UNDERSTANDING and controlling RF interference

Interference is not hard to find; it is actually difficult to avoid, especially in urban areas where the wireless revolution is well underway. By definition, 2/01/1999 7:00 AM Eastern

UNDERSTANDING and controlling RF interference

Feb 1, 1999 12:00 PM, Bill Whitlock

Interference is not hard to find; it is actually difficult to avoid,especially in urban areas where the wireless revolution is well underway.By definition, interference originates from a source external to a signalpath and produces undesired artifacts in the signal. A radio frequency, orRF, is loosely defined as being in that portion of the electromagneticspectrum above audio (about 20 kHz) but below infrared (about 30 THz).Electromagnetic interference (EMI), is a broader term having the same basicmeaning but without frequency limitations. Electromagnetic compatibility(EMC), is a term coming into more widespread use regarding issues ofequipment electromagnetic emissions and susceptibility, especially becauseregulations now require all equipment sold in Europe to carry the CE mark.

Electromagnetic fields, such as radio and TV signals, travel through space(or air) at the speed of light, about 300,000,000 m/sec or 186,000miles/sec. Because wavelength is the physical distance such a signaltravels during a single cycle, as frequency increases wavelength decreases.For example, a 1 MHz AM radio signal has a wavelength of about 1,000 feet(305 m), but for a 100 MHz FM radio signal, it is about 10 feet (3 m), andfor a 12 GHz DSS TV signal, only about an inch (25.4 mm). Any wire canaccidentally become a good antenna if its length happens to be, say, thewavelength of a strong local FM station.

Sources of RF interference fall into two broad categories-intentional andunintentional. Intentional sources include AM, shortwave, FM, and TVbroadcast transmitters as well as ham and CB transmitters, remote controls,wireless phones, cellular phones, commercial taxi/police/aircraft radios,microwave ovens, motion sensors, radar systems, and a myriad of medical andindustrial RF devices.

Unintentional RF sources are most commonly devices that produce anelectrical spark. Sparks are potent RF generators-before vacuum tubes, theywere the heart of radio transmitters-that splatter energy over a widefrequency spectrum. Any wiring connected to the spark source not onlyconducts the RF but also acts as a transmitting antenna to radiate it.Common sparking sources include electric welders, brush-type motors, relaysand switches of all kinds. Less obvious sources include arcing or coronadischarge in power line insulators (common in seashore areas or under humidconditions), malfunctioning fluorescent or neon lighting and automobilespark plugs. Lightning is the ultimate spark and a well known producer ofmomentary interference to virtually anything electronic.

Other unintentional RF generators are devices that abruptly interruptcurrent flow using some form of electronic switching. The most commonexamples are light dimmers, fluorescent lights, TV or computer CRT displaysand any piece of equipment using a switching power supply or "clock"oscillator (computers and other digital devices). The RFI source may be inthe same room as your system or, worse yet, it may be a part of your system.

RFI symptomsThe tolerance of equipment to RFI depends largely on how well it isdesigned. Generally, symptoms will appear when sufficient RF energy reachesan active device-IC, transistor, tube-inside the equipment. The energy canarrive in two ways: radiation or conduction. As it travels through the air,internal equipment wiring can act as a receiving antenna and deliver RFvoltages directly to an active device. This is most common in equipmentwith plastic or wood enclosures that have no RF shielding ability. Becauseany wire can become a receiving antenna, RF energy can also be conductedinto the equipment's active devices via any wire leaving or entering theequipment. Interference can also arrive via any wire coming into thebuilding. Because power, telephone, CATV and even driveway intercom,landscape lighting, or outdoor loudspeaker lines also behave as outdoorantennas, they are often teeming with AM radio signals and otherinterference. The most troublesome sources, however, are frequently insidethe building where the interference is distributed via the power wiring. Athigh frequencies, a building's power wiring behaves like a system ofmisterminated transmission lines gone berserk, reflecting RF energy backand forth throughout the power wiring until it is eventually absorbed orradiated. The RF does not just follow the green ground wire back to theearth ground rod and magically disappear.

