Time-domain Reflectometry(TDR) Measurements

Test Methodologies for Today’s Fastest Digital Data Interfaces

Intel Developer’s ForumAsia PacificSpring 2000

Z1Z2

TDR Test Methodologies forToday's Fastest Digital Data Interfaces

Time-Domain Reflectometry (TDR) test methods are used to measure the signal transmission path characteristics in high-speed digital systems.

This lab will review the basic concepts of TDR, then present some fundamental TDR measurement methods. These methods will next be applied to some of the fastest digital data interfaces used in today’s computer systems

Typical TDR Applications

– Printed Circuit Boards– Connectors– IC Packages– Cables and Interconnects

TDR Measurements Are Used to Characterize the Signal Transmission Properties Of:

Typical TDR Measurements

– Signal characteristic impedance– Differential signal characteristic impedance– Signal-signal Crosstalk– Signal propagation delay– Inductance and capacitance characterization

Types of TDR Measurements used to characterize signal transmission properties include:

When Are TDR Measurements Needed?

– To characterize electrical transmission properties in high bandwidth and high data-rate applications.

– To guarantee the transmission properties meet the system performance requirements.

– To verify manufacturing processes of PC boards, IC packages and connectors.

Typical applications where TDR Measurements are needed include:

– Starting with a transmission line with a characteristic impedance Z equal to Z0

– A fast rise-time step signal is applied at the transmission line input point.

– The step signal will propagate down the line.

StepSource

TransmissionLine

CharacteristicImpedance Z = Z0

TDR Fundamentals - Applied Step Signal

PropagatingStep

– An impedance change in the transmission line will cause a change in the amplitude of the propagating step.

StepSource

CharacteristicImpedance Z1 = Z0

ImpedanceChange Point

TDR Fundamentals - Impedance Change

CharacteristicImpedance Z2 > Z0

– The change in impedance causes some of the power to be reflected back to the source.

– The remainder of the power will be transmitted.

StepSource

TransmittedStep

TDR Fundamentals – Transmitted and Reflected Signals

Z1 = Z0Z2 > Z0

IncidentStep

ReflectedStep

– 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.

StepSource

TDR Fundamentals – Oscilloscope Monitoring

Z1 = Z0Z2 > Z0

OscilloscopeMeasurement

Point

– The oscilloscope waveform will show the impedance change as a change in signal amplitude occurring in twice the propagation time, Tpd, from the incident step.

TDR Fundamentals – Oscilloscope Waveform

Z1 = Z0Z2 > Z0

Propagation time = Tpd

Propagation time = 2Tpd

Time ( Sec/Div )

TDR Fundamentals - Typical TDR System

– A Typical TDR system consists of TDR sampling head that generates fast step signals, samples the incident and reflected signals that are digitized by the oscilloscope.

ZLoadLoad

Transmission Line

Z00

50

Sample-HoldGate

To OscilloscopeMainframe

SMAConnector

TDR Sampling Head

StepSource

TDR Measurements – Typical System

– TDR measurements provide a means to get quantitative characterization data of the transmission system.

– The measurement comparison of the incident and reflected signals provide the data for analysis.

ZLL

LoadTDR SamplingHead

Z0

TDR Measurements - Reflection Coefficient – (rho)

is the ratio of the reflected pulse amplitude to the incident pulse amplitude.

=Vreflected

Vincident

=

can be expressed in terms of the transmission line characteristic impedance, Z0 , and the load impedance, ZL.

=Vreflected

Vincident

=( ZL – Z0 )

( ZL + Z0 )

TDR Measurements - Reflection Coefficient for Matched Load

There are some interesting boundary conditions for the value of the reflection coefficient, .

Vreflected

Vincident

=

ZLL

Load

Z0

– When ZL is equal to Z0 – Matched Load

VReflected = 0 and = 0

= 0

V= 0

Reflected Wave is equal to zero.

No Reflections.

TDR Measurements - Reflection Coefficient Boundary Values

Vreflected

Vincident

=

– When ZL is Infinite – Open Load

= V

V= 1

Vreflected

Vincident

=

– When ZL is equal Zero – Shorted Load

VReflected = -VIncident and = -1

= -V

V= -1

Reflected Wave is equal but negative of incident wave.

Reflected Wave is equal to the

incident wave.VReflected = VIncident and = +1

TDR Measurements – Open and Shorted Load TDS 8000 Display

TDR Measurements - Reflection Coefficient and Impedance

The characteristic impedance, Z0 , or the load

impedance, ZL, can be calculated with the value of

ZL = Z0 * ( 1 + )

Z0 = ZL * ( 1 - ) ( 1 – ( 1 +

ZLLZ0

TDR Measurements – Oscilloscope Waveform Measurement Units

– Oscilloscope TDR Measurements can use units of Volts, Ohms or (Rho) for the vertical magnitude scale.

– The horizontal axis represents unit of time.

Time Units - Sec/Div

Magnitude Units

Volts/Div

Rho/Div

Ohms/Div

-or-

-or-

TDR Measurements – Impedance Measurements With Cursors

– Using a properly calibrated TDR Oscilloscope the horizontal waveform cursors can be used to make impedance measurements.

