Crosstalk Measurements for Signal Integrity Applications. Chris Scholz, Ph.D. VNA Product Manager R&S North America

Transcription

1 Crosstalk Measurements for Signal Integrity Applications Chris Scholz, Ph.D. VNA Product Manager R&S North America

2 Outline ı A brief history of crosstalk ı Introduction to crosstalk Definition of crosstalk Why is crosstalk important? Types of crosstalk Impact of crosstalk on signal integrity ı Measurement methods for crosstalk Time domain measurements Frequency domain measurements What is important when measuring crosstalk: measurement uncertainty 2

3 A Brief History of Crosstalk ı Crosstalk terminology form telephone lines Communication crosses line from the intended user to a victim Crosstalk is frequency dependent Significant contribution to crosstalk identified to be telephone circuit unbalances Crosstalk set measures near-end crosstalk of telephone line at audio frequencies Source: L.P. Ferris, R. G. McCurdy: Telephone Circuit Unbalances. Determination of Magnitude and Location, Pacific Coast Convention of the A.I.E.E.,

4 From Parallel to Serial Data ı Late 1970 s Multiple parallel data lines to increase throughput SCSI, Parallel ATA/IDE, PCI, etc. Multiple flavors (fast, fast-wide, ultra, ultra-wide, etc.) Limited scalability Each line requires termination (passive/active) Crosstalk between lines limits speed ı Late 1990s/Early 2000 Industry switches to serial buses Pioneered by USB and Fibre Channel ı Mid 2000 s Multiple parallel serial lanes PCIe, SAS, SATA, XAUI, WDM, CEI Crosstalk is becoming an issue again 4

5 What is Crosstalk? 5

6 Definition of Crosstalk ı Crosstalk is the interference between signals that are propagating on various lines in the system. ı Crosstalk results from the interaction of electromagnetic fields generated by neighboring data signals as they propagate through transmission lines and connectors. 6

7 What does Crosstalk look like? Serial Bus Ideal Transmission Line Two serial buses Two with serial capacitive busses coupling in parallel 7

8 What does Crosstalk Look like? ı Time Domain View of Crosstalk 8

9 Why Crosstalk is Becoming Important ı Frequency Domain View of Crosstalk: 9

10 Impact of Crosstalk: Eye Diagram Jitter Eye Opening Some authors believe they can identify crosstalk by analyzing the eye diagram Source: Jung CICC 2012 Source: QEOS Wireless Cable 10

11 Impact of Crosstalk: Bit error rate BER = 0.5 Slope of bathtub curve BER Dj 1-2*Tj Dj 0 1 Rj BER Rj BER Tj BER = Dj + alpha BER * Rj alpha is related to the slope of the bathtub curve 11

12 Types of Crosstalk 12

13 Types of Crosstalk ı Near-end Crosstalk (NEXT) The noise induced in the receiving pair due to the signal on the transmitting pair on the same port. (Source: IEEE1394) Spread out over time Sensitive to stubs in TX path Source: Jung CICC 2012 ı Far-end Crosstalk (FEXT) The noise induced in the receiving pair due to the signal on the transmitting pair on the opposite port. Peaking in timedomain Increases with frequency Slow transitions less FEXT Source: Jung CICC

14 Near End Crosstalk (NEXT) ı Single-Ended Coupled Microstrip ı NEXT coefficient K b VNEXT( t) = K b( Vin ( t) Vin ( t 2tf ) t f : propagation time through the trace K C ( C 1 M b = 4 + Total L L M Total ) C M, L M : Mutual Capacitance, Inductance per unit length C Total, L Total : Total Capacitance, Inductance per unit length (Source: Sohn, Advanced Packaging V24(4), 2001) 14

15 Far End Crosstalk (FEXT) ı Single-Ended Coupled Microstrip ı FEXT coefficient K f ı ı V FEXT K f ( t) = K t = 1 2 C ( C f M Total d Vin ( t t dt LM ) L f Total f ) t f : propagation time through the trace C M, L M : Mutual Capacitance, Inductance per unit length C Total, L Total : Total Capacitance, Inductance per unit length (Source: Sohn, Advanced Packaging V24(4), 2001) 15

16 Types of Crosstalk ı Alien Crosstalk (AXT) Crosstalk within a group or bundle of cables Alien Near-End Crosstalk (ANEXT) (IEEE terminology) Alien Far-End Crosstalk (AFEXT) (IEEE terminology) ı Power sum near end crosstalk (PSNEXT) power sum of NEXT of all other wire pairs on crosstalk in one pair (in UTP cables) ı Equal Level Far end crosstalk (ELFEXT) FEXT minus attenuation of cable ı Power Sum Equal Level Far end Crosstalk (PSELFEXT) power sum of ELFEXT of all other wire pairs on crosstalk in one pair (in UTP cables) 16

17 Types of Crosstalk(con ed) ı ICR: Insertion crosstalk ratio ICR = IL PSXT Similar to PSELFEXT but includes NEXT ı ICN: Integrated crosstalk noise Takes into account spectrum of excitation signal (Source: Sercu, DesignCon 2010) 17

18 Sources of Crosstalk Source: Wu, EMC V55(4), 2013 Source: Mukherjee, ECTC, 2013 ı Crosstalk happens even in ideal transmission lines ı Crosstalk on TSVs (through silicon vias) ı Crosstalk in packages ı Launch pattern for BGAs ı Crosstalk on vias in PCB ı Crosstalk through difference in propagations velocity of different modes in coupled stripline/microstrip Source: Lim, EMC V55(4), 2013 Source: Hsu, ECTC,

