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Interconnection Tests For Radiated Emissions Testing

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Interconnection Tests For Radiated Emissions Testing

A splitting ferrite bead (SMB) is made of a metal like aluminium or stainless steel and is used for the purpose of separating two conductors with a gap. The metal strip is pushed between the two conductors by a spring and the gap is filled with air, sometimes by a liquid like argon. The process is not too complicated and it can be handled easily by a novice. Let us see how this works.

Split Ferrite Bead (SMB) has the ability to change state from conductive to non conductive by heating and cooling. Generally, two turns of the power cord are placed between the two wires. The junction at the end is formed by a thin metal strip. The thickness of the strip is selected such that it gives a matching size to the exposed wire. The inherent impedance given by the ferrite bead is roughly equivalent to the combined number of turns in a single turn; therefore, two such beads gives four times the inherent impedance of a single one.

The power cord is connected to both the beads and the junction. Now the power cord is unplugged and a fresh connection is made. The dissimilarities between the two wires induced a current passing through the junction and the resulting resistance is measured as the primary current. The current that flows through the thin metal strip is in the form of its resistance. If the current is much larger than the inherent resistance, then it is assumed that there is some kind of discontinuity in the device.

The measurement of the DC output cable from the input circuit can be performed with the help of a multimeter. If the results of the measurement are consistent, then we assume that the ferrite bead used has been selected appropriately. In case the results are varying, then the device is either under-determined or is having some other problem. Care should also be taken while changing the current connection if the multimeter is operated directly through the external power supply. In such cases, the multimeter can also be operated using a low amperage power supply.

The second type of ferrite bead uses two turns around a core; however, the size of the core will determine the level of undesired signals that can pass through it. To measure the level of the DC current flowing through the wire, a potentiometer should be used. The two turning processes will cancel each other out and the level of undesired signal will be known. A small current passing through a junction produces a DC output signal at a frequency which is quite high. If the desired current level is set, then this will be measured with a potentiometer and the output level of the circuit measured.

In order to perform the measurement, a load is attached to the input terminals of both turning pieces. The load is connected to both wires in the identical way so that they produce polarity currents. Then the output currents will be measured. The DC output voltage will be the maximum value of the output signal when the two turns are compared. Both types of the multimeter are useful in performing the above two types of tests.


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