Patent Application
20140177150
The present invention generally relates to stripline transmission lines. In particular, the present invention relates to techniques for reducing crosstalk in stripline systems.
Printed circuit boards may be used in a variety of computing devices, such as laptop computers, desktop computers, mobile phones, tablet computers, and other computing devices. However, the performance of the computing devices may be negatively affected by crosstalk within the printed circuit boards.
Certain exemplary embodiments are described in the following detailed description and in reference to the drawings, in which:
Embodiments disclosed herein provide techniques for reducing crosstalk in stripline systems. High speed performance in a computing system is limited by the negative impact of crosstalk. On a platform, crosstalk can be divided into two components, namely vertical and horizontal. Vertical crosstalk can be attributed to vertical components, such as vias, connectors, and sockets, while horizontal crosstalk is attributed to horizontal components, i.e., signal line to signal line. The combination of horizontal and vertical crosstalk degrades overall system performance.
Each of the vertical and horizontal crosstalk may further be divided into far end crosstalk and near end crosstalk. Near end crosstalk is crosstalk formed at the same end of a victim signal line at which a stimulus is input on an aggressor line. Near end crosstalk is generally illustrated as a level and is not cancellable. Rather, near end crosstalk generally enters the victim line and terminates. Far end crosstalk is crosstalk propagated on a victim line away from the end at which a stimulus is input on an aggressor. Far end crosstalk generally acts as a pulse and may be cumulative, negatively affecting the performance of a platform, such as a platform of a computing device.
Crosstalk may be reduced by increasing spacing, both between the vertical components and between the horizontal components. However, increasing the spacing may decrease routing density on a board or package, resulting in increased layer count and increased cost. In addition, vertical components cannot be easily spaced out due to size constraints. Vertical crosstalk may be reduced by adding more ground vertical components between the signal vertical components. However, adding more ground vertical components increases both the cost and size of the package and fails to completely eliminate the vertical component crosstalk.
Horizontal crosstalk may occur between horizontal components 102. Vertical crosstalk may occur between vertical components 104. The polarity of the vertical crosstalk is opposite the polarity of a stimulus. The horizontal crosstalk will combine with the vertical crosstalk of the stripline topology. If the polarity of the horizontal crosstalk is also the opposite to the polarity of the stimulus, and therefore the same polarity as the vertical crosstalk, the horizontal and vertical crosstalk will add to each other and the crosstalk of the stripline system will increase. However, if the horizontal crosstalk is the opposite in polarity to the vertical crosstalk, the horizontal crosstalk will destructively combine with the vertical crosstalk and the crosstalk of the stripline topology will decrease, or even cease completely.
A crosstalk reducer 106 may be disposed between horizontal components 102. The crosstalk reducer 106 may be configured to reduce crosstalk. For example, the crosstalk reducer 106 may be configured to cancel at least some of the crosstalk between vertical components 104. In an example, the crosstalk reducer 106 may cancel crosstalk between vertical components 104 by increasing crosstalk between horizontal components 102. In some embodiments, the crosstalk reducer 106 may be a change in the properties of the materials of the circuit board, a variation in the geometry of the horizontal components 102, or some combination thereof.
Dielectrics 206 and 208 may be a single material or a composite material. In an example, the dielectric layer may be a resin-impregnated cloth. In an example, the dielectrics 206 and 208 may be a composite of a glass, such as a woven glass, and a resin. In another example, the cloth may be a fiberglass and the resin may be an epoxy resin. Dielectrics 206 and 208 may be formed of the same material. In another example, dielectrics 206 and 208 may each be formed of a different material. Circuit board 200 may include multiple dielectric layers. For example, circuit board 200 may include two dielectric layers. In another example, circuit board 200 may include four, six, eight, or more dielectric layers.
A first dielectric layer 206 may be considered a laminate or core. The laminate may include a metallic layer overlaying a surface of the laminate. The metallic layer may be patterned, such as by etching, to form signal lines 202. In another example, the laminate may include individual signal lines overlaying the laminate. In a particular example, the laminate may be a fully cured resin/cloth, such as a resin/fiberglass weave, clad or laminated with etched sheets of copper foil. The remaining dielectric layers of the circuit board 200 may be a part of a prepreg. In an example, the prepreg may be partially cured resin-impregnated cloth. In another example, the prepreg may be partially cured epoxy resin impregnated with a fiberglass weave.
The resin 210 may flow between the signal lines 202 from the prepreg in the direction of the arrows 210 during the forming process. In an example, the resin 210 may be an epoxy resin. The resin may have a relative permittivity, ε. The permittivity of the resin may fall within a range, such as within a range of 2-7, 2.8-3.3, or 5-7. The permittivity of the resin may affect the polarity of the horizontal crosstalk. In an example, if the permittivity of the resin falls within a low range, such as 2-3, the polarity of the crosstalk of the circuit board 200 may be opposite that of the stimulus. For example, the polarity of the crosstalk of the circuit board 200 may be negative if the permittivity of the resin is low. This opposing crosstalk polarity may be caused by the difference in the permittivity of the resin as compared to the permittivity of the cloth, such as the resin-impregnated cloth. For example, the permittivity of a glass cloth typically falls within a range such as 5-7, as compared to the typical range of resin permittivity of 2-3. However, if the permittivity of the resin more closely matches the permittivity of the cloth, the polarity of the crosstalk may match the polarity of the stimulus.
