ECEN689: Special Topics in High-Speed Links Circuits and Systems Spring 2012

Transcription

1 ECEN689: pecial Topics in High-peed Links Circuits and ystems pring 01 Lecture 3: Time-Domain Reflectometry & -Parameter Channel Models am Palermo Analog & Mixed-ignal Center Texas A&M University

2 Announcements This Thursday lab time will be used for a make-up lecture No class next Monday Reference Material Posted on Website TDR theory application note -parameter notes

3 Agenda Interconnect measurement techniques Time-domain reflectometry (TDR) Network analyzer -parameters Cascading -parameter models Full -parameter channel model Transient simulations Impulse response generation Eye diagrams Inter-symbol interference 3

4 Lecture References Majority of TDR material from Dally Chapter 3.4, Majority of s-parameter material from Hall Advanced ignal Integrity for High-peed Digital Designs Chapter 9 4

5 Interconnect Modeling Why do we need interconnect models? Perform hand calculations and simulations (pice, Matlab, etc ) Locate performance bottlenecks and make design trade-offs Model generation methods Electromagnetic CAD tools Actual system measurements Measurement techniques Time-Domain Reflectometer (TDR) Network analyzer (frequency domain) 5

6 Time-Domain Reflectometer (TDR) [Agilent] [Dally] TDR consists of a fast step generator and a high-speed oscilloscope TDR operation Outputs fast voltage step onto channel Observe voltage at source, which includes reflections Voltage magnitude can be converted to impedance Impedance discontinuity location can be determined by delay Only input port access to characterize channel 6

7 TDR Impedance Calculation 7 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0.5V 1 If 1 1 TEP i i r i r i r r T T T i r r V V V t V V t V Z t V V t V V Z t k t k Z t Z Z t Z Z t Z V t V t k ( ) ( ) ( ) ( ) υ x t Z x Z t V V t V Z t Z T T T 1 0

8 TDR Waveforms (Open & hort) Open termination Input step at 1ns t d hort termination t d 8

9 TDR Waveforms (Matched & Mismatched) Matched termination Mismatched termination t d Z T > Z 0 Z T < Z 0 9

10 TDR Waveforms (C & L Discontinuity) hunt C discontinuity τ C Z 0 C t d eries L discontinuity t r 10ps t d Peak voltage spike magnitude: V V τ 1 e tr t r τ τ L L Z 0 10

11 TDR Rise Time and Resolution TDR spatial resolution is set by step risetime x > t r υ tep risetime degrades with propagation through channel Dispersion from skin-effect Lump discontinuities low-pass filter the step Causes difficulty in estimating L & C values Channel filtering can actually compensate for lump discontinuity spikes 11

12 TDR Multiple Reflections 1

13 TDR Waveforms (Multiple Discontinuities) A B C Load A B C BAB, CBC BAC, CBCBC, CAB Note: tep comes at 1ns 13

14 Time-Domain Transmission (TDT) [Dally] Can measure channel transfer function ( jω) ( jω) ( jω) Hard to isolate impedance discontinuities, as they are superimposed on a single rising edge H V V 1 TDR TDT 14

15 Network Analyzer timulates network with swept-frequency source Measures network response amplitude and phase Can measure transfer function, scattering matrices, impedance, [Dally] 15

16 Transfer Function & Impedance Measurements [Dally] 16

17 cattering () Parameters Why Parameters? Easy to measure Y, Z parameters need open and short conditions parameters are obtained with nominal termination parameters based on incident and reflected wave ratio [Dally] 17

18 Formal -Parameter Definitions [Agilent] 18

19 Cascading -Parameters Network analysis allows cascading of independently characterized structures However, can t directly cascade s- parameter matrices and multiply Must first convert to an ABCD matrix (or T matrix) 19

