cable testing using tdr and tdt methods presented by christopher skach tektronix dima smolyansky tda...

Cable Testing using TDR and TDT methods

Presented byChristopher Skach

TektronixDima Smolyansky

TDA Systems

Tektronix Net Seminar 2003Slide 2

Agenda

Cable specifications and standards Introduction to TDR and TDT measurements Measurement Techniques Using TDA IConnect® software for frequency

domain measurements Conclusion Questions and Answers


Higher Clock and Data Rate Challenge

Infiniband 2.5Gb/s Firewire 1394b 1.6Gb/s Serial ATA 1.5Gb/s DVI 1 Gb/s High Speed USB 480 Mb/s


Cable and Connector Specifications

Standard committees and trade associations are specifying the exact methods and measurements required to meet the signal integrity and interoperability requirements.

These specifications are developed for compliance testing requirements and used to guarantee interoperability between many vender devices.

Specifications can be found on Trade Association web sites such as:

www.serialata.org

www.usb.orgwww.infinibadta.orgwww.1394ta.org EIA standard testing documents can be found at:

http://www.ec-central.org/PDF/Engineering/EIA364/EIA-364-108.pdf

http://www.serialata.org/



http://www.usb.org/

http://www.infinibadta.org/



http://www.1394ta.org/






Cable and Connector Specifications

Electrical testing required for most cables and connectors: Impedance (for these new standards this is Differential

Impedance and is a TDR measurement) Crosstalk ( This requires TDR & TDT Differential

measurements) Rise Time (Differential TDR & TDT measurements) Skew (Differential TDR & TDT measurements)

All of these measurements can be made using direct TDR/TDT methods.

Loss, Jitter and Eye opening ( Frequency domain measurement) Can be obtained using TDA Iconnect software and TDR

and TDT methods.


Why use TDR/TDT Methods?

TDR can identify mismatches and variations of impedance which cause signal integrity problems. These SI problems can lead to data and logic errors and severe, hard-to-identify reliability problems especially in cables and connectors.

TDR/TDT measurements are easier to setup and use than many other devices such as a VNA.

One instrument can accomplish all tests required Substantially lower test costs!

Automation can accommodate increased testing requirements.

Can also be used as a troubleshooting - design tool.


Example of TDR results on production sample

Initial inspection found excess solder on center pin of SMA connector, shown as yellow

TDR trace

Blue TDR trace is after solder removal


Agenda




So What is TDR?

Time Domain Reflectometry - a measure of reflection in an unknown device, relative to the reflection in a standard impedance.

Compares reflected energy to incident energy on a single-line transmission system. Known stimulus applied to the standard impedance is

propagated toward the unknown device Reflections from the unknown device are returned toward

the source and measured.

FastStepSource

CharacteristicImpedance Z1 =Z0

ImpedanceChange Point

CharacteristicImpedance Z2 > Z0


TDR Fundamentals –Oscilloscope Monitoring

Use an oscilloscope to monitor the transmission line signal at the step source input point.

The oscilloscope waveform will show the combined sum of the incident and reflected propagating signals in proper time sequence.

FastStep

Source

Z1 = Z0Z2 > Z0

Oscilloscope Measurement

Point

Transmission Line


What is TDT?

Time Domain Transmission - a measure of propagation transmission in an unknown device.

A TDR step is propagated down a transmission line and then measured .

Amplitude can be measured to determine the loss of the line or cross talk and other measurements such as rise and fall time can be measured to determine signal integrity through the line.

Requires a TDR step and a separate sampling channel to acquire transmitted signal.


TDT Fundamentals –Oscilloscope Monitoring

Use an oscilloscope to monitor the transmission line signal at the step source input point and at specific points of the transmission line.

FastStep

Source

Second Oscilloscope Measurement

Point

First Oscilloscope Measurement

Point

Time

Amplitude

Transmission Line

Delay/Skew measurements

Rise-Fall time measurements


TDT Measurements – Cross Talk

Mutual coupling and crosstalk between signal lines can be characterized with TDT measurements.

Apply the TDR step on one signal line and measure the signal strength on the other.


