Printed Circuit Design & Fab - October 2008 - (Page 22)

SIGNAL intEGrity The New Wave in HigH-SPeed ModeliNg Unlocking the electrical performance of an interconnect may be as simple as mastering the S-parameter. by Dr. ErIC BOGATIn The fastest way to an answer is not always the direct path. When it comes to characterizing the electrical performance of an interconnect, like a connector, package, interposer or backplane channel, looking at its impact on digital signals is not the most efficient way of describing its high speed digital performance. You may see how it behaves for one specific application, but it is difficult to extend this to other high-speed digital signals (FiGurE 1). The solution is to borrow an approach used in the RF world: that is, to use the interactions of precision sine wave signals with the connector to characterize its performance. While this seems like a step backward for digital applications, it provides a simple, reproducible and general process that has swept the signal integrity world by storm over the last five years to become the de-facto standard in gigabitper-second applications. In the frequency domain, all signals are sine waves with an amplitude and phase. Describing the electrical properties of a connector in terms of how sine waves interact with it, as seen in FiGurE 2, means using the changes in the magnitude and phase of the sine wave as it interacts or “scatters” off the connector. Historically, when sine waves scatter off an interconnect, their response is called the Scattering Parameters, or S-Parameters. All the information pertaining to the behavior of an interconnect is contained in its S-Parameters. Because the S-Parameters of a connector can be used to predict the complete behavior of a connector in a system, the S-Parameters can also be called behavioral models. A port is the connection to the device, and it has a signal and return path, which in its most common form, is a coax connection. When a sine wave is launched into the port to simulate or measure the S-Parameters, by definition, it must be in a 50-Ohm environment. The S-Parameters are a measure of the sine waves that scattered off the digital signals is either device, not always Figure 1. Paradoxically, characterizing an interconnect with the best way of describing its behavior for high speed digital applications. reflecting back from the incident port or scattering into any other ports that might be connected. If a sine wave enters a port, there are only three qualities of it that can change when it comes out again; its frequency, amplitude or phase could be affected. For all linear, passive interconnects, which includes all interconnects except those with ferrites, the frequency of the sine wave coming out will always be exactly the same as the frequency of the sine wave that has entered. That leaves the changes in the amplitude and the phase that will be recorded as the S-Parameters. Figure 1. Paradoxically, characterizing an interconnect with digital signals is not always A of describing device used in high-speed the best waytypical its behavior for high speed digital applications. applications will be part of a differential channel with four connections on the incident ends. To distinguish which port we are transmitted reflected discussing, each is labeled with an index amplitude amplitude number (FiGurE 3). Port 1 isphase input an phase that feeds to Port 2, with Port 3, adjacent to Port 1, feeding intouniversal behavioral description of the electrical Figure 2. Sine waves offer a scalable and Port 4. performance of interconnects. When a sine wave goes into 1 incident reflected transmitted 2 (and their return paths!) 3 4 V S out ,in = out Vin amplitude Figure 1. Paradoxically, characterizing an FiGurEdescribing its behavior for interconnectdigitaldigital signals is not always 1. Paradoxically, with applications. characterizing the best way of high speed an interconnect with digital signals is not always the best way of describincident ing its behavior for high-speed digital transmitted applications. reflected amplitude phase phase FiGurE 2. Sine waves offer a scalable Figure Sine waves offer a scalable and universal behavioral description of the electrical and2. universal behavioral description of performance of interconnects. the electrical performance of interconnects. FiGurE 3. Most common labeling Figure 3. Most for the four endsfour a differenthe of Though scheme common labeling scheme for a coaxialends of a differential pair.and return not shown, each connection or port, is really connection to the signal path the interconnect. tialofpair. though not shown, each connection, or port, is really a coaxial connection to the signal and return path of the interconnect. OCTOBER 2008 amplitude amplitude 22 printEd CirCuit dESign & fAB phase phase 1 3 (and their return paths!) 2 4 V S out ,in = out Vin Figure 2. Sine waves offer a scalable and universal behavioral description of the electrical

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - October 2008

Printed Circuit Design & Fab - October 2008 Contents Our Line Market Watch Around the World Happenings Test and Inspection ROI Tip Jar Interconnect Strategies Final Finish Forum The New Wave in High-Speed Modeling The PCB Design Library Mixed Signal Design Considerations Modeling Conductor Surface Roughness Copper Erosion: The Influence of Metallurgy on Copper Dissolution The Wave of the Future Ad Index Building a Profitable Niche Marketplace Off the Shelf BGA Bulletin

Printed Circuit Design & Fab - October 2008

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