Filtering Out Interference Signals with Cable Ferrites
a ferrite to a cable or looping a cable through a ferrite can
help reduce unwanted high-frequency interference.
of FerriShield Inc., New York City
cable can be looped through a ferrite to increase the ferrite
impedance at a specific frequency.
cabling and wires, by virtue of their length-to-width ratios,
are perfect natural antennas. In the presence of high-speed microprocessor
signals, cables will conduct, radiate, and receive unwanted high-frequency
interfering signals. Control of radio-frequency (RF) interference
can be ensured by the proper placement of an insertion-loss device,
such as a ferrite suppressor. Compared with alternatives such
as in-line filters, onboard suppression circuits, shielded cables,
and expensive filtering circuits, the high resistivity per cubic
volume of ferrites stands out as the most important advantage.
Ferrites have a concentrated, homogeneous magnetic structure with
high permeability. They are consistently stable over time and
over a wide temperature range, and provide RF suppression without
high eddy-current losses.
major application factors used for defining a specific ferrite
solution for a particular interference problem include the following:
Frequency at which maximum attenuation is required.
Amount of attenuation needed.
Ferrite permeability formulation characteristics as they relate
to the frequency range in question, i.e., initial permeability.
Ferrite formulation consistency, i.e., expected range of variation
in attenuation performance.
Installation environment and mechanical attachment requirements
levels found on real-world cables and circuits vary widely as
related to frequency and circuit length. For suppression modeling
purposes, a 50-W source and load impedance
combination is assumed. However, circuit and wire-cable geometries,
especially circuit and cable length, can cause variation from
as little as a few ohms to hundreds of ohms. Characterization
of an actual circuit can be done with a simple impedance analyzer
setup to refine any suppression modeling calculations at given
impedance varies linearly with changes in its overall length
and with changes in the ratio of its outer diameter (OD) to
its inner diameter (ID). One of the most effective techniques
to increase ferrite effectiveness is to size the ID as closely
to the wire size as possible. For a broadband ferrite with a
1.0-in. length and 0.50-in. OD attached to a 0.25-in.-diameter
cable, a 0.25-in. ferrite ID would yield a 20% impedance improvement
over a 0.30-in. ferrite ID (a looser fit). The OD to ID ratios
would be 0.50 in./0.25 in., or 2.0 (about 200 W), and
0.50 in./0.30 in., or 1.66 (or about 166 W), respectively.
modeling procedure to calculate impedance characteristics of
the source and load coupled with the ferrite suppressor is developed
ZA = source impedance, ZB
= load impedance, and ZF = ferrite impedance.
Insertion loss is defined as a measure of the effectiveness
of a filter at a selected frequency. Insertion loss is expressed
in decibels and is described as the ratio of voltage with, and
without, the filter in the circuit.
of radio-frequency interference can be ensured by proper placement
of an insertion-loss device such as a ferrite.
example, see the curve labeled as "one turn" at 44 MHz in Figure
1. If the circuit impedance (ZA + ZB)
is 50 W and the ferrite impedance is 70 W at 44 MHz,
then the insertion loss will be 20 log10 (50
+ 70)/50 = 7.6 dB.
if the same ferrite is used, the attenuation provided by the
ferrite suppressor can change somewhat as the original circuit
impedance varies. The ferrite is more effective when the circuit
impedance is low. For example, by using the same ferrite performance
of 70 W at 44 MHz in a 75-W circuit, the resulting
insertion loss will be 20 log10 (75 +
70)/75 = 5.7 dB.
Placement. For the most part, the impedance effect from
the addition of a ferrite suppressor is constant regardless
of where along the circuit the suppressor is applied. However,
the overall success of the addition relates to the antennalike
length-to-width structure ratio of the cable and its tendency
to receive or emit radio signals. For a cable, it is best to
locate the suppressor close to the cable termination where it
exits the electronic enclosure, thereby negating the cable's
antenna-length effect. This is effective for both emissions
and susceptibility. A suppressor may also be needed on each
end where a cable connects two enclosures containing RF sources.
For circuits within an enclosure, a position close to the RF
source is best; however, other locations along these relatively
short runs are usually just as effective.
1. Typical performance at one, two, and three turns for
the same ferrite configuration.
Size. In applications with high circuit impedance, it may
be possible to increase the effectiveness of the ferrite by increasing
its impedance ZF. A larger- size ferrite
will increase ZF almost on a direct percentage
basis. However, larger ferrites, especially longer ferrites, are
sometimes not an option because of space allowances, weight, aesthetics,
and other packaging considerations.
Looping. An effective alternative is to pass the cable through
the ferrite opening multiple times by looping the cable back through
the ferrite. This increases the effective magnetic pathZF
increases geometrically by the square of the number of turns (N2)
through the ferrite opening. For example, two turns provide (22
= 4) four times the ZF at a given frequency.
drawback to multiple looping is that the characteristic frequency
performance band will become narrower and the frequency at which
maximum impedance is attained will be lower. However, this is
not an operational restriction because ZF
increases so much throughout the (narrower) band that it is
not necessary to be concerned with where the peak of the performance
curve moves. At three or more turns, there are commensurate
(N2) results. Although the performance band compression
and resonant-frequency effects may differ from one core geometry
to another, the ZF increase from multiple
turns around the ferrite should follow the N2 rule.
1 shows that two turns through the same ferrite will yield 280
W at 44 MHz. The ZF of 280
W is the same as the theoretical value
of 70 W X 22. The resulting
attenuation with a 50-W circuit at
the same frequency is calculated as
Figure 1 again for three turns (N = 3) yields a ZF of
604 W. The ZF of 604 W
is close to the theoretical value of 70 W
X 32 = 630 W. The attenuation
with a 50-W circuit is
round- and flat-cable circuits lend themselves to the multiple-looping
technique. Ferrite cores for round cables are typically available
with an ID up to 1.00 in., and cores for flat cables are available
in widths up to 3.25 in. or 64 conductors. One variation is
a multihole ferrite core that allows the cable to be threaded
through the openings without passing around the exterior dimensions
of the ferrite. Multiple looping with flat cables can be accommodated
by using a side-by-side method.
multihole ferrite core. A cable is threaded through openings
in the core without passing around the exterior dimensions.
flat cables can be accommodated by using a side-by-side method.
performance of a ferrite suppressor is consistent over time
and over a wide range of temperatures. With the proper matching,
placement, fit, and sizing, ferrites can provide an effective
solution to unwanted RF interference. If more suppression is
required, looping the cable through the ferrite can increase
the effectiveness of a ferrite without increasing cost. In addition,
multiple looping locks the ferrite into position along the longitudinal
axis of the cable. In some cases, it can serve as the mounting
location mechanism for ferrite positioning. The resulting increase
in packaging integrity is an ancillary benefit. Furthermore,
packaging with the smallest, most cost-effective component to
accomplish the task is certainly an option with this simple
May is the vice president of FerriShield Inc. (New York City).
He has been an electromagnetic compatibility market participant
for the past 23 years and holds many related patents. He can be
reached at firstname.lastname@example.org.