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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:

Specs.gif (11600 bytes)

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:

Noises.gif (9633 bytes)

    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.

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