Generally speaking, Radiated Susceptibility is concerned with the ability of an electronic circuit, a piece of equipment, or a subsystem or system to operate acceptably when subjected to an externally generated electromagnetic field.  Radiated Susceptibility is also known as Radiated Immunity, particularly in European Union nomenclature.

When an electromagnetic field encounters an electrical conductor current is induced on the conductor.  The amplitude of the induced current, and the voltages induced at the circuits at each end of the conductor are a function of numerous variables, as explained in more detail below.  In general, current amplitude increases with increasing field strength.  The field strength that induces sufficiently high current to cause the device to no longer operate acceptably is termed the susceptibility threshold.  Thus, at field levels below the susceptibility threshold the device operates acceptably, and at field levels above the susceptibility threshold the device does not operate acceptably.  The criteria for establishing what constitutes "operating acceptably" are a function of the device and its intended use.  Susceptibility threshold for a given device typically varies as a function of frequency.

Radiated Susceptibility tests are performed on a device to determine whether the device is susceptible to electromagnetic fields having specified amplitude over a specified frequency range.  If the device operates acceptably as the field is applied and swept over the specified frequency range the device is considered to have passed.  If not, it has failed.  In many cases a device that is adversely affected by the field will return to normal operation when the field is removed.

RS Analyst allows the device circuits to be modeled so that current amplitude induced by the electromagnetic field can be calculated at each frequency specified.  It is incumbent upon the engineer to determine what level of induced current or voltage in the circuits may be problematic for the circuit.

Tech Note - Differential Mode and Common Mode Current

When exposed to an electromagnetic field, current is induced on both conductors of the conductors, but not equally.  If the current I1 on one conductor is measured at the same point on the conductors and at the same instant in time as the current I2 on the other conductor, two amplitudes will be measured.  The difference between the two measurements is twice the differential mode current ID, i.e.,

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The common mode current IC is the difference between the measured current and the differential mode current, i.e.,

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In a sense, the current on each conductor can be separated into two constituent currents, the differential mode current and the common mode current.  The differential mode current is equal in each conductor, but oppositely directed (180° out of phase).  The common mode current is equal in each conductor and is similarly directed (in phase).

The differential mode current at the ends of the conductors is the current that flows in the circuit components connected from line to line.  Similarly, the common mode current at the ends of the conductors is the current that flows in the circuit components connected from line to ground.  At frequencies for which the length of conductors is a significant fraction of a wavelength the magnitude of both the differential mode and common mode current will vary at different points along the length of the line.  For most conductors the magnitude of the field-induced common mode current is substantially greater than the magnitude of the differential mode current.

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