EECS240 – Spring 2008Lecture 18: High-Speed FiltersElad AlonDept. of EECSEECS240 Lecture 18 2Link Channels and ISI0 2 4 6 8 10-60-50-40-30-20-100frequency [GHz]Attenuation [dB]9" FR4, via stub26" FR4,via stub26" FR49" FR4• 20-30dB loss at 3GHz• How bad is that?• Two related issues:• (1) Noise and min. signal amplitude• (2) IntersymbolinterferenceEECS240 Lecture 18 3Inter-symbol interference (ISI)• Channel is low pass• Short TX pulses get spread out0 1 2 300.20.40.60.81nspulse responseTsymbol=160psEECS240 Lecture 18 4Impact of ISI• Middle sample is corrupted by 0.2 trailing ISI (from previous symbol) and 0.1 leading ISI (from next symbol) • Total ISI: 0.3 total ISI• Middle symbol incorrectly detected0 2 4 6 8 10 12 14 16 1800.20.40.60.81Symbol timeAmplitudeError!EECS240 Lecture 18 5Equalization• ISI is proportional to TX swing• Generally can’t just boost signal to overcome it• Solution: Equalization• If channel applies filter H(s)• Pass the signal through another filter H-1(s)EECS240 Lecture 18 6Equalization cont’d• Link channel basically low-pass• Equalizer boosts high frequency, attenuates low frequencyEECS240 Lecture 18 7Equalizer RequirementsEECS240 Lecture 18 8Equalizer Requirements cont’dEECS240 Lecture 18 9Equalizer Options and LimitationsEECS240 Lecture 18 10RX EqualizerEECS240 Lecture 18 11RX Equalizer ImplementationsEECS240 Lecture 18 12RX Equalizer ImplementationsEECS240 Lecture 18 13RX Equalizer ImplementationsEECS240 Lecture 18 14RX Equalizer ImplementationsEECS240 Lecture 18 15ProgrammabilityEECS240 Lecture 18 16Aside: Switched Cap. ResistorEECS240 Lecture 18 17Aside: Switched Cap Low-Pass
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