Document 155-1

DataLineFiltering
Introduction
Noise is a major concern among designers of electricalequipment because failure of an electronic design due to
unexpected or indeterminate noise sources is not uncommon. The FCC and other regulatory agencies are rigorously enforcing their noise requirements and this gives
noise an often pivotal role in the ultimate success of apiece of electronic equipment. This paper will addresssome of the important attributes of the noise associatedwith electrical and electronic equipment, particularly low
signal level data line equipment of which digital equipment
is one type. Some simple filtering solutions will also bediscussed.

Externally created noise may emanate from a number ofsources which are beyond design control. High frequency
noise may appear on the power line, riding the 60 Hz signal
usually without affecting the power circuitry, but providing
a real danger to data line transmissions. Radiated noise
may impinge on many parts of a piece of electronicequipment, especially through unshielded cabling, orthrough an ineffectively grounded case. Cabling is an
important factor in the generation of EMI. Cables aregenerally the longest paths between circuit components
and modules, thus providing an optimum situation for
creating a loop antenna for radiating and receiving externally generated noise fields.

Noise Sources

Noise Signal

Power converters are notorious sources of noise; theynormally produce both common mode (occurring equallyon all signal and common lines which refer to earthground) and differential mode noise (occurring between
signal and return paths). Noise from power converters
generally predominates at the harmonics of the switchingfrequency, but some wide band noise is also produced.
Because power converters are often required withinelectrical proximity of low signal level circuitry, they canbe a major factor in determining the overall dependabilityof the data line system involved. Semiconductor devices
may act as noise sources because of temperature (thermal noise), the adjoining of differing materials (contactnoise), and the electron-hole movement of junction devices (shot noise). The magnetic components of modules
which often support a data line system (for example, thetransformer of a power converter, or some other high Qcomponent) may themselves produce noise from ringing.
The logic circuits of equipment are yet another significant
source of noise. At the switching of logic levels, the local
power source can momentarily short to ground, introducing noise directly to both the ground plane and the dc
power supply and affecting the entire electrical system.
Internal noise may be generated from a number ofsources which are electrically near the data line circuitry
such as adjacent printed circuit boards or local magnetics(transformers, mixers, etc.) Semiconductor noises canalso propagate to the threshold sensitive logic lines, toand from digital system clocks, and onto data lines.

Figure 1 shows an actual sine wave of 60 Hz line voltage(110 Vrms) occurring in a typical factory situation. Thewaveform contains random noise voltages of much higher+150.0+75.00.0V-75.0-150.00.0 mS

16.67 mS

Figure 1. Line source in a factory

than line frequencies. Figure 2 shows a 2 mS section ofthe 60 Hz wave with noisy spikes shorter than 5 µS andattaining 50 V peaks. Much higher frequency noise canalso exist beyond the resolution of the waveform plot.
Contaminated line currents are conducted directly intomachines and devices connected to the same line as theoffending machines; noise voltages can also be measured radiating through the air near these machines.
Generally, line filters are placed ahead of sensitive equipment to improve the situation of the 50 V high frequency
factory noise mentioned above. Typically, a line filter iscomposed of a low frequency common mode magnetic
structure which allows (differential) line current to passfreely without saturating the magnetic material (differen-Specifications subject to change without notice.

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+75.0
+37.5
0.0V
-37.5
-75.0
0.0 mS

2.0 mS

Figure 2. 2 mS section of the line

tially induced flux lines cancel one another; this is theprimary aspect of a common mode magnetic design).
However, the noise common to both the source and returnlines is attenuated.

dBm Attenuation

Line filters clean the incoming 60 Hz line signal by
substantially attenuating frequencies through approximately 10 MHZ, with increasing failure to attenuate
beyond this frequency (Figure 3). A real possibility still
exists for higher frequency line aberrations, which often

Certainly many data line devices and equipment are notintended to be situated in the severe environment of afactory. For example, the equipment required to meet FCC
Class B limits do not usually have to deal with 50 V noiseoccurring from the power line; yet, Class B devices are
becoming more and more susceptible to the usually low
level/very high frequency noise common to contemporary
data line equipment environment (Figure 4). Especiallytrue is the sensitivity to noise of equipment which runs athigh clock speeds and baud rates.
The system clocks of data line equipment are one
concern of the noise conscious design engineer. Figure 5ashows a 20 MHZ clock signal with a spurious highfrequency content and potential problem regions; Figure
5b shows a noise-free 20 MHZ clock signal.

