Data Line Filtering

IntroductionNoise is a major concern among designers of electricalequipment because failure of an electronic design due tounexpected or indeterminate noise sources is not uncom-mon. The FCC and other regulatory agencies are rigor-ously enforcing their noise requirements and this givesnoise 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 lowsignal level data line equipment of which digital equipmentis one type. Some simple filtering solutions will also bediscussed.

Noise SourcesPower 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 betweensignal and return paths). Noise from power convertersgenerally 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 devicesmay act as noise sources because of temperature (ther-mal noise), the adjoining of differing materials (contactnoise), and the electron-hole movement of junction de-vices (shot noise). The magnetic components of moduleswhich 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 significantsource of noise. At the switching of logic levels, the localpower source can momentarily short to ground, introduc-ing noise directly to both the ground plane and the dcpower supply and affecting the entire electrical system.Internal noise may be generated from a number ofsources which are electrically near the data line circuitrysuch 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.

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

Noise SignalFigure 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

than line frequencies. Figure 2 shows a 2 mS section ofthe 60 Hz wave with noisy spikes shorter than 5 S 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 mea-sured radiating through the air near these machines.Generally, line filters are placed ahead of sensitive equip-ment to improve the situation of the 50 V high frequencyfactory noise mentioned above. Typically, a line filter iscomposed of a low frequency common mode magneticstructure which allows (differential) line current to passfreely without saturating the magnetic material (differen-

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Specifications subject to change without notice. Document 155-1 Revised 7/19/95

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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.Line filters clean the incoming 60 Hz line signal bysubstantially attenuating frequencies through approxi-mately 10 MHZ, with increasing failure to attenuatebeyond this frequency (Figure 3). A real possibility stillexists for higher frequency line aberrations, which often

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

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 FCCClass B limits do not usually have to deal with 50 V noiseoccurring from the power line; yet, Class B devices arebecoming more and more susceptible to the usually lowlevel/very high frequency noise common to contemporarydata 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 oneconcern of the noise conscious design engineer. Figure 5ashows a 20 MHZ clock signal with a spurious highfrequency content and potential problem regions; Figure5b shows a noise-free 20 MHZ clock signal.

The transmission of data between equipment, whetherwithin a local networking scheme or machine-to-machinecommunication through peripheral equipment like mo-dems, can be disrupted and contaminated by uncheckednoise.

Filtering in GeneralThe 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 almostalways a side effect of a critical module of a design. The

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Figure 5b. A clean 20 MHz clock signal

Figure 5a. 20 MHz clock with high frequency noise

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Specifications subject to change without notice. Document 155-3 Revised 7/19/95

Figure 6. TTL signals

next obvious solution is to eliminate the noise at or as nearthe source as possible. Noise sources may sometimes beobvious. A power supply, for example, is a logical placeto expect ample noise to emerge. Sometimes the sourcesof noise are not at all predictable; moreover, the preciseeffect of a noise source is often illusive and only sporadi-cally evident. The achievable solution to the problem ofnoise is to prevent it from affecting the systems andsubsystems susceptible to it. After locating and filteringthe 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 unman-ageable 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, radiatingnoise to equipment and components which in turn induceconducted 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 conductedand 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 encroach-ment of a noise spike in the electrical vicinity of the cable,

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

Filtering TechniquesA noise spike is caused by transient energy of a highfrequency content. Because noise spikes are composedof such high frequency energy, the drastic attenuation of

all high frequency noise just prior to reception of a logicsignal would preserve the integrity of the original signal.Likewise, attenuating all high frequency components ofthe signal just ahead of a transmission cable would cleanup 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 attenua-tion, safeguarding the data line system from false stateand logic transitions.

Magnetic FilteringThe solution to EMI problems typically arising in cablingis inherently magnetic. Cables are loop antennas createdby a miss-match of a cables 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 cancelthe field effected by the conducted current of the returnline; 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 possiblebreaks up the loop antenna structure into smaller loopantennas, achieving two useful results: 1) a substantialincrease in the resonant frequency of the generated/received fields, and 2) allowing adjacent and invertedfields to cancel one another. Increasing the noise fre-quency is sometimes helpful, but generally not a suffi-ciently complete solution since applicable noise frequen-cies range into the hundreds of megahertz.Ribbon type cabling usually does not lend itself to thetwisting method described, unless special pre-twistedribbon 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 lineassignments is seldom an option provided an EMI engi-neer; usually, a solution to noise problems is a postmortem activity, such basic layouts having already beenmade for an equipments design.Filtering 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 signalpropagating through the wire. By increasing the induc-tance of the wire or cable at an appropriate point along itspath, 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-

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Specifications subject to change without notice. Document 155-4 Revised 7/19/95

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 mate-rials can be used that have tens of thousands of times themagnetic capacity of air. Magnetic materials significantlyincrease the inductance with minimal looping of associ-ated wires and cables.Magnetic beads are often utilized for filtering; binocularcores, often used in balance to unbalanced (Baluntransformers) are also used in common mode filtering. Awire through one hole in a balun effectively constitutes asingle turn (Figure 7). The advantage of using baluns is in

their simplicity: one wire, one bead. Unfortunately, onebead may not suitably attenuate high frequency noise formany data line systems. Several beads may be strungtogether on the same wire for increased attenuation, butstringing beads on a signal wire, or multiple beads on acable of multiple wires can result in both a difficult designand problems achieving repeatable design. Also, beads,each one magnetically isolated from all others, lack theability to attenuate wide band common mode noise whilesupporting 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 sub-systems 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., communi-cating through common signal lines), the common mode

noise from each can become problematic. Fairly largemagnetic 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

may lack the magnetic capability to provide effectiveprotection of cabling because of the physical limitation oflooping the cable through the large toroid more than once(thus increasing inductance).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 connec-tions. The capacitive connectors are typically more ex-pensive than equivalent magnetics; magnetic connectorsoften have the additional advantage of being commonmode by design. Multi-line common mode componentsare also available. Such components are magnetic, arecompatible with ribbon cabling, and can be readily de-signed into a circuit. These multi-line common mode DataLine filters are available in standard DIP and surfacemount configurations.

Rules of ThumbA 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 substan-tially the magnetic field at virtually any critical electricallocation.3) The filter method must provide effective attenuation ofall high frequency noise, as well as broadband commonmode noise.

Figure 7. A binocular filter

Figure 8. Filtering with a large magnetic structure

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4) The filter method must be compatible with standardcabling schemes, including ribbon-type cables, and indi-vidual signal wires.5) Ideally, the filtering method should be easy to imple-ment, readily available, and successfully repeatable.

ConclusionNoise 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 equip-ment. Power converter noise may also be evident outsidethe factory situation, where the more stringent regulatoryagency noise limits come into effect.

The various noises which can destroy the repeatabilityand integrity of sensitive equipment, particularly data lineequipment, are essentially of two types: common modeand differential mode. Common mode noise predomi-nates. To successfully filter both types of noise, a mag-netic scheme is optimum. Common mode noise reductionis effective when afflicted lines are coupled togetherthrough a common magnetic structure. Individual beadsare 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 ofa transmission cable). The filters should offer repeatableresults and be effective through a wide high frequencybandwidth.