bluesman Posted June 30, 2014 Share Posted June 30, 2014 There is no "90 ohm specification" as far as I can determine, Chris. The actual impedance spec for USB 1 and 2 from the 2007 USB.org document is a minimum of 76.5Ω and a maximum of 103.5Ω for the cable. The USB 3 specs only seem to document the need to normalize the resistance across the connection between mated cable assemblies to 90Ω. I don't know if this is the reason many believe there's a standard 90Ω impedance for USB cables, but it makes sense. USB uses a 45-ohm system in which the driver has a source impedance of 45 ohms and the receiver has a termination of 45 ohms to ground. All USB cables and tracks should have a single-ended impedance of 45 ohms - here's a circuit diagram: As both D+ and D- lines terminate into 45 ohm resistors to ground, one could conclude (wrongly) that the cables have a 90Ω impedance. In reality, these resistors are in the devices, not the cable. David Link to comment
Harpy Posted June 30, 2014 Share Posted June 30, 2014 Wouldn't that be that termination spec of the device? 44 ohms to 28 ohms with the (+15%). "The 90ohms +/- 15% impedance has to do with the internal components, the high frequency signals USB transmits to match the other USB devices, and to reduce noise. Each data line uses a 1.5K and a 15K ohms pull down and up resistors respectively." Dahlquist DQ-10 Speakers DQ-LP1 crossover 2 DW-1 Subs Dynaco Mk III Mains - Rotel 991 Subs Wyred W4S Pre Gustard X10 DAC SOtM dx-USB-HD reclocked SOtMmBPS-d2s Intel Thin-mini ITX Link to comment
ar-t Posted July 7, 2014 Share Posted July 7, 2014 ARGH!!!!!!!!!!!!!!!!!!! Yes, the resistors are in the equipment. But, the characteristic impedance, of the cable, has to match the source and load impedance. No, it is not a physical resistance. Oy vey. It is the cable impedance. 90R + /- 15%. And not all stuff has those 1.5k pull-up/-down resistors. Only the really slow stuff. And it has NO termination resistors. The higher speed stuff does not have those pull-up/-down ones, or it would not work worth diddly squat. Which is why it is terminated. http://ar-t.co/ Link to comment
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