Design007 Magazine


Issue link:

Contents of this Issue


Page 24 of 87

October 2017 • The PCB Design Magazine 25 • The characteristic impedance (Zo), is de- fined as the instantaneous impedance of a lossless transmission line. • The most fundamental signal integrity analysis involves defining the board stackup, which is essential to generating accurate results. • Dielectrics, specifically designed to be used for high-speed design, have a very low dielectric constant. • Closed-form equation based impedance calculators are extremely limited in accuracy and the use of a precision field solver is mandatory. • A series resistor can be used to match the driver to the impedance of the transmis- sion line. • Termination is required if the trace length exceeds one sixth of the electrical length of the rising edge rate. • High-speed digital design is all about controlling impedance. PCBDESIGN References 1. Barry Olney's Beyond Design columns: Controlled Impedance Design, Impedance Match- ing: Terminations, Signal Integrity Parts 1, 2 & 3 2. What's the Difference Between Signal In- tegrity and Power Integrity? by Patrick Carrier 3. The Critical Length of a Transmission Line, by Eric Bogatin 4. Bandwidth Basics, Wavelength Electronics 5. IPC-2251–Design Guide for the Packaging of High-Speed Electronic Circuit 6. High-Speed Digital Design, by Howard Johnson Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner. The software can be downloaded from To contact Olney, or read past columns, click here. High-speed digital design is all about con- trolling impedance. The impedance of the transmission lines needs to be matched and maintained at a constant value along the en- tire length of the interconnect. Also, the power distribution network needs to provide a low impedance path, through the planes, across the entire frequency bandwidth of the signal. These seemingly unrelated disciplines control the sta- bility and reliability of the product. Get it right and your high-speed design is off to a great start; get it wrong and you are in for a (real) world of pain. Points to Remember • At high frequencies the PCB trace now behaves as a distributed system with para- sitic inductance and capacitance charac- terized by delay and scattered reflections. • The transmission line effects create under and over shoot resulting in ringing in the signal. • The Fourier series expansion of a square wave is made up of a sum of odd harmonics. • A trace becomes a transmission line sim- ply when the length is comparable to the signal rise time. • All drivers whose trace length (in inches) is equal to or greater than the rise time (in nanoseconds) should be considered critical and treated as high-speed transmission lines. • Impedance is the key factor that controls the stability of a design; it is the core issue of both the signal and power integrity methodology. • For perfect transfer of energy, the impedance at the source must equal the impedance at the load. • Terminations are generally required, at fast edge rates, to match the impedance and limit ringing. • As core voltages drop, rise times become faster and frequency increases, and a lower impedance is more desirable. • The measured input impedance of a transmission line looks remarkably similar to the AC impedance of a plane cavity's resonance which also has no termination. WHEN DO TRACES BECOME TRANSMISSION LINES?

Articles in this issue

Links on this page

Archives of this issue

view archives of Design007 Magazine - PCBD-Oct2017