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Impedance Measurements on a PCB

Tuomo Heikkilä Systems Applications Engineer Network Solutions Applications Project Center. Tektronix Oy Piispantilankuja 2A 02240 ESPOO puh: 09-4783 400 GSM: 040-506 4401 email: tuomo.heikkila@tek.com. p. Impedance Measurements on a PCB.

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Impedance Measurements on a PCB

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  1. Tuomo Heikkilä Systems Applications Engineer Network Solutions Applications Project Center Tektronix Oy Piispantilankuja 2A 02240 ESPOO puh: 09-4783 400 GSM: 040-506 4401 email: tuomo.heikkila@tek.com p Impedance Measurementson a PCB PCB Transmission Lines Impedance and Interconnects Measurements Using TDR-Techniques

  2. Impedance Measurements on PCB • Speed of Signal • Transmission Line on a PCB • TDR Concept • Microstrip Line Impedance Measurement • Differential Line on PCB • The Impedance? • Differential TDR • CrossTalk on PCB • Tektronix CSA8000 – TDR Sampling Oscilloscope

  3. tr 0.5 fknee = tr Speed of the Signal • Edge Rise Time • Circuit design must be extended up to the Knee Frequency!

  4. sin(x)/x ; tr = 0 sin(x)/x [tr = 0] * sin(x)/x [tr = 1.3ns] fknee = 0.5/tr (385MHz)

  5. D Q C tr Signal Speed vs Physical dimension • Edge Length over Substrate • If length of line longer than 1/6 of rise time, line behaves as a Transmission Line • Propagation Delay Time • Change of Impedance causes a Reflection

  6. Lumped model: single component value for the line • short line, long rise time • no indication of location • Distributed model: each section of line separated and valued for the transmission line • long line, short rise time • individual values at each location Two models for transmission lines

  7. PCB Microstrip – Transmission Line Strip Width PCB Thickness e Impedance level of the microstrip line is a function of: • Dielectricity (e) of the substrate • Board Thickness • Strip Width. Ground Plane

  8. Any time when there is Current flow in two Conductor system inside the Magnetic field, there is an Inductance involved Any time when there is Voltage between Planes, there is a Capacitance involved Coupling in Microstrip line Microstrip Line Ground Plane

  9. Connector Open circuit (no termi- nation) Induc- tivedis- continuity CapacitiveDiscontinuity Microstrip – Line Discontinuities Z u u/2 t

  10. Time Domain Reflectometry – TDR u Z u Connector Open circuit (no termi- nation) Induc- tivedis- continuity CapacitiveDiscontinuity u u/2 2*t

  11. Z - Z Reflected Amplitude T 0 Rho (  ) = = Z + Z Incident Amplitude T 0 Where ZT represents the trace impedance Z is a known impedance 0 (the characteristic impedance of the TDR system)  is measured by the oscilloscope TDR Overview - Reflection Coefficient and Impedance Tek CSA800 calculates -profile into display waveform: 1 +   Z = Z T 0 1 - 

  12. TDR Overview - Typical System BW and Rise Time of system must support the resolution : 20GHz = 17.5ps Incident Step Sampler Reflections 50  Step Generator Rise Time = 17.5 ps

  13. Tektronix CSA8000 – with TDR Sampler 50GHz mainframe 80E04: • 20GHz BW • 17ps tr TDR-step generator • Dual-TDR with deskew adjustment • 35ps System- RiseTime • ~ 3mm resolution

  14. TDR – How to set it up? • Open Setup Dialog (Windows-like) • Click the TDR Tab • TDR modules only active • Switch On TDR Step Generator • Separately activate acquisition • Select vertical scale Volt,  ,  • Click Cx (C3) Preset for TDR Autoset

  15. Open end of Transmission Line Transmission Line PCB Edge connector TDR-Step 50  Coax Cable to PCB Front Panel Connector

  16. Ground Plane Differential Line on PCB Two Microstrip Lines close enough to have both inductive and capacitive coupling; They also have coupling to the ground plane.

  17. Differential Line with Coupling to Ground • Modes of Signal • Modal Propagation (dispersion, wave shape distortion) Differential Mode (Odd Mode) + drive Common Mode (Even Mode) - drive Ground Plane

  18. Impedance? Definition of Impedance may vary within various applications > check the application. For TDR-measurements on HW-design, following is used: Zo; Characteristic Impedance Differential Impedance Commom Mode Impedance Odd Mode Impedance Even Mode Impedance

  19. Characteristic Impedance Zo is the Impedance between the conductors when there is no Coupling to Ground This is the same as an ordinary Transmission Line Differential Impedance and applies to Unshielded Twisted Pair + drive - drive

  20. Differential Mode Impedance Differential Impedance is the impedance between the conductors when there is a coupling to Ground. + drive - drive

  21. Common Mode Impedance Common Mode Impedance is the impedance between the short connected conductors and the Ground (when there is a coupling to Ground) Short connection + drive + drive

  22. Virtual Short due to equal voltages on both conductors Even Mode Impedance Even Mode Impedance is the impedance between one conductor and the ground plane when both conductors are driven with same polarity signal against the ground. + drive + drive

  23. Diff Imped divided into two Virtual Ground due to opposite voltages on the conductors Odd Mode Impedance Odd Mode Impedance is the impedance between one conductor and the ground plane when the coductors are driven with opposite polarity signal against the ground. + drive - drive Ground Plane

