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Click on the following bookmarks for further details of an EMC Shielding Tool:

Enclosure Shielding Effectiveness
Waveguide below cut-off
Cavity Resonances of an Enclosure
Twisted Pair Rejection Ratio
Circular/Square EMI Gasket Groove Design
Aperture Shielding Effectiveness


Enclosure Shielding Effectiveness

    The Shielding Effectiveness Tool allows the attenuation of a conductive shield to be calculated and plotted. The attenuation consists of three components: Reflection Loss, Absorption loss and a Re-Reflection correction factor. All three are calculated, along with the resultant total attenuation, and displayed on the form shown below.

Se_form.gif (8481 bytes)

    The shield material is defined in terms of its conductivity, permeability and thickness. As an alternative to conductivity, an ohms per square value can be entered. This can be used with the entered conductivity, in which case the material thickness is calculated for the given ohms/square. Alternatively, the entered thickness and ohms/square can be used to define the material conductivity. A pull-down list of materials provides instant reference to the conductivity and permeability of Aluminium, Brass, Chromium, Copper, Steel, Mumetal, Nickel, Silver, Tin and Zinc.

    The source details are defined in terms of frequency in MHz, and distance to the source.

    The Analyse button produces a spot calculation of the Reflection Loss, Absorption loss and Re-Reflection correction factor, along with the overall Shielding Effectiveness, at the selected frequency. (Skin Depth is also calculated and displayed for that frequency). A Tabulate button allows a table of shielding effectiveness values to be produced over a user-defined frequency range.

    The Plot command those of the above factors selected for plotting by the check boxes on the form. A typical plotted output is shown below.



Se_graph.gif (7893 bytes)

Plot of total shielding effectiveness (Curve 1) for a 0.5mm thick copper shield, radiated with plane waves from a distant source. Note that the reflection loss (Curve 3) falls with increasing frequency, whilst the absorption loss (Curve 2) rises. Re-Reflection loss is insignificant in this example.


Waveguide below cut-off

The effectiveness that holes in an enclosure wall transmit electromagnetic waves into the equipment may be greatly reduced by increasing the depth of the hole. The diagram on the waveguide form below shows a metal spindle fixed behind a hole in the shield, and electrically connected to the shield.

waveguide.gif (9150 bytes)


    The spindle acts as a waveguide, which will not support efficient transmission below a cut-off frequency.

    For a depth (d) to width (w) ratio of 3 to 1, an attenuation of 82dB is obtained. For a width (w) of 10mm, the cut-off frequency of the spindle arrangement is 15GHz.

    The cut-off frequency, Fc is given by:

            Fc = 1.5 * 105 / w      MHz,        where w is in mm.

    The attenuation A at frequency F is given by:

            A = (27.2 * d * ( 1 - ( F / Fc )2 ) 0.5 ) / w       dB,     where d and w are in mm.

    The tool also allows for the effect of multiple apertures in the shield, where these are less than half a wavelength apart. Thus the performance of honeycomb vents, which are commercially available may be estimated. The reduction in shielding caused by multiple apertures is approximated by:

            Reduction = 10 * log 10 (Number of Apertures)

    Thus 10 apertures would have an attenuation 10dB worse than a single aperture, and 100 apertures 20dB worse.

    Up to five sets of data are supported, and selected using the Model Number pull down box.


Cavity Resonances of an Enclosure

    A metal enclosure can behave as a resonant cavity, with modest size enclosures resonating in the frequency range of emc susceptibility tests. The resonance establishes a standing wave, with the electric field peaking in the centre, and the magnetic field peaking at the edges.

    Many modes can be supported, leading to a series of peaks at increasing frequencies. Although the calculation performed by the model is unlikely to be especially accurate, due to the effects of the box contents, it does give an indication of the lowest possible resonance for a particular set of dimensions.

Cavity Resonance.gif (4679 bytes)



    The first sixteen resonant frequencies (of a theoretically infinite number) are listed. Note that when two or more box dimensions are equal, modes with different field patterns, but equal frequencies are created. These are known as degenerate modes.

    The tool requires the dimensions of a rectangular enclosure to be entered, in mm. The Analyse button then lists the first sixteen resonances in a pull down box, sorted in order of frequency.

    Thus the lowest resonant frequency supported by a small enclosure measuring 60mm by 70mm by 80mm would be 2.85GHz.

    The expression used to calculate the series of resonances is as follows:

Fres = 47.75 * ( ((m * Pi) / h) 2 + ((n * Pi) / d) 2 + ((p * Pi) / w) 2 ) 0 .5     MHz

    where h, d and w are the height, depth and width of the box respectively, expressed in metres. m,n and p represent the mode integers for the transverse electric and transverse magnetic waveguide modes TEmnp and TMmnp. Only one of the integers m,n and p may be zero at any one time.



Twisted Pair Rejection Ratio

    A twisted wire pair offers a low cost means of reducing differential mode coupling, and balancing common mode capacitance to ground. Each twist in the wire induces an opposite voltage to the previous twist, and for a uniform external field, cancels the induced voltage. At low frequencies, twisted pair wire is especially effective at reducing magnetic field induced currents.

Twisted pair form.gif (3888 bytes)

   

At higher frequencies however, phase differences between adjacent twists progressively reduce the twist effectiveness. It is this effect which is modelled by the program. The reduction in rejection ratio can be offset at any particular frequency by reducing the separation between twists. As with most other tools, a spot calculation and a graph plot are both available. The only parameters to be entered are the total wire length, the distance between twists, and the spot calculation frequency.

