Frequently Asked Questions
- Q: What are electromagnetic bandgap (EBG) structures?
A: Electromagnetic band-gap (EBG) structures refer to arrays of two-dimensional or three-dimensional periodic objects that inhibit the propagation of electromagnetic waves in a particular frequency band (the bandgap) irrespective of the direction of propagation. WEMTEC has pioneered the development of various types of EBG structures that suppress modes on parallel-plate waveguides.
- Q: What are the broad areas of application for WEMTEC’s technology?
A: The two main areas of application for WEMTEC’s technology are:
- To suppress noise on power distribution networks at the printed circuit board and package level.
- To suppress parasitic modes inside shielded microwave and millimeterwave packages.
- Q: Which end-use products could be improved with WEMTEC’s technology?
A: Printed circuit boards and RF packages are the backbone of a variety of electronic devices including computers, cell phones, automobile electronics, power supplies, and sophisticated defense systems.
- Q: What is special about WEMTEC’s EBG technology?
A: WEMTEC has patented many different EBG structures that suppress modes on parallel-plate waveguides. Some patents teach very low profile designs of EBG structures that are 5x to 10x thinner that previous technology that has the same cutoff frequency. Other WEMTEC patents teach very low frequency EBG structures whose stopbands extend down to 30 MHz, or about 10x lower than competing EBG structures. These low frequency EBG structures have unit cell sizes that are as small as 1/800 of a free-space wavelength. EBG technology is the only known way to suppress parallel-plate mode noise above 1 GHz.
EBG Structures in Power Distribution Networks (PDNs):
- Q: Which is the best EBG structure for power plane noise suppression?
A: Unfortunately, there is no single EBG structure that works best in every application. All have strengths and weaknesses. EBG structures need to be tailored to the application. Some types, such as hybrid EBG structures, can have a 100:1 bandwidth for the stopband, but they are limited in current carry capability to a few amps or less. Fortunately, WEMTEC has developed many different types of parallel-plate EBG structures with a spectrum of features and benefits.
- Q: What challenges or disadvantages might exist for the integration of WEMTEC technology ?
A: There are several key performance parameters that limit the application of internal EBG structures for parallel-plate mode suppression in PDNs.
- The single biggest issue is the size of the unit cell in the EBG structure. Everyone wants a smaller unit cell to fit into their available board or package real estate. And at least a 3 x 3 array of unit cells is required to achieve the filtering properties of EBG structures.
- The next biggest challenge is bandwidth, namely to achieve a wide stopband. The frequency of noise in today’s PCBs covers multiple decades.
- The third challenge is to lower the cutoff frequency of the fundamental stopband. This is the lowest frequency where the EBG structure becomes a bandstop filter. Most PCB designers will tell you that their pain is trying to suppress noise in the 100 MHz to 2 GHz frequency range. Most published EBG structures will not work below 400 to 600 MHz, and even then their unit cell size is about 30 mm square!
- Q: Will the new generation of embedded capacitance layers mitigate the need for EBG structures in PDNs?
A: No. The thin and sometimes high dielectric constant cores that are generically called embedded capacitance layers will function as a higher frequency bypass capacitor than convention chip caps. However, the use of an embedded capacitance core in a PDN will not prohibit the propagation of noise, and it increases the density of resonant modes in a printed circuit PDN due to the relatively high dielectric constant, which may go up to 16 for some brands.
- Q: Are embedded capacitance layers competitive or complementary with WEMTEC’s EBG technology?
A: The embedded capacitance layers are a complementary technology because these layers may be used in most EBG structures to lower the frequency of the stopband or to reduce the unit cell size for a given cutoff frequency. The very thin dimension of the embedded capacitance layer may be used to enhance the capacitance per unit cell for many types of resonant via EBG structures.
- Q: Why is noise suppression important in a power distribution network (PDN)?