20 ABCD Parameters v i v i i i D v i C i v B v v A 1 i v D C B A i v i [Hall]

21 Converting Between & ABCD Parameters [Hall] 1

22 Example: Cascaded Via & Transmission Line Taken from Advanced ignal Integrity for High-peed Digital Designs by Hall

23 Example: Cascaded Via & Transmission Line Using conversion table: Can also use T matrixes to cascade Taken from Advanced ignal Integrity for High-peed Digital Designs by Hall 3

24 -Parameter Channel Example [Peters, IEEE Backplane Ethernet Task Force] 4

25 -Parameter Channel Example (4-port differential) v v a a a a b b b b ( ) ( ) a b a b a a d d dd a a d d dd + + [Hall] Data from 50MHz to 15GHz in 10MHz steps

26 -Parameter Channel Example

27 Impulse Response Channel impulse responses are used in Time domain simulations Link analysis tools y Y ( ω) H ( ω) X ( ω) ( t) h( t) x( t) h( t τ ) x( τ ) h 1 ( t) F { H ( w) } 7

28 Generating an Impulse Response from -Parameters Perform the inverse Fourier transform on the s-parameter of interest h ( t) F 1{ ( ω) } tep 1: For ifft, produce negative frequency values and append to s- parameter data in the following manner ( f ) ( f ) Positive Frequency Negative Frequency 8

29 Increasing Impulse Response Resolution Could perform ifft now, but will get an impulse response with time resolution of For 1ps resolution: zero pad to +/-500GHz 1 f max 1 15GHz ( ) 33.3ps To improve impulse response resolution expand frequency axis and zero pad zero padding 9

30 Channel Impulse Response Now perform ifft to produce impulse response Can sanity check by doing an fft on impulse response and comparing to measured data 30

31 Impulse Response of Different Channels 7 Desktop/0Conn 17 Refined BP/Conn 17 Legacy BP/Conn 31

32 Channel Transient Response * 3

33 Eye Diagrams [Walker] 33

34 Eye Diagrams vs Data Rate 34

35 Eye Diagrams vs Channel 35

36 Inter-ymbol Interference (II) Previous bits residual state can distort the current bit, resulting in inter-symbol interference (II) II is caused by Reflections, Channel resonances, Channel loss (dispersion) ingle Input Bit Output Pulse Response 36

37 II Impact At channel input (TX output), eye diagram is wide open As data pulses propagate through channel, they experience dispersion and have significant II Result is a closed eye at channel output (RX input) INPUT Eye FFE1 10.0Gb/s [OPEN,1e-8] No Xtalk Packaged erdes Eye FFE1 10.0Gb/s [OPEN,1e-8] No Xtalk 500mV DATA RAND Tx 600mVpd AGC Gain -5.48dB XTALK NONE AGC 5.0GHz 0.00dB PKG0/0 TERM 5050/5050 IC 3/3 400mV 300mV ignal Amplitude Vpd 500mV DATA RAND Tx 600mVpd AGC Gain -6.0dB XTALK NONE AGC 5.0GHz 0.00dB PKG0/0 TERM 5050/5050 IC 3/3 400mV 300mV 00mV 100mV -0.0mV -100mV -00mV -300mV HCDR.3.-pre IBM Confidential Date at 01/1/006 1:01 PM PLL0F1V00,C16,N3,O1,L80 FREQ0.00ppm/0.00us -400mV FFE [1.000, 0.000] -500mV -100ps -50ps 0ps 50ps Time 100ps Backplane trace Line card trace Edge connector Via stub ignal Amplitude Vpd 00mV 100mV -0.0mV -100mV -00mV -300mV HCDR.3.-pre IBM Confidential Date at 01/1/006 1:00 PM PLL0F1V00,C16,N3,O1,L80 FREQ0.00ppm/0.00us -400mV FFE [1.000, 0.000] -500mV -100ps -50ps 0ps 50ps Time OUTPUT 100ps [Meghelli (IBM) ICC 006] 37

38 Next Time Channel pulse response model Modulation schemes 38