Electronics Guide

EBG Design Principles

Designing effective EBG structures requires understanding the relationship between geometric parameters and electromagnetic response. The fundamental resonant frequency of an EBG unit cell determines the center of the bandgap, while the coupling between adjacent cells influences the bandwidth. Key design parameters include the patch size, gap spacing, via diameter (for mushroom-type EBGs), and substrate thickness.

The mushroom-type EBG, consisting of metallic patches connected to a ground plane through vias, represents one of the most common configurations for PCB applications. This structure can be analyzed using an equivalent LC circuit model, where the gap capacitance between patches and the via inductance determine the resonant frequency. The relationship is given by:

f₀ = 1 / (2 * pi * sqrt(L * C))

Where the inductance L depends primarily on the via height and diameter, while the capacitance C is determined by the gap geometry and dielectric properties. More sophisticated designs employ multi-layer structures or fractal geometries to achieve wider bandwidths or multiple bandgaps within a single structure.

Surface Wave Suppression

Surface waves propagating along ground planes and power planes are a significant source of electromagnetic interference in high-speed digital systems. These waves can couple to signal traces, radiate from board edges, and transfer noise between circuits. EBG structures provide an effective means of suppressing surface wave propagation within their bandgap frequency range.

When placed strategically on a PCB, EBG structures create regions where surface waves cannot propagate. This property is particularly valuable for isolating sensitive analog circuits from noisy digital sections, preventing noise coupling through the ground plane. The high surface impedance exhibited by EBG structures within the bandgap reflects incident surface waves, confining energy to specific regions of the board.

Design considerations for surface wave suppression include the placement geometry, the number of EBG periods required for adequate attenuation, and the trade-off between suppression effectiveness and board area consumption. Typically, three to four periods of EBG cells provide substantial attenuation, though demanding applications may require more extensive implementations.

Common-Mode Filtering

Common-mode currents on cables and PCB traces are a primary cause of radiated emissions in electronic systems. These currents arise from asymmetries in the circuit and can be particularly challenging to control at high frequencies. EBG structures offer a unique approach to common-mode filtering by presenting high impedance to common-mode currents while allowing differential signals to pass with minimal disturbance.

In differential pair routing, EBG structures can be placed beneath or adjacent to the traces to suppress common-mode energy at specific frequencies. The differential signal experiences minimal impact because the EBG impedance affects both conductors equally, maintaining the differential mode integrity. Meanwhile, common-mode currents see the high surface impedance of the EBG, which impedes their propagation.

Applications of EBG-based common-mode filtering include high-speed serial interfaces, where common-mode noise can cause EMI failures, and mixed-signal systems where noise coupling between analog and digital sections must be minimized. The frequency-selective nature of EBG filtering allows targeting specific problematic frequencies without affecting the entire signal spectrum.

Power Plane Noise Suppression

Simultaneous switching noise (SSN) in digital ICs creates voltage fluctuations on power planes that propagate throughout the PCB, potentially disturbing other circuits and causing radiated emissions. Traditional decoupling capacitor approaches become less effective at higher frequencies due to capacitor parasitic inductance. EBG structures embedded in power planes offer an alternative approach to high-frequency noise suppression.

EBG-enhanced power planes can be designed to exhibit bandgap behavior at frequencies where SSN is most problematic. By etching periodic patterns into one of the power plane layers, engineers create a structure that blocks noise propagation within the bandgap frequency range. This approach is particularly effective in the gigahertz range, where conventional decoupling becomes inadequate.

Implementation strategies include partial EBG coverage around noise sources, complete EBG power planes, and hybrid approaches combining conventional decoupling with EBG structures. The choice depends on the noise spectrum, board size constraints, and power integrity requirements. Care must be taken to ensure the EBG design does not compromise DC power distribution or introduce unacceptable impedance variations.

Antenna Isolation

In devices containing multiple antennas, such as smartphones with cellular, WiFi, and Bluetooth radios, mutual coupling between antennas degrades performance and increases interference. EBG structures placed between antennas can significantly reduce coupling by suppressing surface wave transmission along the ground plane.

The surface wave contribution to antenna coupling becomes dominant when antennas share a common ground plane, as is typical in compact devices. By placing an EBG structure in the ground plane between antennas, the surface wave path is blocked, reducing isolation by 10 dB or more in many practical cases. This improvement enables closer antenna spacing without unacceptable coupling degradation.