Sec/Div

Ohms/Div

Cursor 1

Cursor 2

MeasurementReadouts

Cursor 1 = 50.0

Cursor 2 = 95.3

Delta 2-1 = 4.7

TDR Measurements – Propagation Delay Measurements With Cursors

– The horizontal waveform cursors can be used to make time and propagation delay measurements.

– The one-way propagation delay is half the time measured between the incident and reflected waves.

Sec/Div

Ohms/Div

Cursor 1 Cursor 2

MeasurementReadouts

Cursor 1 = 50.0 pSCursor 2 = 201.5 pS

Delta 2-1 = 151.5 pS

Prop Delay = 75.75 pS

TDR Measurements – Propagation Delay and Dielectric Constant

– The relationship between propagation delay, – Using the medium dielectric constant and the fact the

measured time is twice the propagation time of the signal.

Propagation Delay = TPD = L * ( EFF )1/2

VC

Where:

• EFF is the effective dielectric constant which is a function of materials and type of transmission line, strip-line, micro-strip, etc.

• L is the length of the trace

• VC is the velocity of light

TDR Measurements – Location Calculation with Propagation Delay

– The location of a transmission line impedance change can be calculated.

– Using the medium dielectric constant and the fact the measured time is twice the propagation time of the signal.

2 * ( EFF )1/2

= D = TMeas * VC

Location ofImpedance Changed

Cursor 1 Cursor 2 TMeas

Where:

• D is the distance to the point of the impedance change that caused the reflection.

TDR Measurments – Reflection Diagrams

– Reflection diagrams are useful for calculating and understanding the propagating wave reflections.

Z1Z2

ReflectedWave

IncidentWave

TransmittedWave

Z1 Z2

Time

TDR Measurement – Reflection Diagram Calculations

– Reflection diagram calculations determine the magnitude of each reflection using the values of the reflection coefficient, and the incident voltage.

Z1 Z2

Time

Vreflected Vincident * =

TDR Measurements – Diagrams for Multiple Reflections

– Reflection diagrams are useful for calculating and understanding transmission line with multiple discontinuities.

Z1Z2

Z1 Z2

Z3

Z4

Z3 Z4

Time

TDR Measurements – Waveform for Multiple Reflections

– The TDR waveform for multiple reflections shows the results of the combined reflections from all the impedance discontinuities.

Z1Z2 Z4

Ohms

Time

TDR Waveforms - Shorted and Open Terminations

– Short Circuit Termination

– Open Circuit Termination

V

0

2TP

TP

Z0 ZL = 0

V

0

2TP

Z0 ZL = Open

TP

2V

TDR Waveforms - Matched and Mismatched Load Terminations

– Matched Load Termination

– Mismatched Load Termination

V

0

2TP

TP

Z0 ZL = Z0

V

0

2TP

Z0

TP

ZL <> Z0

ZL < Z0

ZL > Z0V + VR

TDR Waveforms - Capacitor and Inductor Terminations

– Capacitor Load Termination

– Inductor Load Termination

V

0

2TP

TP

Z0 ZL = C

V

0

2TP

Z0

TP

ZL = L

2V

TDR Waveforms - Shunt Capacitance and Series Inductance Discontinuities

– Shunt Capacitance Discontinuity

– Shunt Inductance Discontinuity

V

0

2TP

TP

Z0 C

V

0

2TP

Z0

TP

L

Z0

Z0

TDR Waveforms - Inductance and Capacitance Discontinuities

– Series Inductance – Shunt Capacitance

– Shunt Capacitance – Series Inductance

V

0

2TP

TP

Z0 C

V

0

2TP

Z0

TP

L

Z0

Z0C

L

TDR Waveforms - Multiple Inductance and Capacitance Discontinuities

– Capacitance – Inductance - Capacitance

– Inductance – Capacitance - Inductance

V

0

2TP

TP

Z0 C

V

0

2TP

Z0

TP

L

Z0

Z0C

L

C

L

TDR Waveforms - PC Board Transmission line

– A typical PC board will have impedance controlled PCB micro-strip and strip-line transmission lines.

– The transmission lines will have components, vias, connectors, etc., that will create impedance discontinuities.

Input

TDR Waveforms - PC Board Impedance Model

– These impedance discontinuities can be modeled as inductors, capacitors and resistors.

Input

TDR Waveforms - PC Board Impedance Measurement

– The TDR Oscilloscope waveform will display the reflections created by the impedance discontinuities.

– Measurements can be made to determine how much the impedance deviates from the nominal value.

Rambus TDR Measurements – Channel Impedance Requirements

– The Rambus channel has an impedance specification of 28 Ohms with a 10% tolerance, +/- 2.8 Ohms.

– Meeting this requirement means careful design and testing of the PCB transmission lines.

28 W within10% over

entire system

Rambus TDR Measurements – Multiple Simultaneous Data Transfers

– Multiple bits can be simultaneously propagating on a single Rambus channel data line – multiple clock domains.

– Unwanted signal reflections from one data transfer would effect multiple data transfers on the channel date line.

– Assuring PCB impedances meet specifications is a requirement for successful Rambus design.

