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UNDERSTANDING and controlling RF interference

Feb 1, 1999 12:00 PM, Bill Whitlock


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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 signal path and produces undesired artifacts in the signal. A radio frequency, or RF, is loosely defined as being in that portion of the electromagnetic spectrum above audio (about 20 kHz) but below infrared (about 30 THz). Electromagnetic interference (EMI), is a broader term having the same basic meaning but without frequency limitations. Electromagnetic compatibility (EMC), is a term coming into more widespread use regarding issues of equipment electromagnetic emissions and susceptibility, especially because regulations 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 186,000 miles/sec. Because wavelength is the physical distance such a signal travels 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), and for a 12 GHz DSS TV signal, only about an inch (25.4 mm). Any wire can accidentally become a good antenna if its length happens to be, say, the wavelength of a strong local FM station.

Sources of RF interference fall into two broad categories-intentional and unintentional. Intentional sources include AM, shortwave, FM, and TV broadcast 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 and industrial RF devices.

Unintentional RF sources are most commonly devices that produce an electrical spark. Sparks are potent RF generators-before vacuum tubes, they were the heart of radio transmitters-that splatter energy over a wide frequency spectrum. Any wiring connected to the spark source not only conducts the RF but also acts as a transmitting antenna to radiate it. Common sparking sources include electric welders, brush-type motors, relays and switches of all kinds. Less obvious sources include arcing or corona discharge in power line insulators (common in seashore areas or under humid conditions), malfunctioning fluorescent or neon lighting and automobile spark plugs. Lightning is the ultimate spark and a well known producer of momentary interference to virtually anything electronic.

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

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

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

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

In audio systems, RFI symptoms range from actual demodulation of radio or CB (heard as music or voices) or TV signals (heard as buzz) to various noises or subtle distortions often described as a "veiled" or "grainy" quality in the audio. In video systems, symptoms from intentional transmitters usually cause herringbone patterns of some sort, and power-line related-sources usually cause bands of sparkles that slowly move vertically in the picture. In data connections, RFI generally causes otherwise unexplained behavior or crashes.

Stopping it There are two basic strategies to control RFI. The first prevents it from coupling in the first place by using filters or arc snubbers at the source, relocating equipment or rerouting cables, using signal path ground isolators or adding shielding or ferrite chokes to cables. The second filters out the RF, when possible, after it is coupled but before it reaches a sensitive active device in the equipment. The following recommendations can help prevent or cure most RFI problems.

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

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

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

Maintain good connections. Connectors left undisturbed for long periods can develop high-contact resistance or become metal oxide detectors for RF. Hum or other interference that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors.

Do not add unnecessary grounds. It will generally increase circulating ground noise rather than reduce it. Attempting to short out RFI with heavy ground wires is generally ineffective. At RF, a wire's impedance is proportional 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 (AM broadcast 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 or lightning protection ground to solve a problem-it is both illegal and dangerous.

Use ground isolators in problem signal paths. Ground isolators, whether transformer or optical types, couple signals while completely breaking electrical connections, which stops common-impedance coupling. Commercial isolators are available for audio, video and CATV signals. Because most types have limited bandwidth, they offer inherent RFI suppression. Beware that poor-quality units can often degrade signal quality.

Install RFI filters in the signal path. If the offending RF interference is more than about 20 MHz, ferrite clamshells, which are easily installed over the outside of a cable, can be effective. In most cases, they work best when 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 on the signal line. Schematics for unbalanced or balanced filters are shown in Figure 2. For mic line applications, L should be a miniature toroid to prevent possible magnetic hum pickup. If FM, TV or cell phone is the only interference, a small ferrite bead may suffice for L. In any case, C should be an NP0/C0G type ceramic disc with short leads. For severe AM radio interference, C may be increased to about 1,000 pF maximum.



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