High Speed Topic: What is meant by a 50 ohm impedance?




In our past videos, we have discussed characteristic impedance related to the layer stack-up of the PCB. However, we have yet to address the notion of impedance itself. Impedance has always been part of the signal traces on a PCB but not necessarily a concern. With high-speed designs becoming more prevalent and significantly faster-rising edge times now being common on all digital components (even the classic 7400 logic gate series), the understanding of impedance is becoming a critical concern. Many of the intermittent failures that one may experience can be traced back to impedance issues.

In this deep dive, the focus will be on the fundamentals of impedance:

  • Review the classic approach of the voltage/current relationship and explain why the inductance to capacitance relationship creates the same kind of relationship, hence the term 'characteristic impedance.'
  • Discuss the three functions of impedance: power transfer, reflections, and radiation. Understanding and managing these three critical functions can help avoid attenuation, signal distortion, and cross talk.
  • Review of T-line theory as an infinite transmission line

Sean Kelly will give this presentation. Sean has been in the electronics industry for 30 years, with 20 years of experience in PCB design. Also, Sean is an instructor and consultant on topics such as high-speed design and RF.

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