A: Noise in a PDN can disrupt the otherwise predictable operation of digital electronics. This may result in data loss or the loss of certain digital functions. In RF receivers, power supply noise can reduce sensitivity or even de-sense the receiver such that it will not be able to receive the intended signal. Power supply noise may also cause undesired modulation of various analog and RF signals. Finally, high frequency noise can radiate from printed circuit boards at levels beyond legal limits for radiated emissions.
- Q: What issues impede the adoption of EBG structures in electronic packaging?
A: The issue has been one of market perception that EBG structures are too large or the frequency of operation is too high. This perception is understandable given that most if not all of the early publications (2003-2005) on power plane noise suppression where not of practical value to the PCB designer. Many EBG related papers were presented by university researchers and showed early results which were not ready for industry adoption. More recent developments of hybrid EBG structures mitigate some of these issues.
- Q: Are there any current RF products or applications where EBG structures are being used?
A: Not yet. EBG structures for noise suppression at the board and package level are still quite new. Most PCB and package designers are not aware of the possibilities.
- Q: What are the software tools that traditionally simulate and analyze the PDN noise challenges which WEMTEC’s technology addresses?
A: Software tools for power integrity analysis include SIwave from Ansoft, Power Si from Sigrity, and Power Grid from Apache. Other more general purpose electromagnetic analysis tools may also be used, such as 3D full-wave simulators from Ansoft and CST.
- Q: Is the implementation of EBG structures into printed circuit boards something that can be done with standard PCB design and assembly processes, or would a new tool set need to be developed?
A: Since WEMTEC EBG structures may be implemented as planar printed structures using conventional cores and metal layers, then standard layout tools such as those offered by Cadence or Mentor Graphics may be used for patch and via layout. Layout can be done for organic printed circuit boards, multilayer ceramic modules, or even silicon wafers. However, the design and validation of an EBG structure to satisfy certain frequency goals will require a more specialized tools set. An initial design may be created using effective medium models found in proprietary Mathcad codes developed by WEMTEC. These codes implement fast analytic methods. However, any EBG structure should be modeled in detail using a full-wave electromagnetic simulator prior to fabrication. WEMTEC uses Microstripes, a time domain transmission line matrix code, but other tools such as HFSS from Ansoft, or Microwave Studio from CST will also work well for validation.
- Q: How do power integrity engineers measure the merit of a power distribution system?
A: A fundamental figure of merit is the self impedance at a given point in the power distribution network. A lower self-impedance is better. Low values of self-impedance will inhibit the generation of noise power.
Another fundamental figure of merit is the transfer impedance between two points in the power distribution network. Again, the lower the value the better. A low value of transfer impedance will inhibit the propagation of noise power between those two points.
- Q: What do EBG structures built into a power distribution network do to the self and transfer impedances?
A: The primary function of an EBG structure in a power distribution network may be understood in terms of impedance. Its function is to lower the self and transfer impedance of the network over the frequency range associated with the stopband. Lowering the self and transfer impedances will result in lowering the generation and propagation of noise power, respectively.
- Q: What is often cited as the most difficult frequency range to suppress noise at the printed circuit board (PCB) level?
A: PCB engineers often cite the frequency range of 100 MHz to 2 GHz as the most difficult frequency range to suppress noise. Many waveforms used in digital systems have fundamental and harmonic frequencies that fall in this range.
- Q: What technology does WEMTEC offer to address the 100 MHz to 2 GHz frequency range?
A: WEMTEC has developed hybrid EBG structures which are a combination of resonant via arrays and inductive grids. Many embodiments of hybrid EBG structures are possible including designs with chip capacitor resonant vias in each unit cell. Such designs offer stopband ratios of 30:1 or greater that cover the 100 MHz to 2 GHz frequency range.
- Q: Are all EGB structures inherently narrowband in operating frequency?
A: No, this is a common myth. WEMTEC has developed numerous EBG structures for parallel-plate mode suppression which are inherently wideband. For instance, resonant via arrays typically have between one to three octaves of stopband bandwidth. WEMTEC has developed hybrid EBG power planes with more than 30:1 stopband bandwidth, and a bandwidth of more than two decades (100:1) is possible using hybrid EBG structures with conventional low cost chip caps and core materials.
