On Sat, Jul 14, 2018 at 10:26 AM Gene Heskett <ghesk...@shentel.net> wrote:
> On Saturday 14 July 2018 11:57:10 Jon Elson wrote: > > Preferably as a twisted pair so the radiation of noise largely cancels at > 2x the twist rate distance or more. A tight twist = less radiated noise. > No doubt twisting helps, but I have not seen a derivation of the above 2X the twist rule. On the other hand if you simply use zip cord, with parallel conductors the equation for the electric field on the plane perpendicular to the current is easy to derive. (Note that zip cord and a tightly waisted pair are identical if you look only on the "cut plane") The 2D electric field is simply the field created by one wire (simple inverse square law) added on the other and it basically goes to near zero after 8X the center to center distance of the wires. I always figured that twisting was just a good way to minimize the center to center distance. But it could work out that 2X the twist pitch works out the same because small wires have both a higher twist and smaller center to center distance. I wonder if the twist actually does anything other then just hold the pair of wires closer together? I think it might but I have never seen a mathematical derivation. The 2D slice case is easy using only high school level math but a full analysis of the 3D cable might be much harder. It might be that the twisting is just a mechanical thing and a way to ensure that wire pairs in a cable bundle are never parallel with each other and can't capacitively couple. Has anyone tried to measure the inductance of a twisted pair. Then twist it more and re-measure. I don't find any difference at all. But perhaps My measurement technique is wrong. My conclusion is that we twist wires only for two reasons (1) to keep then closely spaced and (2) to make the path "bumpy" so it is impossible to lay parallel with anything else. So the effect is mechanical not electrical. I could be wrong, but I've never seen mathematical proof. Also when you think about voltages on the grounded return wires, some people do forget that impedance is a complex number, we are not dealing with a special case of a constant DC voltage and a resistive load. You do either want to run all grounds to one and only one common point or galvanically isolated the systems that you interconnect. In the machine tool world many systems choose Galvanic isolation using either opt-iolators or transformers. For a large distributed system isolation is easier. -- Chris Albertson Redondo Beach, California ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
