Click on the following bookmarks for further details of a Field Tool:
Common mode field coupling occurs when cables connected to an item of equipment are subjected to an external field, as shown below.
A loop is formed between the cabling, the equipments and a ground path, the induced loop voltage driving currents in the same direction along the cabling. The loop may often be hard to define in physical terms, depending on a cabling layout which may be variable, and a ground path which is also ill-defined. In addition, the coupling to the ground path, often capacitive, may also not be easily defined.
The Common Mode Field Coupling Tool provides several facilities to investigate the effects on a circuit of this arrangement. With the comments of the previous paragraph in mind, it must be understood that modelling common mode coupling is affected by many variables, and is unlikely to accurately reflect a real situation. Nonetheless, an understanding of the mechanism is vital in mitigating a widely encountered means of coupling interference currents into an equipment. The effects of circuit grounding, and the use of unblanced and balanced inputs are also clearly contrasted.
The Balanced circuit model is shown below: Note how the circuitry, and the equipment boxes can be grounded or capacitively coupled, which has a major effect on Common Mode Rejection Ratio (CMRR).
Spot analyses and swept frequency graphs can be made for the basic common mode loop, and for unbalanced and balanced circuits. Load voltage and CMRR can be calculated and plotted.
Where N is the number of turns, I is the current in the coil, and r is the coil radius. By differentiating B to find dB/dx, and differentiating again to find d2B/dx2, a point of inflexion at x = r/2 is found by equating the result to zero.
If a second coil is placed at distance r from the first coil, a near uniform field is produced in the central region between the coils. For example, two coils of 1m radius, placed 1m apart, produces a field which is uniform to 0.1% of peak field over a distance of 346mm. The Helmholtz Coils Tool allows the field on the axis of two coils to be calculated, plotted and tabulated, and is shown below:
The coils can be placed any distance apart, although for optimum uniformity the Helmholtz condition must be applied, where the distance between the coils equals the coil radius. The Plot button produces the separate fields due to each coil, as well as the total field.
The Analyse button calculates the fields at any point between the coils, whilst the Tabulation button produces a list at the desired distance interval. The Export button shows a filename box; when a filename is provided, the tabulation figures are stored in a comma separated value (CSV) file, which can be opened by spreadsheet applications. Both the plotted graph and the tabulation can be printed.
Electromagnetic Field Formulae
The field formulae form is shown below:
This tool calculates the magnetic field produced near a wire carrying a current, for the two situations of short wire and long wire. The field B due to a short length of wire, carrying current I, at perpendicular distance r from the wire is given by:
Bsw = mI (cos(b) cos(a)) / (4 * Pi * r) Tesla
The field is directed into the the plane of the diagram, as shown below on the Short Wire tab..
For an infinite wire, angle a = 180 degrees and angle b = 0, and the long wire result is found:
Blw = mI / ( 2 * Pi * r) Tesla
The long wire tab is shown below:
Both analyses express the field in microTeslas.
Westbay Technology Ltd, Main Street, Baycliff, Ulverston, Cumbria LA12 9RN, England
Tel: 01229 869 798 (from the United Kingdom) Tel: +44 1229 869 798 (International) Contact by e-mail