Low-Frequency Effects of AC coupling capacitor.pdf

Low-Frequency Effects of AC Coupling Capacitor IEEE P802.3bj May 2012, Minneapolis Yasuo Hidaka (Fujitsu Laboratories of America, Inc.) 1IEEE P802.3bj 100Gb/s Backplane and Copper Cable Task Force, Minneapolis, May 2012 Contributor ?Mike Dudek (QLogic) 2IEEE P802.3bj 100Gb/s Backplane and Copper Cable Task Force, Minneapolis, May 2012 ? AC cap has low-frequency effects of baseline wonder that cannot be represented well by channel S-parameter. ? If channel includes AC cap, AC cap should be shorted (either physically or virtually) in channel S-parameter model, and an equivalent lumped cap should be placed outside of channel in time-domain simulation. Summary 3IEEE P802.3bj 100Gb/s Backplane and Copper Cable Task Force, Minneapolis, May 2012 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 frequency (Hz) | S 2 1 | ( d B ) Low-frequency effect of AC cap ?High-pass characteristics with cut-off freq at kHz order ? E.g. 100nF cap in 50ohm transmission line : -3dB cut-off freq = 15.9kHz 50ohm 100nF 50ohm P1 P2 -3dB @ 15.9kHz 4IEEE P802.3bj 100Gb/s Backplane and Copper Cable Task Force, Minneapolis, May 2012 A problem of AC cap in Channel ?Channel characteristics are often represented by S-parameter up to 40GHz with 10MHz frequency step ? To include the low-frequency effect of AC cap, frequency step must be 100 Hz ~ 1KHz at low frequency ? Because the attenuation curve in low frequency must be captured ?Otherwise, it is not possible to reproduce the effect of baseline wonder ? Besides, discontinuity at low-frequency in S-parameter model often results in causality problem in time-domain simulation ?Channel S-parameter file grows by a factor of 10,000~100,000, exceeding practical limits of many tools ? Some tool may support variable frequency steps, but most tools do not 5IEEE P802.3bj 100Gb/s Backplane and Copper Cable Task Force, Minneapolis, May 2012 A workaround for Simulated Channel ? Short AC cap to avoid discontinuity in low fre