Electromagnetic Compatibility (EMC) is concerned with both the
process of emission of unintentional electromagnetic energy from
an electronic equipment and the ability of this equipment to
function in the presence of interfering energy (immunity to the
electromagnetic environment).
1) Sources of interference
2) Recommendations for effective EMC design
3) Using ferrite cores for EMI suppression
4) Typical ferrite materials characteristics
5) Relationship between impedance vs. frequency characteristic
and insertion loss
6) Finding insertion loss (attenuation, dB)
7) Relationship between the number of turns (windings)
and impedance vs. frequency characteristic.
8) Common mode choke.
9) Difference in impedance vs. frequency characteristic
between split cores and one-piece cores
10) Typical package example
1) Sources of interference
Since natural interference phenomena
do not really affect the modern equipment, the actual problems
today are generated accidentally by the actual operation of
the equipment. Basically, the radiated interfering signals
are classified in a narrow band and broad band. Narrow band
signals occupy a small portion of the radio spectrum and have
the energy concentrated in a single frequency wave. However,
when modulation is introduced, the narrow band signal may generate
side bands of energy which may cover hundreds of kilohertz.
Examples of sources for narrow band signals are radio & TV
transmitters, radio transceivers, cellular telephonic equipment
and Doppler radar. These sources start out with very low harmonic
frequency outputs but can cause harmonic frequencies to be
generated if they are used in areas which can present non-linear
conditions to the RF energy through secondary transmission
of these signals.
Broad band signals are those whose energy is spread over tens
of hundreds of Megahertz. They are generated by narrow pulses
with sharp rise times, characteristic of radars, gas discharge
tubes, engine ignition systems, power line discharges, computer
clocking pulses, motor brushes and switching regulators. The
steepness of the pulses causes problems because the short rise
times mean very high frequencies and that which may appear to
be a low impedance can actually have a high inductive component
and be a high impedance for the rise time.
Oscillator circuits on printed circuit boards can cause energy
to be both conducted and radiated. These oscillator sources may
be part of the power supply (as in switching regulators) or they
may be part of the logic clock circuits. Other source of radiation
could be amplifier circuits with a high slew rate, which can
have very fast rise times and, if not terminated properly, can
cause a large spectrum of noise.
Actually, every frequency source could be a potential source
of interference and all interference signals can be coupled to
the power line and conducted to the power mains, thus creating
problems to other equipments connected close to the signal source
equipment.
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2) Recommendations for effective EMC design
The most efficient design will take
into account from the beginning the possible presence of strong
external influences which may cause the equipment to malfunction,
as well as the need to limit the amount of conducted and radiated
emissions which may influence other equipments. Retrofitting
a solution to a design problem later, in the testing process,
is costly and time consuming.
The best EMC design will keep the strong energies on board,
where generated. In order to keep it "on board", the following
steps are suggested:
- Reduce the effective radiation area by reducing the loop
size or running signal and return on top of each other on
multi-layer PCB.
- Twist wires together or use co-axial cables.
- Shield or multi-shield signal wires.
- Enclose offending circuits in conductive boxes and prevent
conducted energy from escaping the box by installing ferrites
on the lines.
- Add EMI filters to the boards.
- Use ground-signal alternating path runs with ribbon cables,
flex circuits and connectors.In most cases, a good "on board" EMC
design is not enough if the equipment
has external wiring connections or a power supply cable
line. These cables are both a source of emissions and a
perfect antenna for coupling to external radiation.
The following solutions are suggested to minimize these
effects:
- Use ferrite beads over the whole cable at the signal source
end, as common mode choke.
- Add a homogenous shield with a low transfer impedance,
both ends bonded, so that the cable becomes surrounded by
a ground and any common mode current returns by the this
shield instead of the actual common mode ground.
- Terminate the flat ribbon cables with ferrite filtered
connections.
INTERMARK offers a variety of reliable solutions for
an efficient EMI design.
EMI and ESD can be easily controlled
by proper grounding, shielding
and filtering using ferrite products (cores for round and
flat cables, surface mount ferrite chips, multi-hole plates
for connectors, etc.), cable shielding materials (conductive
tapes, shielding jackets, etc.) and grounding fasteners from
INTERMARK. Among these solutions, filtering by using a ferrite
core may be the most cost efficient measure. This method has
a significant merit that it imparts
insertion loss only to unnecessary noise and does not influence
the line signals when used as a common mode choke.
