> * But they have a high parasitic capacitance because the outside conductor has a large surface area.
> So if capacitance is causing too much propagation delay, you can use twisted pair.*
The capacitance is not really "parasitic", it is the circuit element representation of the electric field which is propagating. The dynamic electric field gives rise to a magnetic field, which is the inductive element in the expression 1/sqrt(LC) that you listed.
Twisted pair is no different: There is a capacitive element and inductive element per unit length. As it turns out, the capacitance varies as the log of the distance between the wires, so you can twist the wires and not change the capacitance very much. There used to be lots of "twinax", which kept the two wires at a fixed distance, but the separation plastic was mostly a PITA. (The pics I can find on google don't show the old flat plastic twinax that folks used to use. You can still find that stuff hanging off old television antennas.)
This isn't quite right. Big wires (ideally entire planes) deliver power. Small wires deliver signal. Half and half isn't the right split, and you don't want to make power wires smaller.
The very different requirements of the two is where a lot of the gains come in.
reply