19 Tools to Measure Crosstalk 19

20 Measurement Techniques ı Systems tests Bit error rate test-set Not suitable to evaluate the amount of Crosstalk Real-time Oscilloscopes Some information on crosstalk via eye diagram Statistical correlation between different source are conceptually possible but of limited use ı Channel tests Time domain Reflectometry (TDR) Frequency domain measurements Using Vector Network Analyzer 20

21 Time Domain Channel Characterization ı TDR Measure reflected/transmitted energy and frequency content Based on equivalent time oscilloscopes ı Advantages: Intuitive Measure Information on impedance as function of electrical length ı Disadvantages Low dynamic range Not very accurate at high frequency Calibration is questionable Repeatability 21

22 Frequency Domain Measurements ı S-parameters Frequency/phase response of a channel ı Advantages: Highly accurate Large dynamic range Well-known calibration procedures/embedding/de-embedding Spatial information via IFT Up to 500+ GHz ı Disadvantages Has reputation of being complicated 22

23 How does a VNA work? ı VNA consists of Generator, directional element and receiver ı Generator sends out pure sine-wave ı Incident wave is measured with reference receiver ı Reflected wave is measured with one measurement receiver ı Transmitted wave is measured with another measurement receiver 23

24 Requirements for Crosstalk Measurements ı In principle, 2n port device can me measured with a 2- port VNA True differential measurements require at least 4- ports with 2 coherent sources Modern VNAs provide up to 48 ports via switch matrix ı Accuracy of models depends on accuracy of S- parameter measurements Stability of setup is crucial Connecting/reconnecting cables is error prone Source: Kim, AP V32(1), 2009 Reflectometer 2 Meas. Receiver Ref. Receiver PORT 2 Bias Tee Reflectometer 4 Meas. Receiver Ref. Receiver PORT 4 Bias Tee Reflectometer 1 Meas. Receiver Ref. Receiver PORT 1 Bias Tee DUT Meas. Receiver Reflectometer 3 Ref. Receiver PORT 3 Bias Tee 24

25 Crosstalk Measurements with Multiport VNAs True Multi-Port VNA vs. VNA plus switch matrix ZNB Port 1 ZNB Port 2 SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 25

26 Basics of VNA Measurement/Calibration ı Transform physical VNA to mathematical description of ideal VNA plus error terms ı Measured S-Parameter is S b ~ S e ~ = e e 3 11DUT 11Meas = = e00 + e a1 1 e11s11dut ı Apply open, short and match with known properties ı Solve for the unknown values e e e~ 00, 11, ı Measure Device S e 11Meas 00 S 11DUT = e~ + e 11 ( S 11Meas e 00 ) S 11 S, O 11S, S 11M 26

27 Terminology ı R: Reflection Tracking: R = e ~ = e e Measure of how much generator power is lost in the VNA Ideally 0 db ı S: Source Match (aka. Test Port Match) S = e 11 Measure how well VNA port is matched to DUT Limits accuracy for large values of reflection coefficient of the DUT Ideally infinite db e00 D = ı D: Directivity: R ability to discern between the forward and reflected waves Limits accuracy for small values of reflection coefficient of the DUT Ideally infinite db S 11Meas = e 00 ~ S + e 1 e 11DUT 11 S 11DUT S 11Meas Γ = R D + 1 SΓ DUT DUT 27

28 Uncertainty Estimation Reflection tracking Sourcematch Directivity ı Measurement uncertainty needs to be considered when DUT has high return loss Test set has high internal losses/high crosstalk Values for ZNB20 at 20 GHz: Sourced match: 38 db Directivity: 41 db Reflection tracking: 0.05 db 28

29 Testset with Switch Matrix ZNB Port 1 ZNB Port 2 ı Directivity D : ı Reflection Tracking R : ı Source Match S SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T SP4T Port 6 Port 5 Port 4 Port 3 Port 2 Port 1 D + SΠ11 D 2 SΠ21 R = es ~~ RS S S Π Π22 2 Π21 S S 11Meas 11Meas = e 00 ~ SΠS11DUT SΠ ~ 1 SΠ11S e ~ S ΠS 1 e11 SΠ SΠ11S Γ = R D + 1 S Γ DUT DUT 11DUT 11DUT 11DUT 29

30 Estimate of Uncertainty with Switch Matrix ı 18 GHz S21 = 3 db S11 = 15 db R&S 20 GHz Directivity 41 db 8 db Reflection Tracking 0.05 db 6.05 db Source Match 38 db 15 db R&S ZNB20 with ZV- Z81.66 ı Adding loss to a VNA (or TDR) Significantly reduces directivity Increase measurement uncertainty for 30

31 Summary 31

32 Summary ı Crosstalk is next frontier in conquering high-speed designs Crosstalk increases as data-rates go up Crosstalk increases as ICs/PCBs continue to shrink ı Multiple tools available to measure crosstalk True Multi-port VNAs, VNAs with switch matrix, True Multi-port TDR/TDT, TDR/TDT with switch matrix ı Test Engineers Know the measurement uncertainty of your test system ı Design/Modeling Engineers Before applying advanced mathematical techniques make sure the measurements meet the expectations 32

33 THANK YOU To learn more please go to: 33