- Q: What is the minimum unit cell size for an EBG structure in a PDN?
A: The answer is dependent on the type of EBG structure. Hybrid EBG structures can employ chip caps as part of the resonant vias, and this will dramatically lower the cutoff frequency, or the edge of the fundamental stopband, to well below 100 MHz. In fact, hybrid EBG structures with a 30 MHz cutoff and a ½ inch period have been demonstrated in conventional FR4 printed circuit boards.
EBG Structures Integrated into RF Packages:
- Q: What are the benefits of integrating EBG structures into multilayer RF ceramic packages?
A: There are multiple benefits:
- Parasitic cavity modes internal to the shielded package may be suppressed over the frequency range of the fundamental stopband.
- Crosstalk between internal transmission lines can be suppressed.
- For packages with integrated antenna elements, external EBG structures can be used to help control the pattern characteristics: beamwidth, side lobe level, and front-to-back ratio.
- For packages with an integrated diversity antenna system, an external EBG structure can reduce the mutual coupling between antenna elements.
- Q: What are cavity modes?
A: Many RF packages are shielded for EMI and EMC reasons. Shielded packages contain metalized cavities that resonate at a multitude of discrete frequencies. Resonant modes, or cavity modes, can be excited by interconnects, such as bond wires, or by transmission lines, such as microstriplines, within the RF package.
- Q: Why are cavity modes a problem?
A: Cavity modes are excited with RF power that would otherwise be used to perform a useful function. These parasitic modes show up in RF system measurements as undesired nulls in insertion loss, and as undesired peaks in return loss. Phase response may also vary unpredictably at frequencies near a cavity resonance.
- Q: Why is an EBG structure useful inside an RF package?
A: At frequencies above 10 GHz, EBG structures are small enough to be fabricated into the cover and/or base of a shielded RF package. EBG structures can be designed to cutoff parallel-plate waveguide (PPW) modes. When PPW modes reflect off the walls of a shielded package, they resonate to become cavity modes. When PPW modes are excited by a given signal interconnect or transmission line, and couple into a nearby interconnect or transmission line, that is undesired crosstalk. EBG structures can suppress the propagation of PPW modes over a limited band of frequencies thus improving RF system performance by suppressing crosstalk and eliminating cavity modes.
- Q: What does a typical EBG structure look like in an RF package?
A: EBG structures are periodic, or repeating, metal and dielectric structures. They may be realized by a 2D array of metalized vias that connects a metal plane (ground plane) to an array of printed metal patches. Some EBG structures have multiple arrays of patches on different dielectric interfaces. Multiple layers of overlapping patches increase the capacitance per square of the patch array, resulting in a lower frequency stopband and/or a smaller unit cell size. The patches may be square, hexagonal, or any polygonal shape. The array of vias may have a square or triangular lattice depending on the patch geometry.
- Q: What is the primary figure of merit for an EBG structure that is used to suppress parallel-plate waveguide (PPW) modes?
A: The effectiveness of an EBG structure can be plotted on a 2D graph as attenuation in dB per unit cell versus frequency. The more area under the curve, the better. The frequency span of this curve is the bandgap, or stopband.
- Q: How are attenuation plots calculated?
A: Attenuation plots (dB per unit cell vs. frequency) are calculated as:
where kx is the propagation constant for the Bloch wave that travels through the periodic EBG structure, and P is the unit cell period.
- Q: How is the Bloch wave propagation constant kx calculated?
A: This propagation constant is calculated numerically from a dispersion equation, which is the eigenvalue equation for guided waves in the periodic structure. A straight forward way to derive the dispersion equation is the transverse resonance method where an equivalent transmission line is used to model each layer in the inhomogeneous PPW.
- Q: What is a Bloch wave?
A: A Bloch wave is a electromagnetic mode that propagates within an infinite periodic structure. This wave exhibits passbands and stopbands because of the multiple reflections from neighboring unit cells.