Click on the following bookmarks for further details of an Enclosure Feature Definition:
The Enclosure
Apertures
Access Panels
Panel Mounting Components
Conductor Interfaces
The Enclosure
Two enclosure (or cabinet) styles are available - a basic rectangular
shape, and an alternative with a sloping front. The enclosure material is selected from a
pull-down list, as is the surface finish, if any. These are important parameters, as they
automatically add information about the conductivity and permeability to the project file.
The materials properties can be viewed by clicking the Properties button:
The details of the enclosure seams, or interfaces between each enclosure face are also
completed during definition of the enclosure. Clicking on the Cabinet Seams button
produces the Cabinet Seams form:
The definition of the seams is important, as these can be major sources of electromagnetic field leakage. If the seam is solid, or if the two faces are gasketed, there is little or no leakage. However, if the seam consists of two metal surfaces fastened together by screws, the narrow slot between the two faces, between each pair of screws acts as an aperture, producing leakage. This is taken into account during the Audit stage, along with the maximum distance between fasteners, which determines the frequency where leakage can become a problem.
Apertures
Most enclosures contain a variety of apertures, to allow for example
for the passage of cooling air, to allow displays to be viewed, and for access to
insertable items such as magnetic disks. These will have a much greater effect on the
enclosure emc performance than the basic enclosure material itself. The amount of
attenuation an aperture offers at a particular frequency decreases as its size increases,
and the Auditor program makes use of a standard working design expression that predicts a
linear decrease in attenuation with log(frequency), until no attenuation is provided when
the wavelength is equal to, or greater than twice, the largest aperture dimension. This
expression will approximate real conditions, but will not be exact, as the transmission of
a particular aperture will be a complex combination of source size, orientation and
distance.
The Rectangular Aperture definition form is accessed from the
Definitions menu, and is shown below. (A separate but similar form is used for Circular
Apertures for clarity).
Arrays of apertures such as a series of cooling holes can be defined as a
single object. An array of holes on a pitch of less than half a wavelength reduces the
attenuation offered by a single hole by 10 x log(Number of Holes).
Aperture Shielding Devices
The Aperture definitions form also allows a shielding component to be
fitted to the aperture, such as a mesh shielding window, or a honeycomb panel for a
cooling vent. The Audit process can then take into account the shielding characteristics
of the additional shielding component.
The following components can be added to an aperture:
a) Mesh Display Window or Coated Display Window
Shielded windows utilise either a fine wire mesh encapsulated between
plastic sheets, or a tin oxide conductive coating. In both cases a ground connection must
be made all the way round the edge of the window. Shielded windows cause some reduction in
the visual quality of the display, with coated windows providing less reduction but also
less shielding.
b) Honeycomb Panel
Honeycomb panels consist of an array of diamond shaped tubes, assembled
into a panel. Each tube acts as a waveguide structure, which does not allow the passage of
an electromagnetic field below a cut-off frequency, determined by its dimensions. The
assembly of tubes allows reasonably unrestricted pasage of air. As with the display
window, the panel must be adequately grounded all the way round its periphery. Note also
that the attenuation can vary with orientation of the field to the panel; two layer panels
with cross-oriented honeycombs are available to offer increased attenuation.
c) Single Waveguide Structure
If the depth of the aperture is small compared to the width or height
of the aperture, it behaves as an open aperture. This will normally be the case for
enclosures made from sheet metal. However if the depth of the aperture is greater than the
width AND the height, improved attenuation will be obtained at some frequencies, as the
aperture behaves as a waveguide structure.
d) Display with Internal Shield
An opening for a display can be shielded by providing an internal
grounded shield or bulkhead around the display. Clicking on the dB Level button produces a
form which explains this, and allows the user to specifiy whether the display wires are
filtered.
Clicking on the dBLevel button when a Coated Window, Mesh Window or
Honeycomb Panel is selected produces the Shielding Components Properties form, shown
below:
Several example Shielding Components files are provided, of mesh display windows (*mdw), coated display windows (*.cdw) and a honeycomb panel (*.hcp) . These are accessed by the form's File Open menu. The user can also define shielding components.
Access Panels
The Auditor examines apertures which are covered by a panel as a
separate topic. The narrow slots formed at the edge of a panel are easily disregarded when
making an emc assessment, but can be one of the more significant sources of field leakage.
Depending on how an access panel is grounded around its edges to the enclosure itself, it
can act in one of several ways.
If it is metallic, but totally ungrounded (isolated by a painted
surface for example), its shielding effectiveness will be severely restricted. If it is
grounded at regular intervals around its edges by screws, the gaps between the fasteners
will act as slot apertures, which will tend to leak electromagnetic radiation. The closer
together the fasteners, the higher the frequency for a given leakage level. If an emi
gasket is placed between the panel and the enclosure, a high level of shielding should be
obtained.
The panels definition form is shown below:
The panels sub-form, which collects details of the panel attachment method is shown below.
Many enclosures incorporate a range of panel mounting
components, such as indicator lamps and fuseholders. These are often quite small, and
because they actually fill a hole in a panel, are overlooked for emc purposes. Do they
matter? It depends on the size of the mounting hole, and whether the component is metal
and grounded. If the component does have a metal body which is long compared to its
diameter, and which is grounded all around its periphery, then the aperture is effectively
protected by a waveguide structure.
However, if the component has a plastic body for example, the mounting
hole represents an aperture. A 15mm mounting hole would provide just 20dB attenuation at
1000MHz. This is at the upper end of most emc assessments, but if an internal circuit was
susceptible to this frequency, it would be of importance.
The Panel Mounting Components form is shown below.
Conductors entering and leaving an item of equipment penetrate the emi shield, and so should be considered when reviewing the shielding characteristics of a design. In addition, emi filtering measures should normally be carried out close to the enclosure wall, if not actually mounted at the wall.
The Conductor Interfaces Form, where details of the cable shield
termination type, the connector style, and whether emi suppression filters are used are
recorded.
An important feature of the interfaces definition is the ability to
define the details of any emi suppression filters which may be fitted to conductors at an
interface. The audit process then examines the noise sources which may have been
identified, and assumes some coupling may occur between the noise sources and conductors
connected to an interface. The insertion loss at the noise frequencies for the particular
filter circuit is then calculated.