Patent Application
20150173256
The present invention relates generally to the field of printed circuit board design, and more particularly to reducing EMI in high frequency printed circuit board design.
Electromagnetic interference (EMI) is the generation of undesired electrical signals in electronic system circuitry when an electromagnetic (EM) wave in one electrical circuit, component or part is unintentionally transferred to another electrical circuit, component or part. As electronic systems are evolving, the density of electrical components in these systems is increasing, and the dimensions of circuit elements correspondingly decrease. As spacing decreases between circuit elements, the likelihood of interference from adjacent circuit elements increases.
Circuit elements are effective in radiating electromagnetic energy or spectral components which have wavelengths similar to the radiating element dimensions. Therefore, as electronic system size decreases and electronic system density increases, electromagnetic emissions occur in greater strength from the system. The current market direction for electronic devices with increasing operating frequencies are moving from the gigahertz (GHz) range to 10 GHz and greater while the devices decrease in size, creates significant EM radiation generation opportunities in electronic devices. With the increasing demand for high speed and portable devices, the reduction of EMI is becoming a larger design consideration.
Electronic product manufacturers take several approaches to minimizing EMI for electromagnetic compatibility compliance. Shielding products to prevent electromagnetic (EM) radiation is a common technique. At the system or board level metal cages or metal covers may be installed to prevent the radiation of electromagnetic waves causing EMI. While commonly done, this can be an awkward and costly solution to meeting EMC requirements. In smaller, portable devices running with higher frequencies, metal covers or shields may be hard to implement. In printed circuit board (PCB) design, placement of high frequency devices may be planned to avoid devices sensitive to EMI. Careful device placement may reduce the size of card covers used to reduce EMI. Designs that focus on suppression of unwanted electromagnetic radiation can provide a more cost effective solution to reducing EMI. Suppression of EMI before is it transmitted outside of the system or enclosure is critical to EMC compliance.
The generation of excess EM radiation in devices is problematic to other components within the electronic device and to other nearby electronic devices. Unwanted signals generated from excess EM radiation, from within a device and from nearby electronic devices, leads to EMI in electronic devices. For this reason regulatory committees exist both nationally and internationally to monitor and define limits of allowable EMI for various electronic devices. Requirements and tests are defined for electromagnetic compatibility (EMC) that monitor EM radiation causing EMI between systems and components for electronic devices.
The present invention provides methods and structures to design electronic circuit elements for frequency suppression of electromagnetic waves using a transmission line grating. The method includes determining a frequency to be suppressed. Using the electrical wavelength of the frequency to be suppressed to determine a length of a plurality of equal length sections of the differential transmission lines. Utilizing the frequency to be suppressed and the electronic circuit requirements, determine a set of properties for the plurality of equal length sections of the differential transmission lines. The transmission line grating is created with a plurality of alternating equal length sections of the differential transmission lines.
Embodiments of the present invention provide methods and structures for suppression of electromagnetic waves using a transmission line grating design technique. In particular, some embodiments of the present invention provide methods and structures for suppression of specific frequencies of the common mode electromagnetic waves using a transmission line grating technique. One skilled in the art will recognize that concepts developed in exemplary embodiments of the present invention could be applied to other multilayered circuit substrates, for example laminate chip carrier design, ceramic chip carrier design, flex cables, and other multilayer circuit substrates including semiconductor design applications.
Detailed embodiments of the claimed structures and methods are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments is intended to be illustrative, and not restrictive. Further, the figures are not necessarily to scale, and some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the methods and structures of the present disclosure.
References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the disclosed structures and methods, as oriented in the drawing figures. The terms “overlying”, “atop”, “positioned on” or “positioned atop” mean that a first element, such as a first structure, is present on a second element, such as a second structure, wherein intervening elements, such as an interface structure may be present between the first element and the second element. The term “direct contact” means that a first element, such as a first structure, and a second element, such as a second structure, are connected without any intermediary conducting, insulating or semiconductor layers at the interface of the two elements.
The transmission line grating design technique described in the present invention is used to suppress targeted frequencies of electromagnetic waves. The suppression of targeted electromagnetic wave frequency in a circuit reduces the electromagnetic radiation produced by electronic devices. More specifically, targeted or selected frequencies of the common mode electromagnetic waves in a circuit may be used to reduce electromagnetic radiation produced by the electronic devices. The elimination of a targeted frequency or frequencies that are generating unwanted electromagnetic waves or noise in an electronic circuit aid in the reduction of EMI. EMI reduction is increasingly important in high speed, high frequency electronics products where miniaturization and portability drive much of the marketplace.