RF power line noise is coupled through equipment power supplies into systemground conductors. Therefore, significant noise voltage will inevitablyexist between the chassis grounds of any two devices in AC-powered systems,whether safety grounded or not. This is the dominant noise source in mostsystems, not noise picked up by cables as is so widely believed. When thisnoise flows in the shield of unbalanced signal cables, the voltage dropdirectly adds to the signal as shown in Figure 1.

Unbalanced interfaces generally use single-conductor shielded cable andtwo-contact connectors, such as the RCA or 1/4 inch phone for audio and theRCA or BNC for video signals. Remember that RS-232 data connections arealso unbalanced. Sadly, most commercial equipment has never been tested forsusceptibility to RF interference, whether arriving through the air orcoupled to its inputs, outputs or such other outside world ports as itspower cord. Of course, even well-designed equipment will misbehave ifconfronted with extreme levels of RF interference.

In audio systems, RFI symptoms range from actual demodulation of radio orCB (heard as music or voices) or TV signals (heard as buzz) to variousnoises or subtle distortions often described as a "veiled" or "grainy"quality in the audio. In video systems, symptoms from intentionaltransmitters usually cause herringbone patterns of some sort, andpower-line related-sources usually cause bands of sparkles that slowly movevertically in the picture. In data connections, RFI generally causesotherwise unexplained behavior or crashes.

Stopping itThere are two basic strategies to control RFI. The first prevents it fromcoupling in the first place by using filters or arc snubbers at the source,relocating equipment or rerouting cables, using signal path groundisolators or adding shielding or ferrite chokes to cables. The secondfilters out the RF, when possible, after it is coupled but before itreaches a sensitive active device in the equipment. The followingrecommendations can help prevent or cure most RFI problems.

Locate and treat the offending source. This applies primarily tounintentional power-line-related sources. Because these sources tend togenerate both conducted and radiated wideband RFI, a portable AM radiotuned to a quiet frequency can be useful as a "sniffer" to locate anoffending fluorescent light or dimmer, for example. Then, the offender canbe replaced, repaired or a power-line RF filter installed.

Keep cables as short as possible, and pay attention to routing. A longcable not only increases power line common-impedance coupling (forunbalanced cables), but it also makes the cable a better antenna. Routingcables close to such ground planes as metal racks or concrete floors willreduce antenna effects. Never coil excess cable length.

Use cables with heavy gauge shields. Cables with foil and drain wireshields have much higher common-impedance coupling than those with braidedcopper shields, increasing power line noise coupling. Multiple shieldsoffer no improvement unless they are connected at both ends.

Maintain good connections. Connectors left undisturbed for long periods candevelop high-contact resistance or become metal oxide detectors for RF. Humor other interference that changes when the connector is wiggled indicatesa poor contact. Use a good commercial contact fluid and/or gold-platedconnectors.

Do not add unnecessary grounds. It will generally increase circulatingground noise rather than reduce it. Attempting to short out RFI with heavyground wires is generally ineffective. At RF, a wire's impedance isproportional to its length but nearly unaffected by its gauge. For example,8 feet (2.4 m) of AWG #10 wire has an impedance of 22 V at 1 MHz (AMbroadcast band). Using AWG #0000 wire (about 1/2 inch or 13 mm diameter)reduces it to only 18 V. Of course, never disconnect a safety ground orlightning protection ground to solve a problem-it is both illegal anddangerous.

Use ground isolators in problem signal paths. Ground isolators, whethertransformer or optical types, couple signals while completely breakingelectrical connections, which stops common-impedance coupling. Commercialisolators are available for audio, video and CATV signals. Because mosttypes have limited bandwidth, they offer inherent RFI suppression. Bewarethat poor-quality units can often degrade signal quality.

Install RFI filters in the signal path. If the offending RF interference ismore than about 20 MHz, ferrite clamshells, which are easily installed overthe outside of a cable, can be effective. In most cases, they work bestwhen placed on the cable at or near the receive end. If this is inadequate,or the frequency is lower (such as AM radio), you can add an RFI filter onthe signal line. Schematics for unbalanced or balanced filters are shown inFigure 2. For mic line applications, L should be a miniature toroid toprevent possible magnetic hum pickup. If FM, TV or cell phone is the onlyinterference, a small ferrite bead may suffice for L. In any case, C shouldbe an NP0/C0G type ceramic disc with short leads. For severe AM radiointerference, C may be increased to about 1,000 pF maximum.

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