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
Tektronix Net Seminar 2003Slide 3
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
Tektronix Net Seminar 2003Slide 4
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
Tektronix Net Seminar 2003Slide 5
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.
Tektronix Net Seminar 2003Slide 6
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.
Tektronix Net Seminar 2003Slide 7
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
Tektronix Net Seminar 2003Slide 8
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
Tektronix Net Seminar 2003Slide 9
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
Tektronix Net Seminar 2003Slide 10
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
Tektronix Net Seminar 2003Slide 11
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.
Tektronix Net Seminar 2003Slide 12
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
Tektronix Net Seminar 2003Slide 13
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.
Tektronix Net Seminar 2003Slide 14
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:
Tektronix Net Seminar 2003Slide 15
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.
Tektronix Net Seminar 2003Slide 16
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
Tektronix Net Seminar 2003Slide 17
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.
Tektronix Net Seminar 2003Slide 18
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
Tektronix Net Seminar 2003Slide 19
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
Tektronix Net Seminar 2003Slide 20
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.
Tektronix Net Seminar 2003Slide 21
Differential Impedance Measurement Procedure (TDR)
Cable & Connector Impedance
M1 = Filter(ChA+ChB)
Just Cable Impedance
Use of template and measurement gates to automate test
Tektronix Net Seminar 2003Slide 22
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.
Unused lines terminated with 50 Ohms to Ground
Rt
80E04 Differential TDR Module
Sampler Extender Cable
High Speed SMA cables
TDS 8000B Sampling Oscilloscope
80Exx Sampling Module
DUT Test Cable
Test FixtureTest Fixture
Tektronix Net Seminar 2003Slide 23
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.
Tektronix Net Seminar 2003Slide 24
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.
Tektronix Net Seminar 2003Slide 25
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.
Unused lines terminated with 50 Ohms to Ground
Skew = T2 – T1T1
T2
80E04 Differential TDR Module
Sampler Extender Cable
High Speed SMA cables
TDS 8000B Sampling Oscilloscope
80Exx Sampling Module
DUT Test Cable
Test FixtureTest Fixture
Tektronix Net Seminar 2003Slide 26
Skew Measurement Procedure (TDT)
Connect TDR channels to test fixture using High Quality SMA cables and sampler extender cable.
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.
Tektronix Net Seminar 2003Slide 27
Skew Measurement Procedure (TDT)
Most specified at 50%. Important to find true min max levels. Also important that differential lines are driven simultaneously.
Tektronix Net Seminar 2003Slide 28
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.
Unused lines terminated with 50 Ohms to Ground
CrossTalk = V1
V1
80E04 Differential TDR Module
Sampler Extender Cable
High Speed SMA cables
TDS 8000B Sampling Oscilloscope
80Exx Sampling Module
DUT Test Cable
Test FixtureTest Fixture
Tektronix Net Seminar 2003Slide 29
Crosstalk Measurement Procedure (TDT)
Connect TDR channels to test fixture using High Quality SMA cables and sampler extender cable.
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.
Tektronix Net Seminar 2003Slide 30
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
Tektronix Net Seminar 2003Slide 31
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 Net Seminar 2003Slide 32
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.
Tektronix Net Seminar 2003Slide 33
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
Tektronix Net Seminar 2003Slide 34
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
Tektronix Net Seminar 2003Slide 35
S-Parameters in IConnect® TDR software
22222121
12121111
TDRSTDTS
TDTSTDRS
TDR stimulus on channel 2,response on channel 2
TDR stimulus on channel 2,response on channel 1
TDR stimulus on channel 1,response on channel 1
TDR stimulus on channel 1,response on channel 2
Frequency Dependent Specifications
Tektronix Net Seminar 2003Slide 36
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
Frequency Dependent Specifications
Tektronix Net Seminar 2003Slide 37
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
Tektronix Net Seminar 2003Slide 38
Serial ATA: Differential Insertion Loss in IConnectFrequency Dependent Specifications
Serial ATA data courtesy Molex, Inc.
Tektronix Net Seminar 2003Slide 39
Eye Diagram OptionsTDT and IConnect Eye Diagram
Tektronix Net Seminar 2003Slide 40
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
Tektronix Net Seminar 2003Slide 41
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
TDT and IConnect Eye Diagram
Tektronix Net Seminar 2003Slide 42
Predicted and Measured Eye Diagrams
MeasuredSimulated
MeasuredSimulated
1.5Gb/s (Gen 1)
6.0Gb/s (Gen 3)
MeasuredSimulated
3.0Gb/s (Gen 2)
TDT and IConnect Eye Diagram
Serial ATA data courtesy Molex, Inc.
Tektronix Net Seminar 2003Slide 43
Cable Loss Modeling: Time and FrequencyTDT and IConnect Lossy Lines
Tektronix Net Seminar 2003Slide 44
Example: Extraction Results
Extracted skin effect and dielectric loss parameters
Simulated and measured transmission
TDT and IConnect Lossy Lines
Tektronix Net Seminar 2003Slide 45
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
Tektronix Net Seminar 2003Slide 46
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 Net Seminar 2003Slide 47
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
Tektronix Net Seminar 2003Slide 48
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