2013 belforte caniggia_cst_coax_cable_si_ final_240312


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A comparative study on modeling techniques of coaxial cables.

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2013 belforte caniggia_cst_coax_cable_si_ final_240312

  1. 1. CST coaxial cable models forSI simulations: a comparative study Piero Belforte, Spartaco Caniggia March 24th 2013 1
  2. 2. Outline• Introduction• S parameters in frequency domain• S parameters in time domain• Comparison between measurements and simulations• Ultra Wide Band (UWB) source• Proposal for efficient and accurate simulation of lossy cables• Conclusion 2
  3. 3. Introduction• The task of this report is to show that some important signal integrity (SI) problems arise when Cable Studio (CS) is used to simulate high-speed digital signal transmission with lossy lines (cables or traces in PCB) [1]• An 1.83-m RG58 coaxial cable is modeled by CS and commercial programs: MC10 [2] and DWS , based on Digital Wave Network equivalent of the electrical network [3].• Simulations are compared with measurements• It is shown that CS doesn’t provide good results• A method is proposed to solve the SI problems with CST Cable and Design studio. 3
  4. 4. S parameters in frequency domain 4
  5. 5. S parameter computation• Cable: RG58• Length: 5cm• Frequency range: 0-10GHz• Characteristic Impedance Z0: 49.94Ω• Only ohmic losses are taken into account because dielectric losses with tanδ=0.0002 at 100MHz (Polyethylene) doesn’t give significant changes.• SPICE simulation performed by MicroCap10 (MC10) because of good TL models [2]• DWS (Digital Wave Simulator) analysis because of speed (50X MC10), accuracy and time-domain scattering parameters.• Comparison among CST Cable Studio, CST MWS, MC10 and DWS (Digital Wave Simulator) 5
  6. 6. Equivalent circuit used by MC10 (SPICE) for theoretic S11 & S21 computation (analytic approach) For details, see [1, clause 11.2.3] 50Ω 5-cm RG58: Z0=49.94Ω 50Ω File:S_LOSSYTL_ANALYTICAL_10GHZ.CIR (MC10) Insulator outside: thickness=0.5mm,Solid shield screen type permittivity=3, Loss angle tanδ=0.02 εr rsPermittivity=2.3, Loss Coaxial cableangle tanδ=0.0002 geometry 2rw 6 ts
  7. 7. CST cable studio for S11 & S21 computation RG58: length=5 cm,50 Ω Z0=49.94Ω 50 Ω File: Ex_coax_S_5cm.cstEquivalent circuit to compute Sparameters by CST DESIGNSTUDIO 7
  8. 8. 3D RG58 model by MWSWaveguide port Meshcells=41,515 Time domain solver: adaptive mesh refinement was used 8
  9. 9. S11 • S11 computed by Cable Studio 2010 & 2012 provide the same results • S11 computed by MWS and MC10 provideCable Studio (CS) 2012 similar results and about some dB lowerCable Studio (CS) 2010 •Level differences are due to impedance mismatchingMWS Studio 2012 • Resonance frequencies are slightly higherMC10 2012 for MWS (lower cable delay) 9
  10. 10. S21 MWS CS MC10,DWS • S21 computed by MC10 is the lowest curve (more losses)Cable Studio (CS) 2012 • S21 computed by CST 2012 is too higher than CST 2010 (less losses)Cable Studio (CS) 2010 • S21 computed by MWS is in the middleMWS Studio 2012 between MC10/DWS and Cable studio 2012MC10 2012,DWS 8.4 and close to cable studio 2010RL-TL model 10
  11. 11. Comments on computation of S parameters• S11 computed by MWS and MC10/DWS provide similar values both in time domain and frequency domain• S11 computed by Cable Studio 2010 & 2012 are about 15dB higher than MWS and MC10/DWS due to characteristic impedance mismatching• S21 computed by Cable Studio 2012 provides much less losses than those computed by Cable Studio 2010• S21 computed by Cable Studio 2010 is close to MWS• CST should investigate the last two items 11
  12. 12. S parameters in time domain 12
  13. 13. Lossy line matched at both ends Typical source and load voltage waveforms for an interconnect matched at both ends: lossless TL (dashed line), frequency-dependent lossy TL (solid line) [1, Fig.7.3] Definitions of S parameters in time domain: •S11=VS-1 •S21=VLWhen TL has characteristic impedance different from the loads, distortions occur 13
  14. 14. Voltage computations in time domain• Cable: RG58• Length: 1.83m• Line terminations: 50Ω• Source: step waveform with rise time tr=0.1ns• Frequency range: 0-10GHz• Characteristic Impedance Z0: 49.94Ω• SPICE simulation performed by MC10 [2]• DWS simulations performed by DWS 8.4 [4]• Comparison between CST & SPICE results• DWS results are the same of MC10 14
  15. 15. Coaxial cable structure Z0=49.94 Ω Length:1.83m 50 ΩRamp 50 Ω V1 V2Source Vsource=2 V trise=0.1 ns 15
