Using Magnetics
ENT-AN0098 Application Note Revision 2.1 3
component guaranteeing a successful EMC design, generally the measures involved fall into one of the
two categories: minimization and containment.
2.2.1 Electromagnetic Interference Minimization
Minimization involves addressing the parasitic RF energy sources causing EMC problems. If this energy
is minimized, there will be less need for additional minimization measures. It is not the purpose of this
Application Note to cover all aspects of EMC improvement but it is important to list some of the major
rules used to minimize the energy emitted from interference sources. These rules are as follows:
• Use the slowest practical rise and fall times on digital signals, especially clocks. These edges can
often be slowed sufficiently by inserting a series resistor of a few tens of Ohms close to the signal
source.
• Keep high-speed digital lines, especially clocks, as short as possible. Route them over unbroken
ground or power planes. This will help to avoid turning these signal lines into efficient antenna.
• Avoid routing high-speed lines near lines going off-board. This is especially important for the
10/100/1000BASE-T signals. Capacitive and inductive coupling between traces can couple RF
energy onto the off-board lines and into any cables connected to them.
• When high-speed lines on a circuit board must cross over off-board lines, ensure they do so at right
angles. Where the lines must run parallel to one another, maximize the spacing between them and
minimize the length of the parallel section. This will help to reduce the coupling between the lines.
For integrated Switch/PHY devices (e.g. SparX family) all the logic is integrated into a single device, and
there are no external busses connected that can generate noise. This significantly improves the EMC
performance of the systems.
2.2.2 Electromagnetic Interference Containment
Containment measures serve to prevent RF energy from escaping into the environment. The three basic
techniques of containment are power filtering, signal filtering, and shielding.
2.2.2.1 Power Filtering
Power filtering means that all the high-speed digital and high-power analog modules are provided with
local decoupling capacitors and, where necessary, either series resistors or ferrite beads are provided for
the power rails. This way the noise generated locally is filtered ‘on the spot’ and not spread all over the
entire board to eventually escape into environment. Since high value capacitors are efficient at low
frequencies and vice versa, each power rail should have a combination of high capacitance and low
capacitance ones.
2.2.2.2 Signal Filtering
Signal filtering involves passing signals that must exit a chassis through frequency selective circuits that
attenuate the RF energy sufficiently well to meet EMC requirements. There is often a tradeoff between
providing adequate attenuation of the interference energy and maintaining the integrity of the desired
signal. This is where the distinction between common-mode and differential signals becomes
advantageous.
In 10/100/1000BASE-T systems, differential signals are used to convey the data in each port.
Fortunately, most of the interference energy present on these same ports is in the form of a common-
mode signal. Magnetic devices such as transformers with common-mode chokes do an excellent job of
strongly attenuating common-mode signals while passing differential signals nearly unchanged.
2.2.2.3 Shielding
Shielding makes use of the fact that no electromagnetic field can pass through a perfect electrical
conductor. If a source of RF energy can be completely enclosed within an unbroken conducting
container, then none of that energy will exist outside of the container. If there are holes, seams,
connectors, etc. passing through the container, or if the container is made of a poorly conducting
material, then energy can escape and create interference.
Various shielding methods are used in system design. The primary method is to enclose the circuitry
within a conductive housing or chassis. This can be made of either metal or plastic that has been treated
to have a conductive surface. Individual pieces of the chassis must be in good conductive contact with
