# RE: [SI-LIST] : A basic question

From: Larry Smith ([email protected])
Date: Fri Mar 17 2000 - 09:42:02 PST

Out of the four parameters below, only two are independent. Given
any two of the four parameters, the others are calculated.

Inductance = L, inductance/length

Capacitance = C, capacitance/length

velocity = 1/sqrt(LC)

Impedance = sqrt(L/C)

The best place to start is with the universal constant: the speed of
light. Einstein said that time and distance are not constants (nor
are inductance and capacitance) but the speed of light (3x10**8 M/Sec,
12 inches/nSec) is. Assuming that we have no ferrous (magnetic)
materials in our environment, the velocity of electromagnetics in all
of our structures is:

light_speed/sqrt(eR), eR = relative permeability

As mentioned in a previous note on this thread, it is generally
easier to calculate capacitance than inductance. This is done
for transmission lines and power plane structures. Given the
fundamental velocity above and the capacitance, both inductance
and impedance are calculated. Usually we assume that all the
charge is on the surface of conductors (skin effect, high
frequency approximation) when making capacitance calculations.

Given constant materials (ie FR4 or Ceramic with eR properties),
the velocity is known. Geometry's can be changed that will affect
the capacitance, inductance and impedance. But the 4 parameters
are fundamentally related by the above equations.

regards,
Larry Smith
Sun Microsystems

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