Differential S-Parameters: The New Universal Standard for Interconnects
By Dr. Eric Bogatin
www.BeTheSignal.com
September 2006
All high-speed serial interconnect standards, such as those used in 10-Gigabit Attachment Unit Interface (XAUI), SATA, PCI-X, or InfiniBand systems, leverage differential signaling and are designed for routing on differential pairs. In the Gbps regime, and with interconnects longer than about 20 inches, the electrical properties of the interconnects can literally make or break the connection.
Determining whether an interconnect is suitable for a high-speed serial link, before it is built and tested, is a critical step in any efficient design process. The 4-port differential S-Parameters have become the universal metric for evaluating interconnect performance and determining if an interconnect meets a standard’s specification.
Some aspects of differential S-Parameters have been used for many years. For example, the XAUI specification introduced the idea of specifying a “compliant channel” if the differential insertion loss, SDD21, was above a minimum curve, crossing -11.4 dB at 3.125 GHz. The 10-Gigabit Serial Electrical Interface (XFI) specification for nominal baud rates in the 9.95 to 11.1 Gbps range specify the minimum SDD21 value of -6 dB at 5.5 GHz.
Figure 1. Measured Differential S-Parameters From a XAUI-Compliant Backplane, Displayed Using Agilent PLTS
This sort of specification may work most of the time when the requirement is for a minimum opening of the eye received at the end of the channel. However, SDD21 performance alone does not have all the information needed to determine acceptable performance when transmit and receive signal processing is used.
Most high performance SERDES devices take advantage of transmit and receive signal processing features such as multi-TAP pre-emphasis and equalization. The bit error rate (BER) from the receiver depends on more than the differential insertion loss being above some specified channel value. In the bit rate and interconnect length regime where pre-emphasis and equalization are needed to assure acceptable bit error rates, multiple reflections from impedance discontinuities, mode conversion, and crosstalk also determine whether a channel is compliant.
This has driven the new trend to use the complete 4-port differential S-Parameter matrix in determining the suitability of a channel for an application. Both Synopsis HSPICE and Agilent ADS are widely used to integrate 4-port S-Parameter matrix data, either from measurement or from simulation, into system-level circuit simulations.
Altera Corporation has been demonstrating their pre-emphasis and link estimator (PELE) tool to customers. This tool uses the full S-Parameter matrix as a description of a differential channel, in conjunction with driver and receiver models, to analyze the channel performance and optimize the pre-emphasis and equalization coefficients.
Though there are 16 elements in the 4-port differential S-Parameter matrix, 6 of those carry more value than the others. Examples of 4 of these important elements, measured from a XAUI-compliant backplane, are shown in Figure 1.
SDD21 is the differential insertion loss. This describes the change in the received differential signal, in magnitude and phase, after transmission through the channel. Differential insertion loss relates to how well the channel transmits a differential signal.
SDD11 is the differential return loss. This describes the differential signal that reflects back to the source, when a differential signal is incident at the beginning of the channel. Differential return loss is sensitive to the differential impedance profile of the channel.
SCC21 is the common insertion loss. This describes how a common signal, launched into the channel, is changed by transmission through the channel. Common insertion loss relates to the quality of the transmission of a common signal.
SCC11 is the common return loss. This describes the common signal that reflects back to the source when a common signal is incident at the beginning of the channel. Common return loss relates to the common impedance profile of the channel.
SCD21 is a measure of the transmitted mode conversion. SCD21 relates how much of a differential signal, launched into the channel, is converted into a common signal by the end of the channel. Any asymmetries between the two lines that make up the differential channel, such as line width differences or skew, convert some of the incident differential signal into a common signal.
SCD11 is a measure of the common signal that reflects back to the source when a differential signal is incident to the channel. SCD11 contains information about the location of the asymmetry that converted the differential signal into the common signal.
If you are involved in high-speed serial links, you can be sure S-Parameters are in your future.
Reference papers about S-Parameters are available for download from www.BeTheSignal.com.
Reader Q&A
If you have a question for the SI Doctor, please send it to DoctorIsIn@bethesignal.com.
Question from Bin Wang: I am an engineer from China. If an unshielded twisted-pair cable, such as category-5 cable, is connected to a differential pair on a circuit board, both differential and common signal may be transmitted onto the cable. If there is common current on the twisted pair, where is the return path of the common signal and the differential signal?
Answer: A CAT-5 cable connected to a printed circuit board can carry both a differential signal and a common signal. For the differential signal, each line of the twisted pair carries the return current of the other line. If there is any common current on the twisted pair, the return path for the common current is the nearest conductive surface to the cable. This may be the floor, any metal conduit the cable may be adjacent to, or other cables in proximity. The typical impedance the common signal sees in the circuit board is about 25 ohms. The common signal transmission line in the cable is the twisted pair as the signal path, and the chassis and then the floor as the return path. Given the typical spacing of at least a few inches between the twisted pair and the floor, this is an impedance for the common signal of at least 200 ohms. Most of the common signal reflects, but some of it is transmitted onto the twisted pair/floor transmission line.
Bio: Eric Bogatin is president of Bogatin Enterprises. Many of his papers are available on his website, www.BeTheSignal.com. He is the author of Signal Integrity - Simplified, published by Prentice Hall. Send email to eric@BeTheSignal.com
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