The transmission line grating technique described herein can be applied to numerous applications in electronics. The transmission line grating may be applied to PCB board design, flexible cable design, chip carrier design, chip design or other electronic components that can utilize differential pair transmission lines. The transmission line grating technique described requires little to no change to existing manufacturing lines and little additional cost to the component. The specific embodiments described in this application address the application of this technique in PCB design; however, one skilled in the art can apply this concept to other electronic devices. For example, a flexible cable using a transmission line grating may be design and manufactured. A flexible cable using the transmission line grating technique reduces noise from suppressed frequencies within a system and/or the EMI radiated outside of system when the flexible cable is used externally.
In differential transmission lines, there are both common mode and differential mode currents, both of which determine the amount of radio frequency (RF) energy that is developed and propagated. Both common mode and differential mode of currents usually exist in a differential transmission line. The differential mode current or signals carry the data or signal of interest (information). The common mode current which carries no useful information is a side effect or byproduct of the differential mode signal transmission. The common mode AC current or “noise” generates EM radiation which is troublesome for EMC compliance.
Radiated emission for a differential mode current or signal is the component of RF energy that is present on both the signal and return paths that are opposite to each other in differential pairs. In designs using differential signaling, an inverted signal is precisely established so that the common mode AC currents will be cancelled out.
In most high speed signal interconnects, the use of differential signaling includes a differential transmitter, a differential receiver, and a differential data path including differential signal pairs, vias, and eventually connectors and cables. An ideal differential transmitter sends a set of two identical signals with opposite polarity. When this occurs, the amplitude of the differential mode which is the difference of the two signals is double that of the single signal while the AC amplitude of the common mode signal is always zero.
In most real world environments, many things cause a notable or non-zero common mode AC amplitude. In particular, differences in rise and fall times in single-ended signals and the difference in duty cycle of single-ended signals have been observed to create common mode noise around the even harmonics of the fundamental signal frequency in PCBs. The fundamental frequency of a data signal is the inverse of two times the single bit time. Additionally, the fundamental frequency of a clock is the inverse of the period of the clock which may be used in some embodiments.
Other sources of common mode currents may include ground bounce, power plane fluctuations, and differential signals that are not precisely inverted where the portions of the signal that do not cancel out create common mode currents. The common mode current is also present in both the signal and return paths. The RF field resulting from the common mode current is due to the sum of the currents that exist in both the signal and return path. This sum can be substantial since common mode currents lack differential mode cancellation effects.
The transmission line grating technique described herein targets the suppression of a specific frequency, in particular a specific common mode frequency in a circuit, while protecting the differential mode signal from change. Utilizing this transmission line grating technique within a PCB structure effectively suppresses common mode noise while preserving the signal integrity of the differential mode signal. In particular, it can suppress common mode even harmonics radiation, empirically shown as a common source of EMI. Common mode even harmonics radiation, a common source of EMI, is usually caused by imperfections of the output signals of high frequency signal transmitters that show up as a narrow spike in the emissions spectrum. The common mode radiation constitutes a major concern during electromagnetic compatibility (EMC) testing. EMC testing is required of many electronic products and is regulated by standards for each nation with advisory guidance from the International Electrotechnical Commission. The suppression of common mode even harmonic radiation in PCBs before it is transmitted outside of the enclosure or box is critical to pass required EMC tests.
High frequency designs often require the use of differential signaling and differential signal pairs. A differential signal pair is a pair of identical transmission lines. Differential signal pairs generally are symmetrical to each other and their surrounding structures. In general, a differential signal pair will have a constant spacing between each other and are driven from a differential transmitter. The electrical properties of a differential pair, for example characteristic impedance, are defined by the geometric dimensions including line width, line thickness, line spacing and line distance to reference or ground planes and the dielectric material surrounding the pair. Maintaining a constant differential characteristic impedance which closely matches the differential characteristic impedance of the initial differential transmission lines (w0/s0 in
The present invention will now be described in detail with reference to the Figures.
In the strip-line cross-section, the transmission lines are surrounded by PCB dielectric material or dielectric layers 101 which are usually composed of epoxy/glass although other dielectric materials may be used. The differential transmission lines 120 and the dielectric layers 101 are covered with reference planes 111 and 112, usually made of copper foil. Reference plane 111 is on top of dielectric layers 101 while reference plane 112 is below dielectric layers 101, thus encasing transmission lines 120 in dielectric layers 101 which are covered with reference planes 111 and 112. Each of the transmission lines in the differential transmission lines 120 has width w0 and thickness t0. The pair of transmission lines 120 have a space, space s0, between them. The thickness of the various layers in the PCB cross-section is determined by system requirements and the manufacturing capabilities for the PCB. While the exemplary embodiment of the present invention uses a strip-line design for the PCB cross-section of the transmission line grating, other embodiments may use a micro-strip design for the cross-section.
