What Are Conducted Emissions?
is a term for
current that flows on one or more connected to an electric circuit, or
alternatively, radio frequency voltage between conductors connected to an
electric circuit. Generally, conducted emissions voltage is specified as
the voltage that develops when conducted emissions current encounters a 50 ohm
impedance.
Why 50 ohms? Most test equipment used in performing
radio frequency EMI measurements has an input impedance of 50 ohms.
For the purposes of EMI analysis, conducted emissions are
generally of interest over the frequency range 150 kHz to 30 MHz, because this
is the frequency range over which most regulatory agencies specify conducted
emissions limits. However, CE Analyst allows conducted emissions
over any frequency range to be evaluated.
Conducted emissions are a natural byproduct of active
circuit operation. Simply put, conducted emissions are created whenever
the current drawn by a circuit from the power leads changes over time. The
current waveform produces a corresponding voltage on the power leads equal to
the product of current amplitude and
impedance. Conducted emissions may be differential mode, common mode, or
both.
Tech Note - Differential Mode and Common Mode Conducted
Emissions Current
When
an active circuit operates, the current amplitude changes in the conductors
between the circuit and its or source. The
change occurs in both the power and return conductors but not equally, or the
signal and return conductors but not equally. If the current I1
on one conductor is measured and at the same instant in time and at the same
location the current I2 on the other conductor is measured, two
different 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 conductor is the
current that normally flows in the circuits. The common mode current at
the ends of the conductors is the current that returns through the ground plane,
shield, or another path other than the conductors. At frequencies for
which the length of the 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 circuits the
magnitude of the common mode current is substantially smaller than the magnitude
of the differential mode current. More:
 Conducted Emissions Limits
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