Free Trials

Download a free trial to find out which Altium software best suits your needs

How to Buy

Contact your local sales office to get started on improving your design environment

Downloads

Download the latest in PCB design and EDA software

  • PCB DESIGN SOFTWARE
  • Altium Designer

    Complete Environment for Schematic + Layout

  • CircuitStudio

    Entry Level, Professional PCB Design Tool

  • CircuitMaker

    Community Based PCB Design Tool

  • NEXUS

    Agile PCB Design For Teams

  • CLOUD PLATFORM
  • Altium 365

    Connecting PCB Design to the Manufacturing Floor

  • COMPONENT MANAGEMENT
  • Altium Concord Pro

    Complete Solution for Library Management

  • Octopart

    Extensive, Easy-to-Use Component Database

  • PRODUCT EXTENSIONS
  • PDN Analyzer

    Natural and Effortless Power Distribution Network Analysis

  • See All Extensions
  • EMBEDDED
  • TASKING

    World-Renowned Technology for Embedded Systems Development

  • TRAININGS
  • Live Courses

    Learn best practices with instructional training available worldwide

  • On-Demand Courses

    Gain comprehensive knowledge without leaving your home or office

  • ONLINE VIEWER
  • Altium 365 Viewer

    View & Share electronic designs in your browser

  • Altium Designer 20

    The most powerful, modern and easy-to-use PCB design tool for professional use

    ALTIUMLIVE

    Annual PCB Design Summit

    • Forum

      Where Altium users and enthusiasts can interact with each other

    • Blog

      Our blog about things that interest us and hopefully you too

    • Ideas

      Submit ideas and vote for new features you want in Altium tools

    • Bug Crunch

      Help make the software better by submitting bugs and voting on what's important

    • Wall

      A stream of events on AltiumLive you follow by participating in or subscribing to

    • Beta Program

      Information about participating in our Beta program and getting early access to Altium tools

    All Resources

    Explore the latest content from blog posts to social media and technical white papers gathered together for your convenience

    Downloads

    Take a look at what download options are available to best suit your needs

    How to Buy

    Contact your local sales office to get started improving your design environment

    • Documentation

      The documentation area is where you can find extensive, versioned information about our software online, for free.

    • Training & Events

      View the schedule and register for training events all around the world and online

    • Design Content

      Browse our vast library of free design content including components, templates and reference designs

    • Webinars

      Attend a live webinar online or get instant access to our on demand series of webinars

    • Support

      Get your questions answered with our variety of direct support and self-service options

    • Technical Papers

      Stay up to date with the latest technology and industry trends with our complete collection of technical white papers.

    • Video Library

      Quick and to-the-point video tutorials to get you started with Altium Designer

    Voltage and Current Waveforms on a Series-Terminated Transmission Line

    Kella Knack
    |  July 27, 2020
    Series-Terminated Transmission Line Voltage and Current Waveforms

    In both of our books, Right the First Time, A Practical Handbook on High Speed PCB and System Design, Volumes 1 and 2, as well as in our on-site and public courses, we talk about the value of a series-terminated transmission line. With this type of transmission line, no power is being used unless the line is being charged up. This enables a lot of computing with a minimum of power. But having a clear understanding of how this type of line operates can be confusing and a bit daunting. In particular, visualizing how this operation works can be challenging. The purpose of this article is to clarify the operation of this transmission line and provide graphics that demonstrate it.

    The Basics

    Series-terminated transmission lines are the primary method for connecting CMOS devices with unmatched output impedance. In the following discussion, all the devices referred to are CMOS devices. CMOS devices actually led to ECL technology's demise because with ECL, no matter what was done, the line always drew power and caused severe cooling problems with large machines.

    Figure 1 is a typical 5V CMOS driver with a 50 Ohm transmission line connected to a passive CMOS receiver.

    CMOS series-terminated transmission line
    Figure 1. A Typical Series-Terminated CMOS Transmission Line

    A passive receiver means that it simply responds to the voltage waveform presented at its input. For the purposes of this explanation, the CMOS receivers look like very small capacitors that are considered to be open circuits. Here, the line is about 12” (30 cm) long. The energy in a PCB travels approximately six inches per nanosecond. So, this line is about two nanoseconds long.