The transmission line grating depicted in
In some embodiments of the invention, there may be more than two different widths and respective spacing for the differential transmission lines. Some transmission line gratings may have alternating sections with multiple different line widths for example.
The length of each of the individual sections, length L, is equal to one quarter of the electrical wavelength of the frequency band that is to be suppressed. The frequency to be suppressed and the effective dielectric constant of the material determine the wavelength. For example, in the PCB design shown, the frequency for suppression is the frequency of the second harmonic of the fundamental frequency of the circuit. Accordingly, the length of each section, length L, is equal to one quarter of the wavelength of the second harmonic of the fundamental frequency of the electronic circuit. The steps of a method to create a transmission line grating design are discussed below in
In step 304, the designer determines the frequency to be suppressed. In an embodiment of the present invention, the transmission line grating is designed to suppress the frequency of the second harmonic frequency of the fundamental frequency; however the transmission line grating technique described may be applied to suppress other frequencies.
In step 306, the designer determines the line width and the line spacing of the initial transmission lines in the differential pair before and after the transmission line grating (e.g., w0 and s0 in
In step 310, the designer selects the line width and the line spacing for the narrow line width sections, section 240 of transmission line grating 200. Section 240 illustrates the first section of the sections with line width w1 and line spacing, s1 and length L
In step 312, the designer selects the line width and the line spacing for the wide line width sections, section 260 of transmission line grating 200. Section 260 illustrates the first section of wide line width sections with line width w2 and spacing s2 as shown in
The transmission line grating consists of alternating equal length sections of narrow and wide transmission lines. The width of either the narrow width lines or the wide width lines may be equal to the width of the initial transmission lines connected to the transmission line grating. Both the width of the narrow width lines (w1 in
In step 314, the designer determines the section length for each section of the transmission line grating (e.g., L in
In step 316, the designer determines the number of sections to be used in the transmission line grating. The larger the number of sections, the better the selected frequency suppression. The number of sections may be limited by the available card real estate and the cost associated with a larger PCB. The transmission line grating may be placed on any desired internal layer. In other embodiments of the present invention, the transmission line grating may be used on an external surface. When the transmission line grating is used on an external surface, the designer should take into account micro-strip line design characteristics, for example differing dielectric constants, not present in a strip design.
In step 402, the designer (e.g., a human designer or a programmatic designer, etc.) determines the fundamental frequency of the transmission lines under design in the PCB.
In step 404, the designer determines the frequency to be suppressed. In an embodiment of the present invention, the transmission line grating is designed to suppress the frequency of the second harmonic frequency of the fundamental frequency; however the transmission line grating technique described may be applied to suppress other frequencies.
In step 406, the designer determines line width of the initial transmission lines in the differential pair before and after the transmission line grating (e.g., w0 in
In step 408, the designer determines the line spacing for the initial transmission line of the differential transmission lines (e.g., s0 in
In step 410, the designer selects the line widths for the transmission line grating (e.g., w1 and w2 in
In step 412, the designer determines the spacing for the different line widths in the transmission line grating (e.g., s1 and s2 in
The manufacture and cost of the PCB place some limitations on line width and spacing. For example, narrow transmission lines for the grating may have process limits for the PCB manufacturing plant or processes. Additionally, PCB cost targets may limit minimum line widths and spacing. Similarly, line width and line spacing may be limited by the electrical properties fundamental to a differential pair (too far apart the transmission lines are not a pair electrically) while narrow line widths may also introduce too much insertion loss.
In step 414, the designer determines the section length for each section of the transmission line grating (e.g., L in
In step 416, the designer determines the number of sections to be used in the transmission line grating. The larger the number of sections, the better the selected frequency suppression. The number of sections may be limited by the available card real estate and the cost associated with a larger PCB. The transmission line grating may be placed on any desired internal layer. In other embodiments of the present invention, the transmission line grating may be used on an external surface. When the transmission line grating is used on an external surface, the designer should take into account micro-strip line design characteristics, for example differing dielectric constants, not present in a strip design.
Therefore, considering both
Embodiments of the present invention suppress EMI by a design that uses the transmission line grating to filter out or suppress unwanted frequencies causing EM radiation. The design of a PCB using the transmission line grating reduces the common mode noise normally produced by the PCB. By reducing the common mode noise at a specific frequency in the PCB, the transmission line grating technique depicted reduces PCB related EMI.
