Day 2: Transmission Lines and Characteristic Impedance ()
Yesterday, we talked about reflections. Today, we'll discuss the key physical property that causes them: Characteristic Impedance.
For low-speed signals, a PCB trace is just a simple wire. But for high-speed signals, the trace behaves like a transmission line.
What is a Transmission Line? A transmission line is an electrical structure that has uniform distributed inductance and capacitance per unit of length. Think of it like a chain of tiny resistors, capacitors, and inductors. The signal isn't an instant pulse but a traveling wave of voltage and current.
Characteristic Impedance () Every transmission line has a property called Characteristic Impedance (). This is an intrinsic value determined by the physical dimensions of the trace (its width and thickness), its distance from the reference plane (ground or power), and the dielectric constant of the PCB material.
The Problem: Reflections occur when the signal encounters a sudden change in impedance. For example, if your trace has a 50Ω impedance and it connects to a load with a different impedance (like 100Ω), a portion of the signal will reflect back towards the source. This reflected signal can interfere with the original signal, causing distortion and errors.
The primary job of a high-speed hardware designer is to control and match impedances throughout the signal path to prevent these reflections.
Book Reference for Deeper Study
To dive deeper into this topic and other fundamentals of high-speed design, I highly recommend:
Book: High-Speed Digital Design: A Handbook of Black Magic
Authors: Howard Johnson and Martin Graham
Why it's helpful: This book is considered the bible of high-speed design. It explains complex concepts with practical analogies and real-world examples. It’s an essential read for any hardware designer looking to master this field.
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