Shield Termination Background
Shield terminations are the means by which cable shields
are connected to ground. Cable shields may be grounded at either end or at
both ends, using a variety of methods. EMI Analyst provides models
for four common shield termination methods. Terminations are defined using
the Shield Termination form, which is opened by clicking the Shield
Termin button on the EMI Analyst Main form.
The impedance of the shield termination can have a
significant effect on the cable shield shielding effectiveness. Several
parameters affect the shield termination impedance, which can vary significantly
with frequency. Each parameter is entered using text boxes and drop down
list boxes on the Shield Termination form.
Once parameters have been entered, the shield termination
impedance is calculated by clicking the Calculate button on the toolbar
or selecting menu Tools | Calculate. Calculated values are
displayed graphically upon completion of calculations and can be viewed
numerically in tabular form by clicking the Data
Table button or selecting menu View | Data Table.
Tech Note - Shield Termination Length
The
optimum shield termination provides a 360° peripheral connection between the perimeter
of the shield and the chassis of the enclosure in which the interface circuits
are housed. Anything less degrades shield performance, especially at high
frequencies. type terminations, where a
wire is crimped or soldered to the shield at one end and grounded at the other,
sometimes passing through a connector pin along the way, introduce inductance
and resistance in series with the shield and perturb the current distribution
around the shield near the termination. As a result shield performance is
compromised. The longer the shield termination, the more it affects shield
performance. The shorter the shield termination, and the more directly the
shield is terminated to chassis, the more effective the shield will
be.
More:
 Shield Termination Equivalent Circuit
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