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3) Using ferrite cores for EMI suppression
Based on the specific magnetic
properties of the Nickel-Zinc ceramic ferrite materials,
molded cores, when used as EMI filters, absorb the energy
of the high frequency noise on the line and dissipate
it as quantities of heat. Since the electrical resistance
component of the material is reduced at low levels of
frequency, the ferrite cores provide very low series
impedance and do not affect the normal data signals
on the line.
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4) Typical ferrite materials characteristics
The physical characteristics
of the INTERMARK Ni-Zn ferrite materials are listed
below:
The impedance vs. frequency characteristics
for several Ni-Zn ferrite materials
are shown on Fig. 1 below.
[ Figure 1 ]
The shielding effectiveness provided at
different frequency levels is shown
on Fig. 2 below.
[ Figure 2 ]
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5) Relationship between impedance
vs. frequency characteristic and insertion loss
The impedance Z increases as frequency
increases (Fig. 3).
[ Figure 3 ]
The impedance is dominated by the
R factor (resistance), which has
the main influence on the insertion loss.
At higher frequencies (> 200 MHz) the impedance
characteristic is reversed, due to stray capacities.
Fig. 4 displays the insertion loss
at 50 Ohms load impedance.
[ Figure 4 ]
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6) Finding insertion loss (attenuation, dB)
Examples of calculation:
6.1. Finding insertion loss when inserting two turns of a cable into the Item
No. TRCN-16-8-13 at 100MHz with the circuit impedance (ZA + ZB) being 1 Ohm
(when making two turns at 100MHz, the impedance is 500 Ohms).
The insertion loss (dB) = 20 LOG ((1+500)/1)
= 54 dB
6.2. Same as Example 1 above, with the only exception
of the circuit impedance (ZA + ZB) being 50 Ohm:
The insertion loss (dB) = 20 LOG ((50+500)/50) =
21 dB
When comparing Examples 6.1 and 6.3 above, it is
found that, even if the same ferrite core is employed,
a strikingly different damping factor will be assured
if the impedance of the circuit is different. In other
words, the effects of ferrite cores are increased
at lower circuit impedance. It may be said, therefore,
that the number of turns should be increased or ferrite
cores with larger volume and higher impedance should
be employed if the circuit impedance is high.
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7) Relationship between the number of turns
(windings) and impedance vs. frequency characteristic.
The graphs shown in Fig 6. indicate
performance curves of one to four turns from the bottom
to the top, respectively. As the number of turns is
increased, the impedance is enhanced, but it shown
from this figure that the peaks of impedance are moved
downward as stray capacities between lines are increased.
[ Figure 6 ]
If insertion loss needs to be increased at lower
frequencies, the number of turns should be increased
as common mode choke so that damping may apply only
without any influence upon signals.
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8) Common mode choke.
Should ferrite cores be inserted
as shown on Fig. 7, they will not only impart insertion
loss to common mode noises, but also influence signals.
On the contrary, and inserting method, as shown
on Fig. 8, called common mode choke, does not influence
signals, because magnetic fields produced by signal
currents cancel each other, thereby no insertion
loss being imparted to the signals. Since noise
currents flow for the ground, magnetic fields with
a specific direction are produced, thereby this
method proved to be a convenient way to absorb noises
only.
When ferrite cores are used to eliminate high frequency
noises on a DC line such as a power supply line, the
supply line as well as the grounding return line can
be inserted through the ferrite core.
[ Figure 7 ]
[ Figure 8 ]
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9) Difference in impedance vs.
frequency characteristic between split cores and
one-piece cores
The graph shown in Fig. 9 compares characteristics of separated and non-separated
toroidal cores of the same size. Although impedance of a separated core is lower
in the area less than 30MHz, it is similar to or slightly higher than that of
a non-separated one at higher frequencies.
The merit of using the separated cores is that they
may be easily mounted in an already wired cable or
a cable which has been already connected to a connector
and that magnetic saturation in DC occurs less frequently
in separated cores than in non-separated ones when
large DC current flows through a line.
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10) Typical package example
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Technical
Considerations on EMC 
Ferrite Products 
