High Precision Time Domain Reflectometry (TDR)

Techniques for achieving the highest possible accuracy and resolution in signal integrity impedance measurements

Introduction

High-performance communications systems require a quality transmission path for electrical signals. For efficient signal flow and high signal integrity, the transmission path impedance should be kept as close to a constant, ideal value as possible. Time Domain Reflectometry (TDR) is a well-established technique for verifying the impedance and quality of signal paths in components, interconnects, and transmission lines. 

As data rates increase and component geometries decrease, the precision and resolution of the basic TDR measurement system can be strained. This application note will review measurement system limitations and the sources of measurement errors. Practical techniques and useful methods to enhance precision will be reviewed. 

Specific topics include: 

• Techniques to remove the effects of fixturing (cabling and connections that obscure the analysis of the test component) 
• A methodology for high-accuracy TDR testing of differential transmission systems 
• Deriving one, two, or four-port S-parameters for key insights into component performance and enhanced modeling accuracy

Table of Contents

Time-domain reflectometry: measurement basics

TDR measurement limitations

• The step generator as an error source
• Cables and connectors cause loss and reflection
• Techniques to minimize the effects of fixturing
• Generating the digital filter
• Measurement examples

Using very fast edge speeds for accurate measurements of closely spaced reflections

Performing precision TDR measurements for differential transmission systems

Deriving ‘S’- parameters from traditional TDR results

Time-domain reflectometry: measurement basics

When a signal is launched along a transmission path, ideally, none of that signal will be reflected back to the signal source and all the signal energy reaches its intended destination. This will be the case when the impedance of the entire transmission path and the line termination is equal to the output impedance of the signal source. However, if the signal ever encounters a change in impedance, some portion of the incident signal will be reflected.

A time-domain reflectometer (TDR) is a measurement tool used to measure the impedance profile of a component (device) under test (DUT). The concept is straightforward. Using a step generator and an oscilloscope, a fast pulse edge is launched into the DUT. Whenever there is an impedance discontinuity, a portion of the pulse will be sent back to the monitoring oscilloscope.

The position of the discontinuity is determined by monitoring the time at which the reflected signal arrives back at the oscilloscope (along with the propagation velocity of the pulse within the DUT). The magnitude of the discontinuity can be determined from the size of the reflected pulse compared to the original pulse sent into the DUT. Thus this “echo technique” reveals at a glance any changes in impedance along the line.

Analysis techniques exist to show the nature (resistive, inductive, or capacitive) of each discontinuity along the line and whether attenuation in a transmission system is from series losses or shunt losses. All of this information is immediately available from the oscilloscope’s display. Since the fast pulse step stimulus is broadband, TDR gives meaningful information concerning the broadband response of a transmission system as opposed to testing over the fixed range of frequencies employed by other reflectometer methods.

TDR measurement limitations

The fundamental performance of the TDR system determines the measurement capabilities. Consider the following factors that dictate the overall performance of the TDR system:

The step generator as an error source

The shape of the step stimulus is important for accurate TDR/TDT measurements. The DUT responds not only to the step but also to the aberrations on the step such as overshoot and non-flatness.

If the overshoot is substantial, the DUT’s response can be more difficult to inter¬pret. Impedance discontinuities are observed as changes in the reflected signal. Aberrations in the TDR step may be incorrectly interpreted as DUT imperfections. If the step is flat, guesswork is minimized. The rise time of the step is also extremely important. To determine how the DUT will actually respond, you should test it at edge speeds similar to those it will actually encounter. 

Edge speed is also critical when using TDR to locate the source of a discontinuity along a transmission line. Both the bandwidth of the oscilloscope and the rise time of the step source can limit measurement accuracy. The rise time of the overall measurement system is the combined rise times of the oscilloscope and the step generator.