Design optimization for antenna isolation involves matching the EBG bandgap to the operational frequency bands of the antennas being isolated. Multi-band EBG designs can address systems with multiple frequency bands, though increased complexity and area requirements must be balanced against isolation improvements. Compact EBG designs using miniaturization techniques help address area constraints in portable devices.

Miniaturization Benefits

Conventional EBG structures require unit cells with dimensions on the order of a quarter wavelength, which can result in large implementations at lower frequencies. Various miniaturization techniques have been developed to reduce EBG size while maintaining electromagnetic performance, making these structures practical for a wider range of applications.

Spiral inductors replacing simple vias increase the inductance per unit cell, lowering the resonant frequency for a given cell size. Similarly, interdigitated capacitor structures between patches increase capacitance, providing another path to miniaturization. Combining both techniques enables size reductions of 50% or more compared to conventional mushroom EBGs.

High-permittivity substrates also contribute to miniaturization by increasing the effective capacitance and reducing the propagation velocity within the structure. However, higher permittivity materials may introduce additional losses and manufacturing challenges that must be weighed against the size benefits. Hybrid approaches using localized high-permittivity regions under EBG patches offer a compromise between miniaturization and practical fabrication.

Bandwidth Limitations

A fundamental challenge with EBG structures is their limited bandwidth compared to the broad frequency ranges over which EMC problems can occur. A typical mushroom EBG might exhibit a bandgap spanning 20-30% of the center frequency, leaving significant frequency ranges unaddressed. Understanding and mitigating these bandwidth limitations is essential for successful EBG implementation.

Multi-band EBG designs address bandwidth limitations by incorporating multiple resonant structures within the unit cell or using stacked configurations. These approaches create multiple bandgaps at different frequencies, providing coverage across a wider spectrum. However, multi-band designs increase complexity and may require additional PCB layers or larger unit cells.

Wideband EBG approaches focus on extending the bandwidth of a single bandgap through techniques such as fractal geometries, tapered structures, or active loading. Fractal EBGs use self-similar patterns that resonate at multiple related frequencies, creating overlapping bandgaps that merge into a wider effective bandwidth. While promising, these advanced approaches often require careful optimization and may be sensitive to manufacturing variations.

Fabrication Methods

EBG structures for PCB applications are typically fabricated using standard printed circuit board manufacturing processes. The most common mushroom EBG requires at least two metal layers (patch layer and ground plane) connected by vias. This configuration can be incorporated into existing PCB stack-ups with minimal additional processing, making EBG implementation cost-effective for volume production.

Critical fabrication parameters include via registration accuracy, etching tolerances for gap dimensions, and layer-to-layer alignment. Modern PCB processes readily achieve the tolerances required for EBG structures operating at typical EMC frequencies (hundreds of megahertz to several gigahertz). Higher frequency applications may require tighter tolerances or alternative fabrication approaches.

For applications requiring extreme miniaturization or very high frequencies, advanced fabrication techniques such as LTCC (low-temperature co-fired ceramic) or embedded passives technology may be employed. These processes enable finer features and tighter tolerances than standard PCB fabrication, though at increased cost. Semiconductor fabrication techniques can realize EBG structures for on-chip applications, though the limited available area constrains their EMC utility.

Measurement Techniques

Characterizing EBG performance requires specialized measurement techniques that capture the structure's electromagnetic behavior. Surface wave transmission measurements using probes placed on opposite sides of the EBG region provide direct assessment of bandgap effectiveness. The transmission coefficient (S21) shows significant attenuation within the bandgap frequency range.

Reflection measurements reveal the surface impedance characteristics of EBG structures. Within the bandgap, the surface impedance becomes very high, approaching open-circuit conditions. This property is directly related to the EBG's ability to suppress surface waves and can be measured using waveguide or coaxial probe techniques.

Near-field scanning provides spatial visualization of electromagnetic fields above EBG structures, revealing how energy is reflected, absorbed, or channeled by the periodic array. This technique is particularly valuable for understanding field behavior at boundaries and transitions between EBG and conventional regions. Time-domain measurements can characterize the transient response of EBG structures to pulsed excitation, relevant for applications involving digital switching noise.

Summary

Electromagnetic bandgap structures offer powerful solutions for challenging EMC problems by creating frequency-selective barriers to electromagnetic wave propagation. From surface wave suppression and power plane noise reduction to antenna isolation and common-mode filtering, EBG technology addresses interference mechanisms that are difficult to control with conventional approaches. While bandwidth limitations and area requirements present design challenges, advancing miniaturization techniques and multi-band approaches are expanding the practical applicability of EBG structures in modern electronic systems.