    A series of rejection ratio plots are shown below.

Twisted pair graph.gif (7045 bytes)



    Twisted pair rejection ratio for a 10 metre length of wire, for twist separations of 0.1m (Curve 1), 0.2m (Curve 2) and 0.3m (Curve 3).


Circular/Square EMI Gasket Groove Design

    Conductive gaskets form a convenient means of ensuring the continuity of a shield at an interface between, typically, a box and its lid or other access panel. Flat gaskets can be used, although their compression is then dependant on close control of the torque used to tighten the screws securing the lid. Circular or rectangular gaskets however can be placed in machined or cast grooves, when their compression is independant of the fastener torque, and only dependant on the groove dimensions.

Gasket Diagram.gif (2810 bytes)


    It is important that conductive EMI gaskets are placed in correctly designed grooves, to ensure correct operation. The gasket must be deflected from its round shape by a controlled amount, typically between 10% and 18%. Too little deflection could result in poor emi shielding; whilst too great a deflection could cause gasket damage. The groove depth controls deflection.

    Equally the maximum gasket volume must not exceed the groove volume, or the gasket could be extruded out of its groove, with attendant damage. All mechanical tolerances must be combined to assess the worse case effects, which with small diameter gaskets can produce out of limit deflection or groove over-fill. Finally, if the equipment lid or panel which is compressing the gasket is thin, and the fasteners are widely spaced, the lid could be deflected by the gasket's deflection force. This has the result of reducing the minimum deflection, and is discussed further below.



Gasket_form.gif (10922 bytes)


    The Gasket Groove Design tool provides a complete analysis of the groove design, at nominal and maximum/minimum tolerance conditions. The cross sectional area of the groove is calculated, and compared with the gasket volume, to calculate the groove fill. The maximum and minimum gasket deflection is also calculated. Use of the tool will demonstrate that small gaskets, under 2mm diameter for example, require increasingly tighter tolerances to meet the deflection requirements. Similarly, it is harder to design a satisfactory groove for a rectangular gasket than for a circular gasket. A set of 'industry standard' diameters and tolerances for circular gaskets are provided in a pull-down box, although any diameter can be entered.

    A printed report summarises the analysis, and a summary of the groove design is shown in a Comments section.


Lid Deflection Effect. A thin metal panel used to compress the emi gasket, combined with large fastener spacing can result in a further reduction in the minimum gasket deflection. This can then result in shielding degradation. The effect increases for narrower flange width, thinner panel thickness, and less stiff materials (lower Young's Modulus).

    The contribution of lid deflection is accessed from a separate form, opened from the main gasket analysis form. Several materials are available in a pull-down box with their respective Young's Modulus figure.

Lidbow.gif (2962 bytes)



    The gasket deflection force is quoted in the non-metric units of pounds per linear inch of gasket material, as this is commonly used by gasket manufacturers. The actual figure is typically 2 to 5 lbs/in for small gasket diameters (<3mm), and up to 15 lb/in for 6mm diameters. Manufacturers' data should be consulted for accurate figures, which should be at the minimum deflection point, generally around 10%. The calculated figure assumes constant Gasket Deflection Force, which is acceptable for small reductions in deflection.


Aperture Shielding Effectiveness

    An opening in the wall of a shielded enclosure is a potential source of radiated emissions, and radiated susceptibility problems. The degradation in shielding effectiveness increases progressively as the size of the aperture increases, until no shielding is provided when the aperture's largest dimension equals a half wavelength.  The Data Entry form is shown below; a spot calculation at a given frequency can be made, and a graph of aperture attenuation versus frequency can be generated. Up to five models can be plotted, and a replicate button is provided to copy Model 1 data to the other four models.

Ap_form.gif (6535 bytes)

    The Aperture model requires the shield wall thickness to be thin compared with the aperture size itself. For openings which are not round or square, the largest dimension should be used.

    The attenuation provided by an aperture is calculated from the following equation:

    Attenuation = 20 * log10 ( 0.5 * Wavelength / Aperture size) dB

    Multiple apertures spaced less than half a wavelength apart produce a shielding effectiveness reduction given by:

    Reduction = 10 * log10 ( n ) dB         where n is the number of apertures.

    Use of the tool will show that small openings of the order of a few mm will have relatively little effect at practical frequencies. Larger openings may cause significant emc problems, and techniques such as the use of a waveguide below cut-off, or a conductive window may have to be considered.

    A typical plot of aperture attenuation versus frequency is shown below, for five apertures of varying size. Curve 1 has a largest aperture dimension of 100mm, and is followed by aperture sizes of 50mm, 20mm, 10mm, and finally in Curve 5, 5mm.


Apleak graph.gif (8265 bytes)


    Note the reduced attenuation as the aperture dimension increases, and the reduced attenuation as the frequency increases.

The Aperture Tool can also be used to plot the radiated field due to a Differential Mode Current Loop. The details of this are entered on the DM Current Loop Tool (PCB Menu), and this plot option can be selected by checking the Leakage Field plot box next to the plot command on the Aperture Tool. If values are available from the Fourier analysis of a Trapezoidal waveform, the option to perform a Fourier plot is also offered. Thus three types of plot can be generated from the aperture tool:

* Plot of aperture shielding effectiveness, in dB's.

* Plot of the field emissions from a current loop, attenuated by the aperture, with units of dBuV/m. The plot is continuous with frequency, and is compared with the unattenuated field.

* Plot of the field emissions from a current loop, attenuated by the aperture, with units of dBuV/m. The field is plotted at discrete frequencies which originate from the Fourier analysis tool.



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