High0Low

10Self Resonance
0.0 nS

2050 nSFigure 5a. 20 MHz clock with high frequency noise

30
0.01

0.10

1.00
Frequency (MHz)10.0100HighFigure 3. Typical line filter response

evade line filtering, to affect the integrity of the low voltage
data transmission of data line equipment. Especiallyvulnerable is inter-equipment data transmission. An additional problem lies in the fact that equipment designatedfor factory use need only meet relatively less stringent
noise requirements, although these machines have thepotential for affecting their more sensitive counterparts(Figure 4).

dBm µVClass A60
Class B50

0.0 nS

50 nS

Figure 5b. A clean 20 MHz clock signal

The transmission of data between equipment, whether
within a local networking scheme or machine-to-machinecommunication through peripheral equipment like modems, can be disrupted and contaminated by unchecked
noise.

8070Low40

Filtering in General30
0.10.45

1.0 1.6
Frequency (MHz)Figure 4. FCC noise limits

10.030.0

The obvious solution to the problem of noise is toeliminate the source, but this is often out of the question.
Noise emanating within a piece of equipment is almost
always a side effect of a critical module of a design. The

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next obvious solution is to eliminate the noise at or as near
the source as possible. Noise sources may sometimes beobvious. A power supply, for example, is a logical place
to expect ample noise to emerge. Sometimes the sourcesof noise are not at all predictable; moreover, the precise
effect of a noise source is often illusive and only sporadically evident. The achievable solution to the problem ofnoise is to prevent it from affecting the systems andsubsystems susceptible to it. After locating and filtering
the obviously culpable noisemakers (e.g., the output of apower supply), the next step is to protect the inputs to thesensitive equipment and components. No more noise isthen permitted into the sensitive equipment than theequipment may continuously tolerate. By filtering theinputs of sensitive components, the elusive and unmanageable sources of noise then become much less criticalto the operation of the equipment.
Filtering is the ready solution to the problem of noise.
Digital equipment and sensitive components can be madeto withstand the subtle noise emanating from subsystemsof the equipment as well as its external environment.
Conducted noise often creates external fields, radiating
noise to equipment and components which in turn induce
conducted noise into these equipment and components.
Also, the higher the frequency the less energy required fordisruption. The cabling used to relay data line signals isparticularly sensitive to external noise, both conducted
and radiated. Because data line signals are essentiallysquare waves with predetermine transition levels (e.g.,0.8V and 2.0V for TTL, Figure 6), the sudden encroachment of a noise spike in the electrical vicinity of the cable,

all high frequency noise just prior to reception of a logic
signal would preserve the integrity of the original signal.
Likewise, attenuating all high frequency components ofthe signal just ahead of a transmission cable would clean
up the signal affected by noise within the data line systemitself prior to any transmission. Also, the logic levels of thedigital clock must be precise and unadulterated. Clocksignals can benefit from high frequency noise attenuation, safeguarding the data line system from false stateand logic transitions.

Magnetic Filtering
The solution to EMI problems typically arising in cablingis inherently magnetic. Cables are loop antennas createdby a miss-match of a cable’s signal and return line current.
If the signal and return lines of a cabling scheme werepositioned identically in space, the field created by theconducted current in the signal line would exactly cancel
the field effected by the conducted current of the return
line; of course this 180° perfect current reversal is notrealizable, and we therefore must have a finite quantity ofradiation due to any cabling.
Twisting signal and return lines of a cable where possible
breaks up the loop antenna structure into smaller loop
antennas, achieving two useful results: 1) a substantial
increase in the resonant frequency of the generated/received fields, and 2) allowing adjacent and inverted
fields to cancel one another. Increasing the noise frequency is sometimes helpful, but generally not a sufficiently complete solution since applicable noise frequencies range into the hundreds of megahertz.
Ribbon type cabling usually does not lend itself to thetwisting method described, unless special pre-twisted
ribbon cabling is used (where the ribbon lines are twistedin pairs) but running return lines directly adjacent torespective signal lines is necessary to decrease radiated/
received noise. Determining the actual ribbon cable line
assignments is seldom an option provided an EMI engineer; usually, a solution to noise problems is a post
mortem activity, such basic layouts having already been
made for an equipment’s design.

Figure 6. TTL signals

or radiating near the cable, could cause a data error at thereceiving end of the cable.