    The schematic representation of the transmission line in Figure 1 is shown in Figure 2.

    Figure 2. Schematic representation of a series-terminated transmission line
    Figure 2. Schematic Representation of a Transmission Line

    As can be seen, there is capacitance, resistance, and inductance distributed along the length of the transmission line. As noted in previous articles, these elements are referred to as parasitics, and they establish the behavior of a transmission line with the ratio of inductance per unit length to capacitor per unit length. Note that the substrate's parasitic conductance and loss tangent have been ignored for the moment as they are not so critical to understanding the basic shape of voltage and current waveforms. At sufficiently high frequency, but not so high that we care about skin effects or copper roughness, the inductance and capacitance determine the impedance of the line, as shown in Equation 1.

    Equation 1. Impedance of a series-terminated transmission line
    Equation 1. Impedance of a Transmission Line

    Note: When selecting drivers for series-terminated transmission lines, the driver's output impedance must be equal or less than the impedance of the transmission line.

    In Equation 1, the inductance per unit length is expressed as Lo, and the capacitance per unit length is expressed as Co. (These two variables can be determined for a given transmission line type using a tool such as a 2D field solver). The equivalent circuit at T0 is the voltage source, and Figure 3 is the equivalent circuit when the transition from a logic 0 to a logic 1 begins.

    Figure 3. Equivalent Circuit for the series-terminated transmission line
    Figure 3. Equivalent Circuit of the Circuit in Figure 1 at T0 When Switching Begins

    The voltage divider is formed by the combination of the driver output impedance and the series termination in the upper part and the impedance of the transmission line in the lower part. When the series termination has been properly chosen, the combination of Zout and Zst will be the same as Zo. In this example, both are 50 ohms.

    Figure 4 shows the voltage and current waveforms for the series-terminated transmission line in Figure 1 as the driver switches from a logic 0 to a logic 1.

    Voltage and Current Waveforms on a Series-Terminated Transmission Line while switching
    Figure 4. Voltage and Current Waveforms, Series-Terminated Transmission Line Switching from 0 to 1

    The voltage waveform starting down the series-terminated transmission line is V/2, which represents half the supply voltage. So, outbound, the capacitance is charged to V/2. This is represented by the current waveform shown in the lower portion of Figure 4 and can be shown by V's easy calculation over the two resistances in series.

    Note: Ohm’s law describes the relationship between the current through a resistance and the voltage across it. Basically, the law states that the current in amperes is equal to the voltage in volts across a resistance divided by the resistance in ohms.

    When the current gets to the far end of the transmission line, which is an open circuit with a shunt capacitor, the voltage doubles to the full scale value of V. The EM field is reflected back from the open end of the transmission line, and it is charging the capacitance up the rest of the way to V. When the EM field arrives back at the start of the line, the capacitance is fully charged, and the current goes to zero. This is seen in the lower part of Figure 4.

    Important things to keep in mind about the operation depicted in Figure 4 include:

    • The current waveform on the bottom of the graphic lasts two times the electrical length of the transmission line.
    • The maximum current drawn from the power subsystem is set by the Zo of the transmission line and the supply voltage.
    • The product of the current waveform and the voltage waveform at the input to the transmission line is the power that must be supplied by the power subsystem.
    • The frequency content of the waveform is not set by the clock frequency.

    Summary

    The lowest power consumption method for high-speed logic signaling involves using a series-terminated transmission line. This is the lowest power consumption method because energy is only consumed in the circuit when a logic line is switched from a logic 0 to a logic 1.

    Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium and learn more about making design decisions with ease and confidence. 

    About Author

    About Author

    Kella Knack is Vice President of Marketing for Speeding Edge, a company engaged in training, consulting and publishing on high speed design topics such as signal integrity analysis, PCB Design ad EMI control. Previously, she served as a marketing consultant for a broad spectrum of high-tech companies ranging from start-ups to multibillion dollar corporations. She also served as editor for various electronic trade publications covering the PCB, networking and EDA market sectors.

    most recent articles

    Back to Home