Controller RDRAM

Rambus

Rambus TDR Measurements – Channel Timing and Voltage Requirements

– The Rambus 800Mbps data transfer rate requires timing margins to be maintained with high degree of accuracy.

– Rambus signals use low voltage swings of 0.8V.– Signal reflections caused by impedance discontinuities

reduce timing and voltage margins.– All the Rambus channel signals and clock lines must

also be accurately matched to maintain timing margins.

0.8 V

1.25 nS

Data

Clock

Rambus TDR Measurements – PCB Transmission Line Design

– Determine Microstrip and Stripline PCB designs with consideration of the PCB fabrication materials and constraints.

– Use impedance equations, simulation tools and reference designs for best results – 28 Ohms is low.

– Include a test coupon for testing impedance and monitoring PCB production lots.

Microstrip Stripline

Rambus TDR Measurements – TDR Test Coupon

– A TDR test coupon can be designed on to PCB for testing impedance and monitoring PCB production lots.

– The coupon should use the same routing guidelines used for the Rambus channel to represent process.

– The probe land pattern used with the coupon needs to match the type and method of TDR probing used.

– The far end of the coupon should be open – no pad or via.

Open end

Probepads

Ground

Rambus TDR Measurements – Instrumentation Setup

– The basic TDR instrumentation for Rambus TDR consist of a sampling oscilloscope with a TDR sampling head, with TDR probe and cable.

– Other methods of connecting to the circuit board can be used including SMA connectors, specialized RIMM modules, probe fixtures, etc.

Rambus TDR Measurements – TDR 28 Ohm Calibration

– The TDR measurement system should be calibrated with a 28 Ohm standard.

– This could be a certified test coupon, specialized calibration air line, etc.

– TDR instrumentation are 50 Ohm systems. So a 28 Ohm impedance will be displayed as a downward step.

50

28

50 TDR Sampling Head

28 Standard

Rambus TDR Measurements – 28 Ohm Calibration TDS8000 Display

Rambus TDR Measurements – TDS8000 Display of Rambus Impedance Range

Rambus TDR Measurements – TDR Probing

– One method to measure the circuit board trace impedance is to probe the trace with a TDR probe.

– The probe is used to apply the TDR step and measure the reflections.

– The probe ground lead needs to be as short as possible and connected to trace ground plane. TDR probe with

short ground lead probing trace

Rambus TDR Measurements – Minimizing Probing Aberrations

– When making 28 Ohm measurements aberrations from the probe launch can be minimized with careful probing techniques.

Rambus TDR Measurements – Probing 28 Ohm Trace TDS8000 Display

I used my finger to lower bandwidth and

ringing

Rambus TDR Measurements – Rambus Differential Clock

– There are two pairs of 400MHz differential clock lines, ClockToMaster and ClockFromMaster.

– Clock and data travel in parallel to minimize timing skew.– An RDRAM sends data to the memory controller synchronously

with ClockToMaster.– The controller sends data to the RDRAMs synchronously with

ClockFromMaster.

– Data and Clock transmission lines must be matched in impedance and length to maintain timing margins.

28 W within10% over

entire system

Rambus TDR Measurements – Differential Clock TDR Measurement

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

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

Rambus 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 there characteristic impedance as a pair.

– Proper characterization of the differential impedance of the Rambus clock traces to maintain voltage and timing margins.

Rambus 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 line launch point.

Rambus TDR Measurements – TDS8000 Differential TDR Display

Rambus TDR Measurements - Propagation Velocity Measurements

– TDR is used to make propagation velocity measurements to compare electrical signal length of the Rambus data and clock lines.

– Matching the electrical signal length – propagation time is important to maintain voltage and timing margins.

Sec/Div

MeasurementReadouts

Dielectric K = 4.50

Distance = 58.17 mm

Delta 2-1 = 822.6 pS

Cursor 1

Cursor 2

More TDR Measurements – Rambus Coupling and Crosstalk

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

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

Sampling Oscilloscopes - Low Voltage Differential Signaling (LVDS)

– Low voltage differential signaling (LVDS) is being used in many high-speed interface applications.

– LVDS using low voltage swing, high-speed, differential signaling – similar to Rambus.

– High speed buses are formed by using multiple sets of LVDS data lines along with a differential clock.

– Transmit and Receive pairs for the bi-directional bus.

TX

RX

RX

TX

Sampling Oscilloscopes - Low Voltage Differential Signaling (LVDS)

– Sampling oscilloscopes have the features required to characterize LVDS interfaces.

– LVDS requires differential transmission line characterization - impedance, coupling, etc.

– LVDS also requires timing and jitter characterization measurements – eye patterns, noise, etc.

– Sampling oscilloscopes have very high timing accuracy and low system jitter.

Conclusion – Other TDR Measurement Considerations

– TDR improved accuracy with baseline level and incident Amplitude correction.

– Fast TDR step rise-time allows for high resolution measurements to characterize short transmission line segments, connectors, etc.

– Parametric modeling with third-party software available to develop SPICE model, etc.

– Easy to determine the transmission line characteristics and Instrumentation for measurements can be applied to manufacturing environments.