  16. 16. Circuit and model used in MC10 and DWS (RL-TL approach) Coaxial cable matched at both ends and modeled as a cascade of 610 3-mm RL-TL cells including the skin effect,] RL-TL model: RL parameters were computed by vector fitting technique starting from analytic expressions for ohmic losses, see [1, clause] Step signal V1=VS V2=VLRemark: the cascade of RL-TL cells provides the same S11 and S21 infrequency domain computed by the analytic approach used in the previous 16section, see Fig.7.22 of [1]
  17. 17. Circuit and cable model used in CSTVinit: 0.0Vpulse: 2.0Tdelay: 1e-9Trise: 0.1e-9Thold: 100e-9 RG58 model withTfall: 0.1e-9 length 1.83 mTtotal: 200e-9 File: Ex_coax_S_1_83_10GHz.cst 10GHz Skin effect only 17
  18. 18. Voltages V1 & V2 (cst 2010) MC10 (SPICE) CST V1 V2 V1 V2 Samples 5001 in Samples 1001 in transient1 task transient1 task ns ns ? ? V1 V1 V2 V2 ns nsMC10 and CS have the same losses except the oscillations provided by 18CST 2010 that should not occur
  19. 19. Voltages V1 & V2 (cst 2012) MC10 (SPICE) CST • CST cable studio 2012 provides less losses than MC10 and CS 2010, as evidenced by frequency computation of S parameters. • Oscillations remain • Using normal or very high accuracy the results do not change 19
  20. 20. With 1-GHz model computed by CST 2012 Oscillations increase! 20
  21. 21. DWS 37-cell model vs CST MWS: S11 •It can be noted that MWS computes about half losses than DWS. •S11 of MWS was obtained calculating the integral of the reflected wave (o1,1) as response to a step source. DWS MWS 21
  22. 22. Comments on computation of V voltages• V1: the voltages at source end computed by MC10 (SPICE)/DWS and CST 2010 are in good agreement.• V2: the voltages at load end computed by SPICE/DWS and CST 2010 are in good agreement except for the oscillations in CST waveform.• V1 and V2 computed by CST CS 2012 are not in agreement with MC10/DWS, less losses are computed by CST 2012 and unrealistic oscillations on V2 remain.• CST should investigate these two last items• Time domain S11 from CST MWS is lower (about half) of that from RL-TL model simulated with DWS as already noticed in return loss vs frquency 22
  23. 23. Comparison betweenmeasurements and simulations 23
  24. 24. Comparison between measurements and simulationsThe measurements performed on 1.83-m RG58 cables are compared with three simulation methods:1. CST cable studio.2. MC10, based on SPICE [2] and using a cascade of 610 3-mm RT-TL unit cells.3. DWS models using both 366 X 5mm RL-TL chain of cells and a 3660 X .5mm RL-TL chain inserted in actual CSA803 measurement setup. 24
  25. 25. CST model (Step source)Vinit: 0.0Vpulse: 2.0Tdelay: 1e-9Trise: 0.1e-9 50-Ω RG58 model with lengthThold: 100e-9Tfall: 0.1e-9 1.83 m (very high accuracy,Ttotal: 200e-9 ohmic losses in CS) Open•V1 (or VP1) voltage at the input of the cable was computed and measured•Dielectric losses are neglected for SPICE (MC10) and CS (Cable Studio 2012) 25
  26. 26. DWS (4) cable cell on Spicy SWAN (5)(Due to DWS sim speed, even a .5mm cell has been tried) 26
  27. 27. Example of Spicy SWAN (DWS) circuit for S-parameter cable characterization using a chain of cells(Due to DWS sim speed, even a chain of 3660 X .5mm RL TL cells has beenutilized, getting practically the same results of the 366X 5mm cell model) 27
  28. 28. RG58 CU (TASKER) specs 28
  29. 29. Measurement set-up (CSA803) 29
  30. 30. Measurements with cable open at far-end voltage1.2 V1 The measurements were performed by Piero Belforte0 on two commercial 1.83-m RG58 cables: Tasker and Reflected edge GBC. ns-1 501.2 V1 Comparison of the Tasker reflected edge of the GBC two cables: very little differences. ns0 4 30
  31. 31. VP1:voltage at cable inputV1 CST 2012 Measurement MC10 ns DWS (including TDR setup)V1 ns 31
  32. 32. VP1 voltage detailsV1 ns• There is good agreement on reflected edge among RL-TLmodel using both MC10 and DWS simulators (DWS is CST 201250X faster than MC10) and measurements. Note thatdielectric losses were neglected in the RL_TL model and Measurementactual cables have stranded conductors (not solid) MC10• CS reflected edge is affected by not acceptable 32 DWSoscillations
  33. 33. S-parameters measurements and comparison with 366 RL_TL model in the actual setup (DWS) S21 S21 S11 S11 33
  34. 34. Actual S-parameter measurements: considerations• Actual cable (stranded conductors) shows significant distributed impedance discontinuities• S11(S22) in time domain shows larger values than model• Actual S11 and S22 are not identical (not symmetrical) due to impedance discontinuities• S21(S12) edge is slightly slower from 0 to 50% due probably to dielectric losses• S21(S12) edge is slightly faster from 50% to 100% due probably to stranded conductors (lower skin effect losses at high frequency) 34
