Doug McKean wrote:
> 
> Anyone have a general equation for calculating the
> characteristic impedence for a twisted pair?
> 


>From "High Speed Digital Design - A Handbook of Black Magic",
by Howard Johnson and Martin Graham, 1993, Prentice-Hall, p. 186,

   Zo =   120    *  ln (2s/d)
        --------
        sqrt(er)

   where, d = diameter of conductor
          s = separation between wire centers (s > d)
          er = effective relative permittivity

   For widely separated wires, use er = 1.
   
   For wire pairs with the insulation touching,
   use the er of the insulating material.

   Also, propagation delay (ps/in):  85 * sqrt(er)

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