  24. CSA8000 TDR – Which does it measure? To CSA waveform CSA8000 TDR measures: • TDR-Profile • Only against Ground Sampler 50 Ohm Terminator Transmission Line under test

  25. TDR CH1 CSA8000 TDR – Differential TDR? CSA Differential TDR measures: • ODD Mode TDR-Profiles: When conductors are driven with opposite polarity steps that are simultaneous in time • EVEN Mode TDR-Profiles: When conductors are driven with same polarity steps that are simultaneous in time TDR CH2

  26. ”T” p CSA8000 – Averaged Differential Impedance From the ”T” and ”p”-models following can be calculated: Averaged Differential Impedance = TDR odd1 + TDR odd2 Averaged Common Mode Impedance = TDR even1 // TDR even2 These can be directly acchieved by the CSA8000 by: • setting the TDR Polarity • adjusting the TDR Skew • Waveform Math

  27. CSA8000 Waveform Math Dialog

  28. Crosstalk – Why to consider? While speed in Digital HW increases, one of the consequencies is, and will be more and more in the future, that Crosstalk becomes (one of) the major new bottleneck in succesfull product launches. HW designers everywhere meet, or will soon meet a need to measure Crosstalk in PCB’s, in Buses, and Cable Sets.

  29. Modeling CrossTalk in Microstrip Bus • Mutual Inductance and Mutual Capacitance are causes • Data Edge travelling in the Transmission Line generates: • Capacitively coupled CrossTalk • Inductively, ie Transformer coupled CrossTalk

  30. u Aggressor Line Victim Line u Capacitively Coupled CrossTalk • Data Edge arrives an empty capacitor at the Aggressor Line • Spike divides to Two: one travels to Reverse, other to Forward direction • Edge couples via C onto the Victim Line • Victim Line has a Spike of Rise Time length • Polarity is the same

  31. u i i u u Inductively (Transformer) Coupled CrossTalk • Data Edge arrives an empty location • Current spike fills the location • Magnetic Field Spike is generated • Transformer couples the voltages on to Victim line • Positive polarity travels to Reverse direction • Negative polarity travels to Forward, along with the aggressor spike

  32. Aggressor Line Victim Line Forward CrossTalk: Faster Edge -> Higher Amplitude. Amplitude grows up while travelling the line. Capacitive and Inductive cancel out if L and C in balance. Destination receives a spike. Reverse CrossTalk: Amplitude is low and only based on mutual impedance value. Capacitive and Inductive sum up. Source receives a pulse that equals length of line. Two Types of Crosstalk • Forward CrossTalk, to the destination direction • Reverse CrossTalk, to the source direction

  33. Effect of CrossTalk • Depends on: • is it Forward or Reverse • drive source impedance • destination termination impedance • Consequencies are: • Unwanted spikes are generated • Edges suffer shape distortion and jitter • Noise level increases • Reflections are generated if improper terminations

  34. Measuring CrossTalk with the CSA8000

  35. Example 1: Forward with all Terminated Aggressor Line Termination 50  Victim Line Termination 50 Termination 50  CSA8000 Filter – function in CSA8000 will display the TDR step Gen aggregated CrossTalk at the amplitude of ”Filtered Output Equivalent Aggregated” XrossTalk

  36. Example 2: Reverse with all Terminated Aggressor Line Termination 50  Victim Line Termination 50  CSA8000 Termination 50  Filter – function in CSA8000 will display the TDR step Gen aggregated CrossTalk at the amplitude of ”Filtered Output Equivalent Aggregated” XrossTalk

  37. Aggressor Line Victim Line Termination 50  CSA8000 Termination 50 Ordinary forward CrossTalk Line amplitue reaches full value if aggressor is not terminated. Similar Edge starts propagation to the source. Secondary Reverse CrossTalk from backreflection on Aggressor line Example 3: Forward with no Termination at the end of Aggressor Line

  38. Example 4: Reverse at the end of Victim with Low- Drive at Source Aggressor Line Termination 50Ohm Victim Line Termination 50  CSA8000 ECL source Z = 15..25 Ohms Line Z = 50 Ohms. • Reverse pulse will reflect with more than half amplitude and opposite polarity. • Noise Level Increases. If destination is not properly terminated, portion of reflected reverse pulse will reflect third time. • Noise Level increases more

  39. Aggressor Line Open end reflects original pulse back. Victim Line Open end reflects alll pulses back. Termination 50  CSA8000 Original Reverse CrossTalk Forward from Reflected Pulse added with Reflection of Original Forward CrossTalk Reflected Reverse CrossTalk from Reflected Original Pulse Example 5: Reverse CrossTalk with Unterminated Lines

  40. Aggressor Line Open end reflects original pulse back. Open source reflects all CrossTalk pulses back. Victim Line Termination 50  CSA8000 Several reflections will occur due to CrossTalk between lines from earlier CrossTalk signals Example 6: Unterminated Lines

  41. Tektronix Solution: The CSA8000 TDR • CSA8000 provides with 80E04 dual TDR capability • Can resolve Differential Line TDR Profile • Measures Odd and Even Mode TDR profiles • Common Mode and Differential Impedance Profiles can be calculated by Waveform Maths • High Accuracy by 17ps Rise and Skew Adjustment • CrossTalk Measurements are easy • Filter transforms from TDR BW down to actual BW with no errors in LTI-circuits (LTI=Linear Time Invariant)

  42. Tektronix CSA8000

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