Printed Circuit Design & Fab - August 2008 - (Page 21)

also solved by Mathcad, which provides an economical21 approach for generating dependable results and design guidance.21 Application of Mathcad for simulating such singleended signaling is demonstrated by FIGURES 11 and 12. In FIGURE 11, Bw symbolizes the transmission line bandwidth in Hz. Zo equals line impedance (ohms). Tb represents the input pulse’s bit period in seconds. Rr is the load series resistance and Cr the load capacitance (Farad). Rr is usually on the order of milli-Ohms, and Cr includes C_comp and C_pkg.22 FIGURE 11 involves transmission of a series of bits across the interconnect. The data pattern is encoded with industry-standard non-return-to-zero (NRZ) format. In NRZ, the binary 1 and 0 are a positive and a negative square wave pulse respectively. The uni(t) function aids in forming the NRZ bit waveform. The load impulse response ignoring propagation delay is impr(t). The vstp(t) models the transmission line as a first-order low-pass filter. The pulse waveform just before the receiver equals vstp(t) – vstp(t-Tb). The convolution integral of this waveform and impr(t) yields the output voltage Voutp(t), which is sectioned into Vout1(t) and Vout2(t). The difference Voutp(t) – Voutp(t-Tb), in turn, produces the received pulse Vrec(t). Finally, vnrz(t) is the NRZ 50-bit stream, which is expressed as summation s1(t) + s2(t)+… +s10(t). vnrz(t) as depicted by FIGURE 12A consists of : 1010110011100010101011110110000101011110011 1110001 An eye pattern at the receiver can be constructed via bitby-bit superposition of the received bits. FIGURE 12B shows an eye pattern formed by graphing 16 bits on the same graph. The plotted bits embrace vnrz(t+Tb), vnrz(t+2Tb), ……….vnrz(t+15Tb), vnrz(t+16Tb). Plotting several Y-axis expressions versus the same X-axis in Mathcad requires entering the first expression followed by a comma. A placeholder will then appear below the first expression. When the second expression is entered followed by a comma, another placeholder results for entering the next expression. This process can be repeated to graph multiple expressions. Eye diagrams are frequently generated employing a large number of pseudorandom bit sequence (PRBS), such 23 as a PRBS [(2^7) – 1]. However, eye patterns created by Mathcad based on a limited number such as 16 bits can be still valuable particularly for examining trends.21 For instance, it can be useful for analyzing effects on the eye pattern caused by variations in bandwidth or load capacitance. Trends based on 16 bits can be quite similar to those associated with a much greater number of signal bits. As an example, decreasing the line bandwidth (Bw) (in FIGURE 11) from 500 MHz to 100 MHz will reveal degradation in eye opening of FIGURE 12B. December 2-4, 2008 Orlando Metropolitan Resort Orlando, FL www.pcbshows.com/orlando Interested in exhibiting? An exciting addition to the PCB Shows portfolio of events for designers and fabricators Bringing you the best of our renowned and respected conferences and seminars to Orlando, FL Featuring • 3-day targeted technical conference • 1-day tabletop exhibition • Great networking opportunities Brought to you by AUGUST 2008 PRINTED CIRCUIT DESIGN & FAB 21 http://www.pcbshows.com/orlando http://www.orlandometropolitanresort.com http://www.pcbshows.com/orlando http://www.pcbshows.com/orlando http://www.upmediagroup.com

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Printed Circuit Design & Fab - July 2008 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar Software Performance Interconnect Strategies Final Finish Forum Product Development Challenges in a Global Market Innovative Modeling Supports Co-Design of the Power Supply Chain, Part 2 Low-Loss Fluoropolymer Copper Clad Laminate Qualifying PCBs Outsourced in Asia Copper Plating and Microvia Fill for Advanced PCBs Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - August 2008

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