Transcript ppt
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 36: December 7, 2011 Transmission Lines Implications 1 Penn ESE370 Fall2011 -- DeHon Transmission Line Agenda • • • • • • • • • Where arise? General wire formulation Lossless Transmission Line See in action in lab Impedance End of Transmission Line? Termination Implications Discuss Lossy Penn ESE370 Fall2011 -- DeHon 2 Transmission Line • Data travels as waves • Line has Impedance • May reflect at end of line Penn ESE370 Fall2010 -- DeHon 1 c0 w LC r r L Z0 C R Z 0 Vi Vr R Z0 2R Vi Vt R Z 0 3 Some Transmission Lines Characteristics arise form their geometry 4 Penn ESE370 Fall2011 -- DeHon Coaxial Cable • Inner core conductor: radius r • Insulator: out to radius R • Outer core shield (ground) R L ln 2 r 1 R Z 0 ln 2 r • RG-58 Z0= 50W – networking, lab • (RG-59 Z0= 75W – video) Penn ESE370 Fall2010 -- DeHon 5 Printed Circuit Board • Stripline – Trace between planes w t b r W 1 1 b Z 0 ln t W 4 b b 6 Penn ESE370 Fall2010 -- DeHon Printed Circuit Board • Microstrip line – Trace over single supply plane 0 r w t h 1 4h ln Z 0 2 0.475 r 0.67 0 0.536W 0.67t 7 Penn ESE370 Fall2010 -- DeHon Twisted Pair • Category 5 ethernet cable – 100W – V=0.64c0 Source: http://en.wikipedia.org/wiki/File:Cat_5.jpg 8 Penn ESE370 Fall2010 -- DeHon Termination Cases • Parallel at Sink • Series at Source Either: destination sees source voltage after delay. Penn ESE370 Fall2010 -- DeHon 9 Example • 25meter category-5e cable (100W, 0.64c) • Supporting 1Gb/s ethernet – 4 pairs at 250Mb/s • Time to send data from one end to the other? • Time between bits at 250Mb/s? • Bits in the cable? 10 Penn ESE370 Fall2011 -- DeHon Pipeline Bits • For properly terminated transmission line – Do not need to wait for bits to arrive at sink – Can stick new bits onto wire 11 Penn ESE370 Fall2010 -- DeHon Limits to Bit Pipelining • What limits? (why only 250Mb/s) – Risetime/distortion – Clocking • Skew • Jitter – For bus • Wire length differences between lines 12 Penn ESE370 Fall2010 -- DeHon Eye Diagrams • Watch bits over line on scope – Look at distortion – “open” eye clean place to sample • Consistent timing of transitions • Well defined high/low voltage levels http://en.wikipedia.org/wiki/File:On-off_keying_eye_diagram.svg http://focus.ti.com/analog/docs/gencontent.tsp?familyId=361&genContentId=41762&DCMP=ESD_Solutions&HQS=Other+OT+esd 13 Penn ESE370 Fall2010 -- DeHon Bad “eye” http://archive.chipcenter.com/knowledge_centers/asic/todays_feature/showArticle.jhtml?articleID=12800254 14 Penn ESE370 Fall2010 -- DeHon Termination / Mismatch • Wires do look like these transmission lines • We are terminating them in some way when we connect to chip (gate) – Need to be deliberate about how terminate, if we care about high performance 15 Penn ESE370 Fall2010 -- DeHon Where Mismatch? • • • • • • Vias Wire corners? Branches Connectors Board-to-cable Cable-to-cable http://www.fpga4fun.com/Hands-on_Flashy.html http://wiki.altium.com/display/ADOH/An+Overview+of+Electronic+Product+Development+in+Altium+Designer 16 Penn ESE370 Fall2010 -- DeHon Impedance Change • What happens if there is an impedance change in the wire? Z0=75W, Z1=50W – What reflections and transmission do we get? 17 Penn ESE370 Fall2010 -- DeHon Z0=75, Z1=50 • At junction: – Reflects • Vr=(50-75)/(50+75)Vi – Transmits • Vt=(100/(50+75))Vi R Z 0 Vi Vr R Z0 2R Vi Vt R Z 0 18 Penn ESE370 Fall2010 -- DeHon Impedance Change Z0=75, Z1=50 19 Penn ESE370 Fall2010 -- DeHon What happens at branch? 20 Penn ESE370 Fall2010 -- DeHon Branch • Transmission line sees two Z0 in parallel – Looks like Z0/2 21 Penn ESE370 Fall2010 -- DeHon Z0=50, Z1=25 • At junction: – Reflects • Vr=(25-50)/(25+50)Vi – Transmits • Vt=(50/(25+50))Vi R Z 0 Vi Vr R Z0 2R Vi Vt R Z 0 22 Penn ESE370 Fall2010 -- DeHon End of Branch • What happens at end? • If ends in matched, parallel termination – No further reflections 23 Penn ESE370 Fall2010 -- DeHon Branch Simulation 24 Penn ESE370 Fall2010 -- DeHon Branch with Open Circuit? • What happens if branch open circuit? 25 Penn ESE370 Fall2010 -- DeHon Branch with Open Circuit • Reflects at end of open-circuit stub • Reflection returns to branch – …and encounters branch again – Send transmission pulse to both • Source and other branch • Sink sees original pulse as multiple smaller pulses spread out over time 26 Penn ESE370 Fall2010 -- DeHon Open Branch Simulation 27 Penn ESE370 Fall2010 -- DeHon Open Branch Simulation 28 Penn ESE370 Fall2010 -- DeHon Bus • Common to have many modules on a bus – E.g. PCI slots – DIM slots for memory • High speed bus lines are trans. lines 29 Penn ESE370 Fall2010 -- DeHon Multi-drop Bus • Ideal – Open circuit, no load 30 Penn ESE370 Fall2010 -- DeHon Multi-Drop Bus • Impact of capacitive load (stub) at drop? – If tight/regular enough, change Z of line L Z0 C 31 Penn ESE370 Fall2010 -- DeHon Multi-Drop Bus • Long wire stub? – Looks like branch • may produce reflections 32 Penn ESE370 Fall2010 -- DeHon Lossy Transmission Line • How do addition of R’s change? – Concretely, discretely think about R=0.2W every meter on Z0=100W • what does each R do? 33 Penn ESE370 Fall2010 -- DeHon Lossy Transmission Line • R’s cause signal attenuation (loss) – Voltage divider R Z0 – Reduced signal swing at sink – Limits length of transmission line before need to restore signal 34 Penn ESE370 Fall2010 -- DeHon Transmission Line Noise • Frequency limits • Imperfect termination • Mismatched segments/junctions/vias/connectors • Loss due to resistance in line – Limits length 35 Penn ESE370 Fall2010 -- DeHon Admin • Proj3 due Friday – Will take until 11:59pm without late penalty – Late penalty 20% per day • Last lecture Friday – One reason to move due so can show up alert at class at noon 36 Penn ESE370 Fall2010 -- DeHon Idea • Transmission lines – high-speed – high throughput – long-distance signaling • Termination • Signal quality 1 c0 w LC r r L Z0 C R Z 0 Vr Vi R Z 0 Penn ESE370 Fall2010 -- DeHon 37