PSpice Application Notes

8 At high frequencies the real part is or The following sections discuss the physical causes of line loss, skin effect, dielectric loss, and proximity effect. Skin Effect Skin effect results from the fact that currents tend to concentrate on the conductor surface. Current density continuously increases from the conductor center to its surface. For classical skin effects, the penetration depth is given by where K=1/sqrt(**), =magnetic permeability of the conducting material expressed in henries per unit length, and =conductivity of the conducting material. For SI units and for a copper conductor =5.85x10 7 (ohm-meter) -1 and =410 -7 (H/meter). The skin effect reduces the equivalent conductor cross-sectional area, which causes the effective resistance per unit length to increase with increasing frequency. Dielectric Loss Dielectric losses result from leakage currents through the dielectric material, which causes an increase in the shunt conductance per unit length. This results in signal attenuation. For frequencies below 250 MHz, this loss can usually be neglected. Skin effect losses will dominate up through RF frequencies. Proximity Effect This is a current density redistribution in a conductor due to the mutual repulsion (or attraction) to currents flowing in nearby conductors. This current density redistribution reduces the effective cross-sectional area of the conductor, thereby increasing the series resistance. No general rules of thumb have been proposed due to its complicated nature. This effect is a function of the conductor diameters, separation of conductors, and frequency. Influence of Loss Effects on Primary Line Parameters Resistance Per Unit Length, R For coaxial lines, skin effect losses dominate and the resistance per unit length is described by ) )( ( C j G L j R C L G L C R 2 2 2 2 0 0 GZ Z R f K D

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