Filtering Techniques
A noise spike is caused by transient energy of a highfrequency content. Because noise spikes are composedof such high frequency energy, the drastic attenuation ofFiltering of a data line system, particularly when cablingand other signal wires are the objects to be safeguarded,is generally and most successfully realized throughexternal magnetics. Creating a magnetic field around awire can divert the high frequency energy from the signal
propagating through the wire. By increasing the inductance of the wire or cable at an appropriate point along its
path, a magnetic field can be created and magnified.
Forming the cable or wire into a number of loops is thesimplest method of increasing inductance. However, toobtain any useful inductance (thus appropriate attenua-Specifications subject to change without notice.

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tion of noise frequencies), many tens, or even hundreds,of loops would be required. Many loops are required toobtain the useful magnetic field because air alone isutilized to contain the field. Fortunately, magnetic materials can be used that have tens of thousands of times themagnetic capacity of air. Magnetic materials significantly
increase the inductance with minimal looping of associated wires and cables.

noise from each can become problematic. Fairly large
magnetic toroidal structures may be used in a similarmanner to a bead for filtering cables by fitting the structurearound the entire cable. (Figure 8). While using a single,
large structure allows common mode noise attenuation, it

Magnetic beads are often utilized for filtering; binocularcores, often used in balance to unbalanced (“Balun”transformers) are also used in common mode filtering. A
wire through one hole in a balun effectively constitutes asingle turn (Figure 7). The advantage of using baluns is in

Figure 8. Filtering with a large magnetic structure

may lack the magnetic capability to provide effective
protection of cabling because of the physical limitation oflooping the cable through the large toroid more than once
(thus increasing inductance).
Figure 7. A binocular filter

their simplicity: one wire, one bead. Unfortunately, one
bead may not suitably attenuate high frequency noise formany data line systems. Several beads may be strung
together on the same wire for increased attenuation, but
stringing beads on a signal wire, or multiple beads on acable of multiple wires can result in both a difficult design
and problems achieving repeatable design. Also, beads,
each one magnetically isolated from all others, lack theability to attenuate wide band common mode noise while
supporting differential signals.
The problem of common mode noise is very real andsomewhat elusive. Because it occurs equally and inphase on all signal lines with relationship to the groundplane, it becomes evident within an electronic systemonly when measured with reference to earth ground.
Within a single piece of equipment, common mode noise,
though very real, may be difficult to detect. The subsystems of a piece of electronic equipment, each withsemi-isolated ground planes, may display symptoms ofcommon mode noise, a loss of signal integrity and afailing electrical reference. Likewise, when isolated piecesof equipment, having independent common references,
are used in conjunction with one another (e.g., communicating through common signal lines), the common mode

Another method of protecting a data line system fromnoise is the use of special cable connectors which areeither predominately capacitive or magnetic by design.
The protection thus provided is limited to external connections. The capacitive connectors are typically more expensive than equivalent magnetics; magnetic connectors
often have the additional advantage of being commonmode by design. Multi-line common mode components
are also available. Such components are magnetic, arecompatible with ribbon cabling, and can be readily designed into a circuit.These multi-line common mode “DataLine” filters are available in standard DIP and surface
mount configurations.

Rules of Thumb
A successful filtering program of a data line systemshould have the following features:
1) The filter method must provide for the maintenance ofa signal integrity.
2) The filter method should be able to increase substantially the magnetic field at virtually any critical electricallocation.
3) The filter method must provide effective attenuation of
all high frequency noise, as well as broadband common
mode noise.

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4) The filter method must be compatible with standard
cabling schemes, including ribbon-type cables, and individual signal wires.
5) Ideally, the filtering method should be easy to implement, readily available, and successfully repeatable.

Conclusion
Noise sources are prevalent in a number of environments.
Particularly severe is the factory situation where highfrequency/high level noise transients intrude onto thepower mains. To a lesser degree, such noise also may befound in the power stage of a piece of sensitive equipment. Power converter noise may also be evident outside
the factory situation, where the more stringent regulatory
agency noise limits come into effect.

The various noises which can destroy the repeatability
and integrity of sensitive equipment, particularly data lineequipment, are essentially of two types: common modeand differential mode. Common mode noise predominates. To successfully filter both types of noise, a magnetic scheme is optimum. Common mode noise reduction
is effective when afflicted lines are coupled together
through a common magnetic structure. Individual beads
are only effective against differential mode noise.
A successful filtering scheme should have filters in placeprior to transmission as well as prior to the reception ofsensitive signals (e.g., at the beginning and at the end of
a transmission cable). The filters should offer repeatable
results and be effective through a wide high frequency
bandwidth.

Specifications subject to change without notice.

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