tdr & crosstalk measurements – april 2001, tuomo heikkilä slide 1 pcb transmission lines...

TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä

Slide 1

PCB Transmission Lines Impedance and Interconnects Measurements

Using TDR-Techniques

Impedance Measurementson a PCB

Tuomo HeikkiläSystems Applications

EngineerNetwork SolutionsApplications Project Center

Tektronix OyPiispantilankuja 2A02240 ESPOO

puh: 09-4783 400GSM: 040-506 4401email: [email protected]

p


Slide 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


Slide 3

Speed of the Signal

– Edge Rise Time

– Circuit design must be extended up to the Knee Frequency!

fknee = 0.5

trtr


Slide 4

sin(x)/x ; tr = 0

sin(x)/x [tr = 0] * sin(x)/x [tr = 1.3ns]

fknee = 0.5/tr (385MHz)


Slide 5

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


Slide 6

Two models for transmission lines

– Distributed model: each section of line separated and valued for the transmission line

–long line, short rise time

–individual values at each location

– Lumped model: single component value for the line

–short line, long rise time

–no indication of location


Slide 7

PCB Microstrip – Transmission Line

Ground Plane

Strip Width

PCB Thickness

Impedance level of the microstrip line is a function of:• Dielectricity () of the substrate• Board Thickness• Strip Width.


Slide 8

Coupling in Microstrip line

Ground Plane

Microstrip Line

Any time when there is Voltage between Planes, there is a

Capacitance involved

Any time when there is Current flow in two Conductor system inside the Magnetic field, there is an Inductance involved


Slide 9

Microstrip – Line Discontinuities

u

Z

u/2

Connector

CapacitiveDiscontinuity

Induc-tivedis-

continuity

Open circuit(no

termi-nation)

t


Slide 10

Connector

CapacitiveDiscontinuity

Induc-tivedis-

continuity

Open circuit(no

termi-nation)

u

Z

Time Domain Reflectometry – TDR

u/2

u

u

2*t


Slide 11

TDR Overview - Reflection Coefficient and Impedance

Rho () =Reflected Amplitude

Incident Amplitude=

Z T - Z0

Z T + Z0

Where ZT represents the trace impedance

Z0 is a known impedance (the characteristic impedance of the TDR system)

is measured by the oscilloscope

ZT = Z0

1 + 1 -

Tek CSA800 calculates -profile into display waveform:


Slide 12

TDR Overview - Typical System

Incident Step

50

Step GeneratorRise Time = 17.5 ps

Reflections

Sampler

BW and Rise Time of system must support the resolution : 20GHz = 17.5ps


Slide 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


Slide 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


Slide 15

TDR-Step

Front Panel Connector

50 Coax Cable to PCB

PCB Edge connector

Transmission LineOpen end ofTransmission Line


Slide 16


Slide 17

Differential Line on PCB

Two Microstrip Lines close enough to have both inductive and capacitive coupling;

Ground Plane

They also have coupling to theground plane.


Slide 18

Differential Line with Coupling to Ground

+ drive

- drive

Ground Plane

Differential Mode(Odd Mode)

Common Mode(Even Mode)

– Modes of Signal

– Modal Propagation (dispersion, wave shape distortion)


Slide 19

Impedance?

• Zo; Characteristic Impedance• Differential Impedance• Commom Mode Impedance• Odd Mode Impedance• Even Mode Impedance

Definition of Impedance may vary within various applications > check the application.

For TDR-measurements on HW-design, following is used:


Slide 20

+ drive - drive

Characteristic Impedance

Zo is the Impedance between the conductorswhen there is no Coupling to Ground

This is the same as an ordinary Transmission Line Differential Impedance and

applies to Unshielded Twisted Pair


Slide 21

+ drive - drive

Differential Mode Impedance

Differential Impedance is the impedance between the conductors when there is a coupling to Ground.


Slide 22

Common Mode Impedance

+ drive + drive

Common Mode Impedance is the impedance between the short connected conductors and the Ground (when there is a coupling to Ground)

Short connection


Slide 23

Even Mode Impedance

+ drive + drive

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


Slide 24

Odd Mode Impedance

+ drive - drive

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.

Ground Plane

Virtual Ground due to opposite voltages on the conductors

Diff Imped divided into two


Slide 25

CSA8000 TDR – Which does it measure?

50 Ohm Terminator

Sampler

To CSA waveform CSA8000 TDR measures:

• TDR-Profile• Only against Ground

Transmission Line under test


Slide 26

CSA8000 TDR – Differential TDR?

TDR CH1

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


Slide 27

CSA8000 – Averaged Differential Impedance

”T”

From the ”T” and ””-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


Slide 28

CSA8000 Waveform Math Dialog


Slide 29


Slide 30


Slide 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.


Slide 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


Slide 33

Capacitively Coupled CrossTalk

– Spike divides to Two: one travels to Reverse, other to Forward direction

u

uAggressor Line

Victim Line

– Data Edge arrives an empty capacitor at the Aggressor Line

– Edge couples via C onto the Victim Line

– Victim Line has a Spike of Rise Time length

– Polarity is the same


Slide 34

– Data Edge arrives an empty location

Inductively (Transformer) Coupled CrossTalk

– Negative polarity travels to Forward, along with the aggressor spike

– Current spike fills the location

– Magnetic Field Spike is generated

– Transformer couples the voltages on to Victim line

– Positive polarity travels to Reverse direction

i

i

uu

u


Slide 35

Two Types of Crosstalk

– Forward CrossTalk, to the destination direction– Reverse CrossTalk, to the source direction

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.

Aggressor Line

Victim Line


Slide 36

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


Slide 37

Measuring CrossTalk with the CSA8000


Slide 38

Example 1: Forward with all Terminated

Termination 50

Termination 50

Aggressor Line

Victim LineTermination 50

CSA8000

Filter – function in CSA8000 will display the TDR step Gen aggregated CrossTalk at the amplitude of ”Filtered Output Equivalent Aggregated” XrossTalk


Slide 39

Example 2: Reverse with all Terminated

Termination 50

Termination 50

Aggressor Line


CSA8000

Filter – function in CSA8000 will display the TDR step Gen aggregated CrossTalk at the amplitude of ”Filtered Output Equivalent Aggregated” XrossTalk


Slide 40

Termination 50

CSA8000

Termination 50

Aggressor Line

Victim Line

Example 3: Forward with no Termination at the end of Aggressor Line

Ordinary forward CrossTalk

Secondary Reverse CrossTalkfrom backreflection on Aggressor line

Line amplitue reaches full value if aggressor is not terminated.Similar Edge starts propagation to the source.


Slide 41


Slide 42

Example 4: Reverse at the end of Victim with Low- Drive at Source

Termination 50OhmAggressor Line

Victim Line

ECL source Z = 15..25 OhmsLine 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

Termination 50

CSA8000


Slide 43

Example 5: Reverse CrossTalk with Unterminated Lines

Aggressor Line


CSA8000

Open end reflects original pulse back.

Original Reverse CrossTalk

Forward from Reflected Pulse added with Reflection of Original Forward CrossTalk

Reflected Reverse CrossTalk from Reflected Original Pulse

Open end reflects alll pulses back.


Slide 44

Example 6: Unterminated Lines

Aggressor Line


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


Slide 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)


Slide 46

Tektronix CSA8000

tdr & crosstalk measurements – april 2001, tuomo heikkilä slide 1 pcb transmission lines...

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