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

    RF Signal Chain Design for FMCW Chirped Radar Systems

    Zachariah Peterson
    |  September 25, 2019

    Brahminy starling chirping

    If you could hear the output from your chirped radar system, it might sound like a brahminy starling

    In automotive and UAV radar applications, radar signals are amplified throughout the signal chain. Amplification is critical for ensuring the reflected signal can be accurately detected and for maximizing the range and resolution of your radar system. Although there are some ICs that integrate an entire signal chain into a single package, these integrated packages may not meet your particular needs. In such a case, you’ll need to think about designing your own signal chain for a chirped radar system.

    RF Signal Chain Basics for FMCW Chirped Radar Systems

    In FMCW chirped radar systems, the frequency sent to the Tx antennas is synthesized with a linear ramp rate (e.g., with a VCO). If you look at the frequency over time, the ideal graph will look like a sine wave. In real systems, the output frequency graph can look much closer to a stairstep waveform when the emitted frequency is synthesized at discrete values.

    Range and resolution are two important design points in any chirped radar system. In an FMCW chirp system with linear ramp rate (such as that used in automotive and UAV radar), the equation below shows how to calculate the maximum usable range as a function of your desired SNR value and Tx power output from the antenna:

    Range equation in chirped radar systems with FMCW emission

    Maximum range in FMCW chirped radar systems

    Note that the noise figure (NF) is equal to the logarithm of the Rx SNR divided by the Tx SNR. The range resolution in FMCW radar can also be easily calculated in terms of the chirp bandwidth (e.g., 4 GHz in 77 GHz automotive radar):

    Resolution equation in chirped radar systems with FMCW emission

    Range resolution in FMCW chirped radar systems

    FMCW chirped radar can be used to determine the speed of an oncoming object by extracting the frequency shift of successive chirps with heterodyne detection. This frequency shift is due to the Doppler effect, which provides a simple way to calculate the speed of target. When combined with directional emission from a phased array antenna, you can also use your radar system to calculate the target’s heading. This aspect is more of a signal processing topic, and as such is outside the scope of this article. Instead, we want to focus on how the particular characteristics of RF amplifiers affect signal integrity in the signal chain.

    Intermodulation Products and Harmonic Distortion

    On the Tx side in FMCW radar, the synthesized frequency will not be a single frequency. In frequency synthesis, the circuit or nonlinear element used to generate the desired modulated signal may also generate other higher order harmonics in addition to sidelobes. These components then enter the Tx amplifier. The power amplifier on the Tx side is generally operating near saturation and the output quickly becomes nonlinear in order to produce the desired power output and satisfy your range requirement. This generates intermodulation products, which appear in the output of the amplifier as the frequency is ramped. This is similar to what happens in passive intermodulation.

    These higher order harmonics and intermodulation products should be filtered from the signal on the Tx side prior to entering the amplifier stage if possible. Harmonics and intermodulation products will have lower intensity thanks to the finite bandwidth of the amplifier and the antenna. This will reduce the strength of higher order harmonics and intermodulation products that are sent to the antenna and emitted.

    These same higher order harmonics and any intermodulation products in the emitted signal will reflect from the target and can be detected at the receiver. This means the Rx side should also contain a filter to remove higher order harmonics and any intermodulation products. Ideally, the bandwidth of any filters should overlap with the chirp bandwidth, although this is not always possible. Any intermodulation products and harmonics effectively increase the noise floor in the signal chain, and particular intermodulation products can interfere with extraction of the beat frequency.

    RF signal chain in FMCW chirped radar

    Example showing how harmonics and intermodulation products can be generated in the RF signal chain. Note that the width of the upper left spectrum due to modulation is omitted for clarity.

    Among the various intermodulation products that can be generated, the 3rd order product (IM3) is the most important for two reasons. First, this particular pair of frequencies tends to fall very close to the frequency of the desired signal and is likely to fall within the bandwidths of downstream components in your signal chain.

    Second, the 3rd order intermodulation product will determine the maximum input level of the fundamental harmonic on the receive side. As the fundamental harmonic increases in strength, the 3rd order harmonic also increases in strength, and the two signal levels eventually become equal. This point is known as the 3rd-order intercept point (3OIP), which determines the highest input signal level that can be reliably used in the Rx side while maintaining linearity of the amplifier stage and ensuring the desired signal can be extracted.

    Removing higher order harmonics from your input signal on the Tx side is quite easy; just use a very high order bandpass filter. Any residual frequency-modulated higher order harmonics can generate their own set intermodulation products at lower frequencies that are near your desired frequency band. Removing any intermodulation products near your desired band requires very precise filter design, which is not always feasible.

    Harmonic Balance and Load Pull for Nonlinear Amplifier Analysis

    In order to maximize power transfer from your amplifier stages to downstream components in RF design, you’ll need to use load-pull analysis for impedance matching for your Tx amplifier’s output impedance in your signal chain. This is particularly important for examining the behavior of an amplifier operating at large input signals (i.e., your Tx amplifier) as typical DC/AC sweeps produce incorrect results at high input signal levels.

    If you want to get an idea of how spurious harmonics affect signal integrity in your system, then you need to use a technique like harmonic balance analysis to determine how any higher order harmonics present in an amplifier’s input will appear at the output. Note that, for an amplifier operating in the linear regime (ideally on the Tx side), the output can be determined using the amplifier’s transfer function, which can then be determined by applying a frequency sweep in a SPICE-based simulation.

    Harmonic balance analysis is uniquely designed for determining how higher order harmonics present on an input signal in a nonlinear circuit will propagate to the output. We won’t go into the finer points of harmonic balance analysis here, but there are a number of simulation packages you can use for harmonic balance with SPICE or IBIS models.

    There are other important design guidelines to consider when working with microwave and mmWave frequencies in general. These include routing and transmission line layout guidelines (see this article for guidelines in 77 GHz automotive radar), manufacturing considerations, and substrate material selection.

    The layout, component selection, and simulation tools in Altium Designer give you a broad toolset for working with chirped radar systems. The simulation and modeling tools are very useful for determining the best circuit design and layout choices for your next chirped radar system.

    Now you can download a free trial of Altium Designer and learn more about the industry’s best layout, simulation, and production planning tools. Talk to an Altium expert today to learn more.

    About Author

    About Author

    Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies.

    most recent articles

    Back to Home