TDR Measurements – Differential TDR Measurement

Higher noise immunity due to common mode rejection

Less radiated noise due to canceling fields More precise timing characteristics Less crosstalk due to noise immunity and less

radiated energy Less power supply noise due to current transients

With the signal integrity issues many designs have gone to differential transmission lines to achieve:


TDR Measurements – Differential Clock Coupling

Attempting to measure the two halves of the differential pair separately can produce misleading results.

Two traces in close proximity tend to read a lower impedance than their characteristic impedance as a pair.

Proper characterization of the differential impedance of the transmission line to maintain voltage and timing margins.


TDR Measurements – Differential TDR Measurement

A differential TDR measurement is performed much like a single-ended TDR measurement

Use two TDR sampling head channels with the simultaneous step generators set to opposite polarities


TDR Measurements – Differential TDR Step Timing Skew

Another important consideration when making differential TDR measurement is the alignment of the TDR step pulses.

The positive and negative going TDR steps must be adjusted so there is not any time skew between them at the transmission launch point.


Agenda




Impedance Measurement (TDR)

Measure Characteristic Impedance of Cable and\or Connector.

Considerations: Specified as differential pair. Some applications specify a specific rise time of the

incident pulses. Test fixture required.

TDS 8000B Sampling Oscilloscope

Sampler Extender Cable

80E04 Differential TDR Module

Test Fixture

High Speed SMA cables

DUT Test Cable

Unused lines terminated with 50 Ohms to Ground

Test Fixture

Impedance = Z1

Z1


Differential Impedance Measurement Procedure (TDR)

Connect differential TDR channels to test fixture using High Quality SMA cables and sampler extender cable so sampler can be close to the DUT.

Create differential TDR pulse with specified rise time if required.

De-skew differential incident pulses at the connector of the test fixture.

Identify near and far end connector positions. Measure cable, making sure not to measure

connector artifacts. Document Results.


Differential Impedance Measurement Procedure (TDR)

Cable & Connector Impedance

M1 = Filter(ChA+ChB)

Just Cable Impedance

Use of template and measurement gates to automate test


Rise Time Measurement (TDT)

Measure Rise Time at far end of cable or connector.

Considerations : Specified with differential TDR applied. Requires a TDR and an electrical sampling module.


Rt





80Exx Sampling Module

DUT Test Cable

Test FixtureTest Fixture


Rise Time Measurement Procedure (TDT)

Connect TDR channels to test fixture using High Quality SMA cables and sampler extender cable.

Create differential TDR pulse with specified rise time if required.

De-skew differential incident pulses at the connector of the test fixture.

Connect High quality SMA cable to far end of DUT cable and high bandwidth sampling module.

Measure cable DUT, by measuring Rise Time from incident TDR pulse at the connector to connector on far end of cable.

Document Results.


Rise Time Measurement Procedure (TDT)

Most specified at 20% to 80%. Important to find true min max levels. Also important that differential lines are driven simultaneously.


Skew Measurement (TDT)

Measure time skew from near end differential pair to far end of cable or connector.

Considerations : Specified in time to a specific tolerance. Requires care taken to de-skew the differential pairs.


Skew = T2 – T1T1

T2






DUT Test Cable



Skew Measurement Procedure (TDT)


Connect High quality SMA cable to far end of DUT cable and high bandwidth sampling module.

De-skew differential TDR at near end of connector. Measure cable DUT, by measuring skew in time from

each differential line at far end of cable. Document Results.


Skew Measurement Procedure (TDT)

Most specified at 50%. Important to find true min max levels. Also important that differential lines are driven simultaneously.


Crosstalk Measurement (TDT)

Measures crosstalk on adjacent lines to driven lines on near end of cable or connector.

Considerations : Specified in amplitude percentage of input TDR pulse to a specific

tolerance or in dB. Requires a Differential TDR and an electrical sampling module.


CrossTalk = V1

V1






DUT Test Cable



Crosstalk Measurement Procedure (TDT)


Connect High quality SMA cable to near end adjacent lines and high bandwidth sampling module.

Create differential TDR with specific rise time if required.

De-skew differential TDR at near end of connector. Measure cable DUT, by measuring amplitude of TDR

input taking care to remove effects of the connector and fixture if possible when measuring a cable.

Repeat for each differential pair available. Document Results.


Crosstalk Measurement Procedure (TDT)

Important that differential lines are driven simultaneously and lines are de-skewed properly on both ends.

