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lecture14_ee689_noise_sources

lecture14_ee689_noise_sources - Sam Palermo Analog&...

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Unformatted text preview: Sam Palermo Analog & Mixed-Signal Center Texas A&M University ECEN689: Special Topics in High-Speed Links Circuits and Systems Spring 2011 Lecture 14: Noise Sources Agenda • Noise source overview • Common noise sources • Noise budgeting • References • Dally Ch 6 • Stateye paper posted on website 2 Noise in High-Speed Link Systems • Multiple noise sources can degrade link timing and amplitude margin 3 [ Dally ] Noise Source Overview • Common “noise” sources • Power supply noise • Receiver offset • Crosstalk • Inter-symbol interference • Random noise • Power supply noise • Switching current through finite supply impedance causes supply voltage drops that vary with time and physical location • Receiver offset • Caused by random device mismatches 4 • Crosstalk • One signal (aggressor) interfering with another signal (victim) • On-chip coupling (capacitive) • Off-chip coupling (t-line) • Near-end • Far-end • Inter-symbol interference • Signal dispersion causes signal to interfere with itself • Random noise • Thermal & shot noise • Clock jitter components Bounded and Statistical Noise Sources • Bounded or deterministic noise sources • Have theoretically predictable values with defined worst-case bounds • Allows for simple (but pessimistic) worst-case analysis • Examples • Crosstalk to small channel count • ISI • Receiver offset 5 • Statistical or random noise sources • Treat noise as a random process • Source may be psuedo-random • Often characterized w/ Gaussian stats • RMS value • Probability density function (PDF) • Examples • Thermal noise • Clock jitter components • Crosstalk to large channel count • Understanding whether noise source is bounded or random is critical to accurate link performance estimation Proportional and Independent Noise Sources • Some noise is proportional to signal swing • Crosstalk • Simultaneous switching power supply noise • ISI • Can’t overpower this noise • Larger signal = more noise 6 • Some noise is independent to signal swing • RX offset • Non-IO power supply noise • Can overpower this noise NI S N N V V K V + = Total noise Proportional noise constant Signal swing Independent noise Common Noise Sources • Power supply noise • Receiver offset • Crosstalk • Inter-symbol interference • Random noise 7 Power Supply Noise • Circuits draw current from the VDD supply nets and return current to the GND nets • Supply networks have finite impedance • Time-varying (switching) currents induce variations on the supply voltage • Supply noise a circuit sees depends on its location in supply distribution network 8 [ Hodges ] Power Routing 9 Bad – Block B will experience excessive supply noise Better – Block B will experience 1/2 supply noise, but at the cost of double the power routing through blocks Even Better – Block A & B will experience similar supply noise Best – Block A & B are more isolated [ Hodges ] [ Hodges ] Supply Induced Delay Variation...
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