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High Speed Data Transmission for Non-EE's - Part 8 – The Rocky Road of Connectivity

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b2ap3_thumbnail_Blogpost8-Image-1.jpgGrowing up in Vermont, I experienced my fair share of pot holes in roads.  When riding my bike, while I always tried to avoid pot holes, but when there was no other option but hit one, I’d either take a jolt to the backside, or ideally absorb the impact by standing on the pedals, and keep moving forward with the bike.  With my limited skateboarding skills, the pot hole would stop the skateboard cold and send it backward as my inertia would carry me tumbling forward.

The same concept can be applied to a high speed square wave moving across a transmission with variations in the characteristic impedance (like a road with pot holes and cracks). Think of the size of the wheel being relative to the wavelength of the sine waves that makes up the square wave.

The base frequency F=X has a long wavelength, and therefore a big wheel.  It easily passes over the crack and pothole with minimal energy lost (like the thud of a tire hitting a pot hole takes some of the mechanical energy of the wheel and creates an audible sound that radiates out from the impact).  At F=4X, the equivalent wheel becomes small enough that none of the energy moves forward down the transmission line.  It is either radiated out like sound waves (to generate crosstalk on other transmission lines), or it reflects back to where it came from, and can combine with other square waves coming up behind it.  The result is that the nice square wave that came into the connector transmission line is not as sharply defined when it comes out of the connector transmission line.

Stay tuned for Part 9, our final segment, where we look at insertion loss.

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  • Guest
    Nick Perlick Friday, 06 June 2014

    Dear Mr. Selfridge;
    I find your technical mastery coupled with a profound universal understanding of life invaluable to folks like me who don't really understand what you're explaining but still get it on a deeper level. If not for awesome contributors like you to our increasing ability to understand how our planet works, we would still be twisting tree twigs for warmth. Thank you.

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