In some cases, fixture is difficult to remove but is usually minimal effect.

Math for dB display


Measurement Considerations

Care taken with connector and cable connections, use Sampler Extender cable if required to keep sampler close to DUT and perform ESD protection procedures.

True Differential TDR capability. Care taken to de-skew signals properly. Care taken to find min-max levels. Care taken to get best resolution. Automated testing will increase repeatability.


Tektronix is Enabling Innovation

Open Choice – Open Windows Platform enables automation and repeatability.

Custom apps enable easy setup and accurate performance.

Enable use of external applications to compliment tasks without the requirement for more equipment.


Agenda




Frequency Domain Specifications

Insertion loss Serial ATA: <6dB to 4.5Ghz

Return loss Crosstalk in frequency domain

Serial ATA: lower than 26dB

EASILY ACHIEVABLE WITH IConnect® TDR software running directly on TDS8000B.

Frequency Dependent Specifications


S-Parameters in IConnect® TDR software

22222121

12121111

TDRSTDTS

TDTSTDRS

TDR stimulus on channel 2,response on channel 2






Differential S-parameters in IConnect

ddcccccccdcdcdcd

cccccccccdcdcdcd

dcdcdcdcdddddddd

dcdcdcdcdddddddd

TDRSTDTSTDTSTDTS

TDTSTDRSTDTSTDRS

TDRSTDRSTDRSTDTS

TDTSTDRSTDTSTDRS

22212122222121

1212111112121111

2222212122222121

1212111112121111

Differential TDR stimulus,differential response

Common mode TDR stimulus,common mode response

Differential TDR stimulus,common mode response

Common mode TDR stimulus,differential response



IConnect Correlation with Network AnalyzerFrequency Dependent Specifications

-25

-20

-15

-10

-5

0

FR

EQ

, Gh

z

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

VNA SDD11, dB

IConnect®S11.wfm(dBMag). NoTDR calibration

Correlation to 10 Ghz


Serial ATA: Differential Insertion Loss in IConnectFrequency Dependent Specifications

Serial ATA data courtesy Molex, Inc.


Eye Diagram OptionsTDT and IConnect Eye Diagram


Predicted and Measured Eye Diagrams

2^10-1 measurement in IConnect based on TDT

2^10-1 measurement

TDT and IConnect Eye Diagram

Data courtesy FCI


Eye Diagram Degradation in Interconnects

Interconnect losses Pattern-dependent, crosstalk induced jitter Method to improve the eye

Equalization Pre-emphasis and de-emphasis Other signal conditioning techniques



Predicted and Measured Eye Diagrams

MeasuredSimulated

MeasuredSimulated

1.5Gb/s (Gen 1)

6.0Gb/s (Gen 3)

MeasuredSimulated

3.0Gb/s (Gen 2)


Serial ATA data courtesy Molex, Inc.


Cable Loss Modeling: Time and FrequencyTDT and IConnect Lossy Lines


Example: Extraction Results

Extracted skin effect and dielectric loss parameters

Simulated and measured transmission

TDT and IConnect Lossy Lines


Agenda




Conclusion

TDR/TDT measurements are sufficient for electrical cable testing.

Differential TDR is required. Automation increases productivity and reliability of tests. Keeping connections close to DUT reduce setup and increase

reliable measurements. Resolution is important when making precise measurements. Software solutions such as IConnect® compliment

measurement capabilities and enables frequency domain measurements


Tektronix TDS/CSA8000B

TDR rise time: 35 ps reflected 25 ps typical

8 acquisition channels 8-port TDR 4-port True Differential TDR

TDR/TDT measurements High resolution and

measurement repeatability Open Choice- Open Windows

Platform


TDA IConnect® Software

IConnect TDR software for gigabit interconnect… Signal integrity modeling and

analysis Accurate impedance

measurements Cost effective eye diagram

testing Easy S-parameter analysis,

differential and single ended

Efficient, easy-to-use and cost-effective!

www.tdasystems.com

cable testing using tdr and tdt methods presented by christopher skach tektronix dima smolyansky tda...

Documents

mbs slide

tdr step

tektronix net seminar

time measurements

tdr measurement crosstalk

solder removal slide

tdrtdt measurements

transmission line signal