Usb Cables

[jkeny]

I just found this impedance calculator which we could use for calculating the recipe to achieve 90ohm impedance of DIY USB cables.
It is specific for twisted pair of wires so I would construct the use cable twisting the 2 signal wires together, not braiding the ground wire into it. That way you ensure a fairly constant spacing between the 2 signal wires - in a braided configuration, the spacing between the two signal wires probably varies?

http://www.mantaro.com/resources/impeda ... _impedance

So you need 3 values to input to the calculator:
- diameter of wire
- spacing between wire
- dielectric constant of the insulation

Using some figures:
- 24AWG silver wire has a diameter of 0.511mm
- dielectric constant of Teflon = 2.1
- spacing from centre of one wire to centre of other wire = 0.85mm (which means the insulation should be 0.17mm thickness i.e 0.85-0.511 = 0.34/2 gives 0.17mm)
Gives an impedance of 90ohm