1. Impedance Measurements on a PCB
PCB Transmission Lines Impedance and Interconnects Measurements Using TDR-Techniques
Tuomo Heikkilä
Systems Applications Engineer
Network Solutions
Applications Project Center
Tektronix Oy
Piispantilankuja 2A
ESPOO
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GSM: p

2. Impedance Measurements on PCB
4/26/2017
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. 0.5 fknee = tr tr Speed of the Signal Edge Rise Time
Circuit design must be extended up to the Knee Frequency!
fknee =
0.5 tr tr

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

5. Signal Speed vs Physical dimension
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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
D Q C tr

6. Two models for transmission lines
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

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

9. Microstrip – Line Discontinuities
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Microstrip – Line Discontinuities Z
Induc-
tive dis-
continuity
Open circuit
(no
termi-
nation) u
Capacitive Discontinuity
Connector
u/2 t

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

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

12. TDR Overview - Typical System
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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
4/26/2017
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
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Open end of
Transmission Line
Transmission Line
PCB Edge connector
TDR-Step
50  Coax Cable to PCB
Front Panel Connector

16. 4/26/2017

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

18. 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

19. 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

20. 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

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

22. 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

23. 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.
Virtual Short due to equal voltages on both conductors
+ drive
+ drive

24. 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.
Diff Imped divided into two
Virtual Ground due to opposite voltages on the conductors
+ drive
- drive
Ground Plane

25. 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

26. 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
TDR CH1

27. 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
”T” p

28. CSA8000 Waveform Math Dialog
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CSA8000 Waveform Math Dialog

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31. 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.

32. 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

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

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

35. Two Types of Crosstalk Forward CrossTalk, to the destination direction
Reverse CrossTalk, to the source direction
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.

36. Effect of CrossTalk Depends on: Consequencies are:
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

37. Measuring CrossTalk with the CSA8000

38. 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

39. 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

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

42. 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 = 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

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

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

45. 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)

46. 4/26/2017
Tektronix CSA8000