Select the Curve Fit tab when the construction of the
shield is not known, but the Inflection Frequency and DC Transfer Impedance are
known. The program will calculate a curve that satisfies these two parameters.
At
low frequencies shield transfer impedance is relatively constant and is equal to
the dc transfer impedance. As frequency increases skin depth decreases,
resulting in lower (better) shield transfer impedance. This effect
typically begins between 10 kHz and 100 kHz for woven wire shields.
Between several hundred kilohertz and a few megahertz, leakage through the
apertures created by the weave begins to have the effect of increasing shield
transfer impedance. As frequency increases so does shield transfer
impedance. Shield cutoff frequency is the frequency at which the shield
transfer impedance, having decreased somewhat due to skin depth effects, and
then increased due to aperture effects, exceeds the dc transfer impedance.
Above the cutoff frequency, shield transfer impedance typically increases
approximately in proportion to increasing frequency.
