Column Control DTX

Signal Integrity Analysis Series Part 1: Single-Port TDR, TDR/TDT, and 2-Port TDR

Application Notes

Introduction

Interconnect Analysis is Simplified with Physical Layer Test Tools

The Time-domain Reflectometer (TDR) has come a long way since the early days when it was used to locate faults in cables. Time Domain Reflectometry can be used for more than 40 characterization, modeling, and emulation applications, many of which are illustrated in this application note series.

If your applications involve signals with rising times shorter than one nanosecond, transmission line properties of the interconnects are important. TDR is a versatile tool to provide a window into the performance of your interconnects to quickly and routinely answer the three important questions: does my interconnect meet specifications, will it work in my application, and where do I look to improve its performance?

The TDR is not just a simple radar station for transmission lines, sending pulses down the line and looking at the reflections from impedance discontinuities. It is also an instrument that can directly provide first-order topology models, S parameter behavioral models, and with up to four channels, characterize rise time degradation, interconnect bandwidth, near and far-end cross talk, odd mode, even mode, differential and common impedance, mode conversion, and the complete differential channel characterization.

To provide a little order to the wide variety of applications we explore in this application note series, we divide the series into three parts covering four general areas. Part 1: Those which use a single-port TDR, those which use TDR/TDT, and those which use 2-port TDR. Part 2: Those which use 4-port TDR or 4-port Vector Network Analyzer (VNA) with Physical Layer Test System (PLTS). Part 3: Those which use advanced signal integrity measurements and calibration. The principles of TDR and VNA operation are detailed in other application notes and references listed in the bibliography. We concentrate this application note series on the valuable information we can quickly obtain with simple techniques that can be used to help us get the design right the first time.

Table of Contents

  • Single-Port TDR
    • Overview
    • Measuring characteristic impedance and uniformity of a transmission line
    • Measuring time delay of a transmission line
    • Accurate measurement of signal speed in a transmission line
    • Extracting bulk dielectric constant of the laminate
    • Building a model of a discontinuity such as a corner, test pad, gap in the return path, SMA launch, terminating resistor
    • Building a high bandwidth model of a component
    • Directly emulating the impact on a signal with the system rise time from a discontinuity
  • 2-Port TDR/TDT
    • Overview
    • Introduction to TDR/TDT
    • Measuring insertion loss and return loss
    • Interconnect modeling to extract interconnect properties
    • Identifying design features that contribute to excessive loss
  • 2-Port TDR/Cross Talk
    • Overview
    • Measuring NEXT
    • Measuring FEXT
    • Emulating FEXT for different system rise times
    • Identifying design features that contribute to NEXT
    • Exploring the impact of terminations on NEXT and FEXT
    • Measuring ground bounce
    • Identifying design features that contribute to ground bounce
    • Emulating ground bounce noise for different system rise times
  • 2-Port Differential TDR (DTDR)
    • Overview
    • Measuring each of the five impedances associated with a differential pair
    • Measuring the degree of coupling between lines in a differential pair
    • Measuring the differential impedance of a twisted pair cable
    • Measuring the reflected noise of a differential signal crossing a gap
    • Measuring the mode conversion in a differential pair
    • Identifying specific physical features that contribute to mode conversion in a differential pair
  • References
  • Web Resources

Single-Port TDR

Overview

In this section, we will look at the seven most important applications of a 1-port TDR. The first two refer to the complete characterization of a uniform transmission line, extracting the characteristic impedance and time delay.

But, we can get more than this if we use specially designed test structures. We can also get a fundamental, intrinsic property of the transmission line, the velocity of a signal, and from this, the intrinsic bulk dielectric constant of the laminate.

When the line is not uniform but has discontinuities, we can build first order, topology-based models, right from the front screen. If this isn’t high enough bandwidth, we can bring the measured data into a simulation tool such as ADS, and build very high bandwidth models which can then be used in simulations to evaluate whether this interconnect might be acceptable in a specific application.

Finally, we can emulate the final application system’s rise time with the TDR to directly measure the reflection noise generated by physical structures in the interconnect and whether they might provide a potential problem, or equally value, might be ignored.

Measuring characteristic impedance and uniformity of a transmission line

Historically, the most common use of the TDR has been to characterize the electrical properties of a transmission line. For an ideal, lossless transmission line, there are only two parameters that fully characterize the interconnect: its characteristic impedance and its time delay. This is the easiest and most common application for the TDR.

The TDR sends a calibrated step edge of roughly 200 mV into the device under test (DUT). Any changes in the instantaneous impedance the edge encounters along its path will cause some of this signal to reflect back, depending on the change in impedance it sees. The constant incident voltage of 200 mV, plus any reflected voltage, is what is displayed on the screen of the TDR.

×

Please have a salesperson contact me.

*Indicates required field

Preferred method of communication? *Required Field
Preferred method of communication? Change email?
Preferred method of communication?

By clicking the button, you are providing Keysight with your personal data. See the Keysight Privacy Statement for information on how we use this data.

Thank you.

A sales representative will contact you soon.

Column Control DTX