  35. 35. DWS BTM (Behavioral Time Model) of 1.83m cable using Spicy SWAN 1 cellsS frommeasurements 1V 366 cells 50 ns of RL-TL 1V 0.035 BTM BTM RL-TL 12 ns RL-TL 50 ns 35
  36. 36. Comments on measurements and simulations• MC10 (SPICE) and DWS open cable and S21 are in good agreement with measurements despite the stranded (not solid) conductors of actual cable.• S11 of measurements takes into account slight distributed impedance mismatching along the cable therefore more accurate models should be needed for a high level of accuracy.• Dielectric losses are much less important than ohmic losses and can be neglected for most applications• CST cable studio provides not realistic oscillations (distorted waveforms) as verified by measurements 36
  37. 37. Ultra Wide Band (UWB) source 37
  38. 38. Coaxial cable with source an UWB signal• The same coaxial cable of previous example was tested by using as a source an ultra wide band (UWB) signal instead of a step waveform.• The signal is introduced into design studio as imported file. 38
  39. 39. MC9 model (UWB source)Coaxial cable matched at both ends and modeled as a cascade of 610 cellsincluding the skin effect: comparison between measured (dashed line) andcomputed (solid line) waveforms [1, chapter7] Model Validation 39
  40. 40. CST model (UWB source)Imported file:New_uwb_input_by2.txt RG58 Ohmic losses File: Ex_coax_UWB.cst 40
  41. 41. Comments on coaxial cable with UWB source• SPICE (MC10) runs in some minutes and gives waveform on 50-Ω load in good agreement with measurement• CST runs with very long time and the simulation was aborted. 41
  42. 42. Proposal for Signal Integrity of lossy cable 42
  43. 43. Method• Define the cable by its geometrical and electrical parameters• Choose between two unit-cell models: 1. RL-TL: the unit cell should be electrically short for the frequencies of interest. It is modeled as a network of resistances and inductances to take into account the ohmic and electric losses (analytic expression in frequency domain) computed by vector fitting technique in series with an ideal transmission line (TL) as reported in chapter 7 of [1]. Simulator: SPICE with good TL model [2], DWS (50X faster) [3]. 2. S-parameter: the unit cell should have a length to satisfy the rule that the rise-time excitation should be less than 1/10 the unit-cell delay. It is modeled by using S-parameters in time domain (2D or 3D computation) as defined in [1,3]. Simulator: DWS only [3]• Model the line by a cascade of unit cells.• Perform simulations in time domain by using SPICE [2] or DWS (more accurate and 50X faster) [3] to get the voltage or current waveforms. Remark: the method can also be used for interconnections in PCB such as microstrip and stripline traces 43
  44. 44. Flow chart Define the cable Define an unit-cell cable Vector fitting unit cell 2D/3D S-parameter unit cell to set RL TL network computation S1RL Which 1RL solution ? S2 1 RL-TL Model S-parameter Model (DWS) (SPICE, DWS) Cascade of unit cells Results obtained by SPICE or DWS time domain simulations 44
  45. 45. Conclusion• The 2D (TL) modeling in CST CABLE STUDIO should be revised because it provides unexpected oscillations on signals when the source is a step waveform.• CST Cable Studio 2012 provides less losses than CST 2010.• CST Cable Studio results are not in agreement with MWS, SPICE and DWS simulations and measurements.• There are instability problems in CST when the source is an ultra wide band signal imported as external file.• We suggest to use the method presented at the end of this document that consists of a cascade of unit-cable cells simulated by SPICE or DWS (50X faster).• DWS supports fast simulations of both time domain s-parameter and RL-TL chain of cells.• BTM (Behavioral Time Model) method supported by DWS is the fastest and most accurate if unit-cell S-parameters are taken from actual measurements. 45
  46. 46. References[1] S. Caniggia, Francesca Maradei, “Signal Integrity and Radiated Emission”, John Wiley & Sons, 2008[2] www.spectrum-soft.com[3] P.Belforte “Time domain simulation of lossy interconnections using wave digital networks” ISCAS 1982 Rome[4] DWS (Digital Wave Simulator) user manual http://www.slideshare.net/PieroBelforte1/dws-84- manualfinal27012013[5 ] Spicy SWAN : www.ischematics.com http://www.slideshare.net/PieroBelforte1/spicy-swan-concepts- 16663767[6] DWS and SWAN, ( Simulation by Wave Analysis) are trademarks of Piero Belforte http://www.linkedin.com/in/pierobelforte 46