What is Radiated Susceptibility?
Generally speaking, 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 , 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 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.,
 .
The common mode current
IC is the difference between the measured current and the
differential mode current, i.e.,
 .
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. More:
 Radiated Susceptibility Limits
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