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Module 2: MeasurementTopic 7: Time Domain Reflectometry

OGI EE564

Howard Heck

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Where Are We?

1. Introduction

2. Transmission Line Basics1. Transmission Line Theory

2. Basic I/O Circuits

3. Reflections

4. Parasitic Discontinuities

5. Modeling, Simulation, & Spice

6. Measurement: Basic Equipment

7. Measurement: Time Domain Reflectometry

3. Analysis Tools

4. Metrics & Methodology

5. Advanced Transmission Lines

6. Multi-Gb/s Signaling

7. Special Topics

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Contents

TDR IntroductionAnalysis MethodSources of ErrorTime Domain TransmissionSummaryReferences

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Time Domain Reflectometry (TDR)

A time domain reflectometer is just a fast step generator with an oscilloscope.

DUT

ZL

Oscilloscope

SamplerCircuit

PulseGenerator

To use it: Inject a fast (< 35 ps) edge onto the line from a 50 source. Observe the reflected waveform back at the scope. Use your knowledge of circuits and transmission lines to

characterize the circuit under test.

ViVr

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TDR #2

For example, the following waveforms show a TDR driving a 50 transmission line with unknown termination conditions.

How is each terminated?

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TDR #3

TDR can also be used to characterize reactive elements:

What are these?

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TDR Usage

More on how to use TDR: Identify regions of the TDR plot:

• Flat regions are transmission lines• Upward spikes or bumps are inductances• Downward spikes or bumps are capacitances

Starting at the sourceDetermine the values of Z0, td, L, or C for the nearest

element.Simulate it to validate your finding and to determine

the tr seen by the next element. (Iterate if needed.)

Move to the next element.

You don’t need to create a model that has more resolution than your fastest rise time.

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TDR Example

Same circuit when driven with tr = 200 ps.

5 nH

1 pF

45/0.5ns50 term

60/0.5ns

5 nH

5 nH

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Sources of Error in TDR

Equipment Scope (bandwidth, sampling rate) Cable loss Probe discontinuities (including ground loops) Lack of standard

Other Sources Coupon design: must replicate board “upstream” elements Measurement region (settling effects)

Scope

Cable Standard

Probe

Total error = Errorscope

+ Errorcable

+ Errorprobe [2.7.1]

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TDR Error Sources: Coupon Design

Coupon design must replicate board “upstream elements.” L’s & C’s low-pass the signal, increasing the rise-time. This affects the reflections from down-stream elements:

Slows the rising edge Spreads out response (convolution with slow edge) L & C responses don’t go full swing

This makes it hard to extract exact L and C values impossible to measure very small discontinuities

However, if the TDR can’t see them, neither can the receiver.

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TDR Error Sources: Measurement Region

For the Direct Rambus DRAM (RDRAM) channel design, Intel redefined the measurement window to reduce impedance measurement errors: Spec = 28 10% Initial measurement errors 3 With improved methodology, errors were reduced to < 0.5

TDR Response for 28 6" coupon

25

35

45

55

65

0.25 0.75 1.25 1.75 2.25

Time [ns]

Z0 [

]100% Window

Settling Region 50-70%Region

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Time Domain Transmission (TDT)

This is a TDR, which is operated in TDT mode: Terminate the line at the load/far end. Launch at the source with a 50 probe. Probe at both source and load with low capacitance, high

impedance probes.

Used primarily to measure propagation velocity. Velocity is more difficult to measure than impedance, and is

more susceptible to measurement errors. Accuracy depends strongly on test structures, measurement

procedures, and probe types. Difficulty: determining exactly where on the signal edge to make

the measurement. Picosecond rise times are needed. Micro probing is highly

recommended.

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TDT Examples

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Summary

TDR offers a way to characterize the elements of your design for modeling purposes.

Scope, cable, probe, test structures, etc. all add to measurement error.

Use TDT to measure velocity.

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References

S. Hall, G. Hall, and J. McCall, High Speed Digital System Design, John Wiley & Sons, Inc. (Wiley Interscience), 2000, 1st edition.

W. Dally and J. Poulton, Digital Systems Engineering, Chapters 4.3 & 11, Cambridge University Press, 1998.

H. Johnson and M. Graham, High Speed Digital Design: A Handbook of Black Magic, PTR Prentice Hall, 1993.

R. Poon, Computer Circuits Electrical Design, Prentice Hall, 1st edition, 1995.

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ReferencesTDR & TDT Hewlett Packard Corp., “Time Domain Reflectometry Theory,

Application Note 1304-2, May 1988. D. Smolyansky and S. Corey, “PCB Interconnect

Characterization from TDR Measurements,” PCB Design, May 1999, pp. 18-25.

Intel Corporation, PCB Test Methodology, August 1999, http://www.developer.intel.com.

D.J. Dascher, “Measuring Parasitic Capacitance and Inductance Using TDR,” Hewlett Packard Journal, Article 11, August 1996.

J. McCall, “Successful PCB Testing Methodology,” PCB Design, June 1999, pp. 10-16.

Tektronix, Inc., “AWG 610 Arbitrary Waveform Generator,” Product Data Sheet 76W-12991-0, April 1999.

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References

TDR & TDT Hewlett Packard Corp., “Crosstalk and Impedance

Measurements of PCB Board Patterns,” Application Note 339-3, September 1986.

Hewlett Packard Corp., “Characteristic Impedance Measurement of PCB Board Patterns,” Application Note 339-2, June 1986.

Hewlett Packard Corp., “Electronic Characterization of IC Packages,” Application Note 1255-5, June 1994.