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SATA/USB cable construction and impedance?


numlog

Question

With RF cables  you have special connectors that can be used terminate the cable in the correct impedance, so cable construction itself is more flexible.

Do SATA and USB cables use a similar feature in their connectors? AFAIK they dont.

looking at many of these cables, gauge can vary irrespective of length.

e.g 3 SATA cables: 25CM with 26AWG conductors, 45cm also with 26AWG conductors, and 20CM with 30AWG.

same with USB cables, lots of varying lengths with almost all 26AWG conductors.

Can anyone explain how these cables are made to the correct specifications with the same wire gauge and vastly different lengths?

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The characteristic impedance of a cable doesn't depend on the length. It is determined by the shape of the conductors and the space between them (distance and material). If a significantly thicker wire is needed, the characteristic impedance can be maintained by adjusting the spacing to compensate. If the wires are thin compared to the distance between them, their actual thickness doesn't matter much.

 

Characteristic impedance is not to be confused with DC resistance. The latter obviously scales with cable length and wire gauge. For USB 2.0 it must be at most 3.5 Ω. USB 3.2 doesn't specify an absolute limit for DC resistance. Instead, cables are required to meet certain loss requirements at various frequencies. In practice, such cables also have a DC resistance of at most a few ohms.

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