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Signal Integrity Analysis Series 4-Port TDR/VNA/PLTS

Application Notes

Table of Contents

  • 4-Port Techniques
    • Complete differential pair characterization 
    • 4-Port single-ended S parameters 
    • S parameters in the time domain
    • 4-Port differential S parameters 
    • High-speed serial links applications 
    • Differential impedance profile 
    • Impact from stimulus rise time
    • Differential or single-ended measurement
    • Common impedance profile 
    • Time delay and dispersion 
    • The bandwidth of an interconnect 
    • Rise time degradation 
    • Eye diagrams 
    • Impulse response and pre-emphasis taps
    • Mode conversion and EMI 
    • Modeling differential channel interconnects
    • Summary
  • References 
  • Web Resources

The Vector Network Analyzer (VNA) has come a long way since it was used to test antenna arrays for military applications. Vector Network Analysis can be used to perform more than 100 critical characterization, modeling, and emulation applications for high-speed, digital design, many of which are illustrated in this application note series.

Introduction

If your application requires the measurement of very low-level signals such as near-end and far-end crosstalk, using a test system with a high dynamic range becomes very important. Unlike a wide bandwidth TDR, a VNA allows the user to set a narrow receiver bandwidth (known as Intermediate Frequency Bandwidth or IF bandwidth). This highly accurate tool provides a window into the performance of high-speed digital interconnects which propagate signals with rising times of 500 ps or shorter. The VNA will expand the design validation capability of all signal integrity laboratories around the world and answer important questions such as: what is the limitation of my current design, where do I need to focus my attention to increase my data rate, and will my interconnect survive the next generation application? 

The VNA is no longer limited to microwave applications traditionally utilized for aerospace and defense work. Today’s commercial electronic designs push the limit of what can be achieved on copper and the power of s-parameters are critical to assure proper performance of these components and systems. Interconnect analysis using a VNA is now simplified with a popular software application called Physical Layer Test System (PLTS). Utilizing a graphical user interface designed for digital designers enables the power of the VNA with the ease of use of a TDR. The ultimate in test accuracy can be provided for topology models, S parameter behavioral models, characterization of rising 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 divided the series into three parts: 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. Part 3:Those which use advanced signal integrity measurements and calibration. The principles of TDR, VNA, and PLTS 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

4-Port Techniques.

Complete differential pair characterization

How a TDR can provide valuable signal integrity characterization information about interconnects was reviewed in Part 1. Figure 1 summarizes the various applications for 1-port and 2-port TDR configurations. Though we are able to obtain some information about a differential pair from two-port measurements, the complete characterization of a differential pair requires four ports

In a 4-port configuration, each of the instrument’s ports is connected to each end of the transmission lines in the differential pair. In practice, it is a coaxial connection made at each end of each transmission line, so that each port has a signal and return the connection to the transmission line

In the configuration shown in Figure 2, everything important about a differential pair can be extracted. This includes the differential and common return and insertion loss and all forms of mode conversion. From these measurements, details of the differential or common impedance profiles, material properties, and asymmetries can be extracted. A 4-port measurement can be performed in the time domain, using a 4-port TDR, or in the frequency domain, using a 4-port VNA. To first order, there is exactly the same information content in both measurements. There are differences in the dynamic range or the noise floor of the measurement, so if higher accuracy measurements are required, you should utilize a Vector Network Analyzer (VNA).

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