The permittivity of the resin may be increased to match or exceed the permittivity of the glass. For example, the permittivity of the resin may be increased to greater than 5, such as within the range of 5-7. In an example, the permittivity of the resin may be increased to match the permittivity of the cloth. In another example, the permittivity of the resin may be increased to a larger permittivity than the cloth. In a further example, the permittivity of the resin may be raised, such as to above 5, while the permittivity of the cloth is lowered in order to prevent having to change the geometry of the signal lines. The permittivity of the resin may also be increased to more closely match or even exceed the permittivity of the laminate.
Decreasing the spacing between signal lines may be combined with increasing the permittivity of the resin to affect the polarity of the crosstalk. For example, increasing the permittivity of the resin may flip the polarity of the crosstalk, but the magnitude of the horizontal crosstalk may not be large enough to cancel the vertical crosstalk. However, by also decreasing the spacing between the signal lines, the magnitude of the now positive horizontal crosstalk may increase enough to substantially cancel at least some, if not all, of the vertical crosstalk.
The spacing between signal lines may be decreased by changing the geometry of the signal lines. For example, the geometry of the signal lines may be modified by the addition of stubs disposed on each of the signal lines. The addition of the stubs may create a stubby signal line. The stubby signal line may include longitudinal lengths interrupted by latitudinal increases to form the stubs. The stubs may be disposed on the signal lines such that the stubs extend from the signal lines in a variety of directions. In another example, the stubs may extend from the signal lines in a single direction. The signal lines may include a longitudinal length. The stubs may include a longitudinal section and a pair of latitudinal sections, and the stubs may be disposed along the length of the longitudinal signal lines such that the longitudinal sections of the stubs are parallel to the longitudinal signal lines. The length of the stubby signal line may be significantly increased compared to a non-stubby signal line. The stubs of the signal line may interlock with the stubs of an adjacent signal line. By interlocking the stubs of adjacent signal lines, the signal lines may be brought closer together over a greater length. This increase in closeness may cause the polarity of the horizontal crosstalk to flip, such as from positive to negative.
At block 804, the first signal line may be coupled, such as electrically coupled, to a first vertical component and the second signal line may be coupled to a second vertical component. The vertical components may be via, sockets, packages, or similar components. In an example, each signal line may be coupled to a single vertical component. In another example, each signal line may be coupled to multiple vertical components, such as two vertical components.
At block 806, a crosstalk reduction element may be disposed between the first signal line and the second signal line. In an example, a crosstalk reduction element may be disposed between each set of signal lines. For example, if a circuit board includes three signal lines, the circuit board may also include two crosstalk reduction elements, disposed between the three signal lines. In another example, the crosstalk reduction element may be a single element which affects the entire circuit board. For example, the crosstalk reduction element may be an increase in the permittivity of the resin within the circuit board. In another example, the crosstalk reduction element may be decreasing the spacing between signal lines. The spacing may be decreased by physically moving the signal lines closer together. In another example, the spacing may be decreased by disposing stubs on the signal lines. The stubs of a first signal line may interlock with the subs of a second signal line, such as an adjacent signal line.
At block 808, at least some crosstalk between the vertical components may be cancelled with the crosstalk reduction element. For example, the crosstalk reduction element may increase the horizontal crosstalk and cancel the vertical crosstalk with the horizontal crosstalk. The crosstalk reduction element may reverse the polarity of the horizontal crosstalk in order to cancel at least some of the vertical crosstalk with the horizontal crosstalk.
In the foregoing description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.
An embodiment is an implementation or example. Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” “various embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. The various appearances of “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. Elements or aspects from an embodiment can be combined with elements or aspects of another embodiment.
Not all components, features, structures, characteristics, etc. described and illustrated herein need be included in a particular embodiment or embodiments. If the specification states a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, for example, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.
It is to be noted that, although some embodiments have been described in reference to particular implementations, other implementations are possible according to some embodiments. Additionally, the arrangement and/or order of circuit elements or other features illustrated in the drawings and/or described herein need not be arranged in the particular way illustrated and described. Many other arrangements are possible according to some embodiments.
In each system shown in a figure, the elements in some cases may each have a same reference number or a different reference number to suggest that the elements represented could be different and/or similar. However, an element may be flexible enough to have different implementations and work with some or all of the systems shown or described herein. The various elements shown in the figures may be the same or different. Which one is referred to as a first element and which is called a second element is arbitrary.
In the preceding description, various aspects of the disclosed subject matter have been described. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the subject matter. However, it is apparent to one skilled in the art having the benefit of this disclosure that the subject matter may be practiced without the specific details. In other instances, well-known features, components, or modules were omitted, simplified, combined, or split in order not to obscure the disclosed subject matter.
While the disclosed subject matter has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the subject matter, which are apparent to persons skilled in the art to which the disclosed subject matter pertains are deemed to lie within the scope of the disclosed subject matter.
While the present techniques may be susceptible to various modifications and alternative forms, the exemplary examples discussed above have been shown only by way of example. It is to be understood that the technique is not intended to be limited to the particular examples disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.
