designers in search of improved performance at higher frequencies. A future blog will return to this topic with a closer look at some of the other stripline and microstrip circuit structures that are finding application in high-frequency circuits, including torroidal inductor structures using holes in the ground plane, substrate integrated waveguide (SIW), and split-ring resonators as used in terahertz-frequency metamaterials. And for those involved in automotive electronics, the next blog will explore the role of PCB materials in helping automotive manufacturers meet their performance and cost budgets for automotive electronic systems.
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