Click on the following bookmarks for further details of an Field Definition Feature:
Emissions Specification
Internal Noise Sources
Emissions Specification
An important aspect of the emc assessment of an equipment enclosure is
to obtain a knowledge of the level of circuit emissions compared to the required
specification. The Shielding Auditor uses two forms to define the noise signal levels and
the maximum allowed signal levels.
Defining the Emissions Limits
The emissions limits are defined using the form shown below:
Two standard emissions specification levels are supplied with the Auditor, namely
EN55022 Class A and Class B limits. A specification is added to a project by clicking the
Add Specification to Project button.
The user may add his own emissions specifications to the emissions
file.
Internal Noise Sources
A powerful feature of the Shielding Auditor is the ability to specify
the frequency and level of sources of radiated interference.
How can the level of emissions be known? Firstly, estimates can be made
through theoretical analyses of current paths within the equipment. If a current path in
the form of a loop can be identified, and the signal details are known, an estimate of the
field emissions can be made. The Westbay Tools software package provides two analyses of
differential mode and common mode radiation, which allow field levels to be stored in a
file format (*.nos) that can be imported into Westbay Auditor.
Secondly, measurements on an actual system, not contained in a shielded
enclosure, will provide evidence of noise levels and frequencies. These can be entered
into a noise file, as described below, and subsequently merged into new projects which use
the same, or similar circuitry.
The noise sources form is shown below:
The Audit process takes field strengths from the noise file, applies
attenuation due to apertures etc, plus attenuation due to any shielding windows for
example, and then compares the expected external field strength with the emissions limit,
and reports accordingly.