[SI-LIST] Re: Help Explaining Microstrip

At 01:44 PM 10/17/2007, you wrote:

>Nearly two hundred years ago Oersted and Ampere figured out that if you
>have
>two conductors carrying current in the same direction, they would would
>to pull
>in close to each other whereas if you had two conductors carrying
>current in
>opposite directions, they would want to separate.


The forces you are talking about are stationary magnetic forces 
around the traces. In DC, they have no bearing on where the current 
is flowing. In DC circuits, current follows the path of least 
resistance, which can be anywhere (or uniformly everywhere, perhaps)

  In AC circuits, there is a CHANGING magnetic field which induces a 
current in the opposite direction. These two currents will follow the 
path of least IMPEDANCE (re: inductance). As the frequency of change 
increases, the impedance distribution changes more sharply, with the 
lowest impedance path being where the currents are closest together. 
Thus, the current DENSITY within the conductors shifts to the path of 
lowest impedance (inductance.)

The difference in the two effects you are concerned about is the 
difference between DC and AC.

Doug Brooks



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Check out UltraCAD's PCB trace and impedance calculators at
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