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DEALING WITH RFI

RFI is an abbreviation for Radio Frequency Interference. Radio frequencies are broadly defined as electromagnetic waves ranging in frequency from the 1/01/2001 7:00 AM Eastern

RFI is an abbreviation for Radio Frequency Interference. Radio frequencies are broadly defined as electromagnetic waves ranging in frequency from the top end of the audio spectrum to the near infrared. We live in an increasingly “wireless” world. This makes RFI in sound and video systems difficult to avoid, especially in crowded cities. Interference, by definition, is something which originates from a source external to our system and produces undesired artifacts when it enters the signal path.

Electromagnetic waves, such as radio and TV signals, travel through space (or air) at the speed of light. Wavelength is the physical distance such a signal travels during one of its cycles. As frequency goes up, wavelength gets shorter. For example, a 1MHz AM radio signal has a wavelength of about 1,000 feet (300 meters), while a 100MHz FM radio signal has a wavelength of about 10 feet (3 meters) and a 12 GHz satellite TV signal has a wavelength of about an inch (0.025 meter). Any wire in our audio or video (or computer) system can accidentally become a receiving antenna if its length just happens to be, for example, half the wavelength of a strong local FM station.

SNEAKING IN

Equipment's susceptibility to RFI varies widely. Some equipment is nearly “bulletproof,” and some is legendary for its ability to receive local radio or CB transmissions at just the wrong time. Although it's not the only way RF can cause interference in the signal path, the “crystal set” explanation is the easiest to understand. All semiconductors, such as transistors and integrated circuits, will behave as diodes. Diodes are widely used to rectify RF signals, thereby recovering the rapid variations in their strength or “modulation.” AM radio is so named because the audio signal is an amplitude modulation of the RF carrier. So any device containing transistors or integrated circuits can become an unintentional radio receiver if a sufficient amount of RF manages to reach an internal semiconductor. When that semiconductor is in the signal path, we'll hear or see the effect as the demodulated RF is added to our audio or video signal.

The RF energy can arrive by either radiation or conduction. As RF radiates through the air, external or internal equipment wiring can act as a receiving antenna and deliver RF voltages directly to an active device. Metal enclosures or RF-shielded plastic or wood enclosures will generally keep RF from reaching internal wiring. But external cables (signal or power) can act as antennas, conducting energy into the equipment. Well-designed audio equipment will have low-pass filters at all inputs (and sometimes at outputs, too) to prevent frequencies over 100 kHz from entering. Remember that RF interference can be brought into a building by power, telephone, CATV and even outdoor speaker lines because they act as huge outdoor antennas.

THE ENEMY WITHIN

Sometimes the most troublesome sources of RF interference are actually inside the building and the interference is conducted (distributed within the building) by the power wiring. At high frequencies, a building's power wiring behaves like a system of poorly-terminated transmission lines gone berserk, reflecting RF energy back and forth and coupling it from one branch circuit to another until it is eventually absorbed or radiated.

Most often these “under-your-own-roof” sources are power line operated devices, which internally produce small sparks. Sparks are very potent RF generators that splatter energy over a wide frequency spectrum. In the pre-vacuum-tube era, they were actually the RF energy source for radio transmitters! 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 (shavers, blenders, etc.), relays (refrigerators, heating and air-conditioning systems, etc.), malfunctioning fluorescent or neon lights and switches of all kinds. A related source (but outside the building) is arcing or corona discharge in power line insulators, which is common in seashore areas or under humid conditions. Of course, lightning is the ultimate spark and a well-known producer of AM radio interference. Other devices, although they don't spark, generate RFI by causing very rapid changes in current flow in the power line.

The best known of these is the inexpensive light dimmer. As shown in Figure 1, this device uses a technique called “phase control” to hold off and then rapidly turn on current (within microseconds) to an incandescent lamp every half-cycle. It is the steepness of the turn-on current which generates the RFI in bursts 120 times per second. The interference is heard as a buzz and will be worse when the dimmer is set for half brightness. Some dimmers contain small inductors and/or capacitors to suppress some of the RF, but it is often not very effective.

SUSCEPTIBILITY AND CURES

In general, unbalanced audio (RCA) and video (RCA or BNC) interconnections are far more vulnerable to RFI than balanced systems. RF power-line noise is coupled through equipment power supplies into the ground of each piece of equipment and ultimately into the signal as the current flows in the cable's shield. The audio-frequency portion of conducted power-line noise will be heard as a hum or buzz, but there may be enough RF energy on the power line to be demodulated as additional noise. Radiated RFI symptoms can range from hearing music or voices from AM or CB radio or a buzz from broadcast TV signals, to various noises or subtle distortions. In video systems, low-frequency (the equivalent of audio hum) is seen as a light or dark “bar” which moves slowly upward if the video is NTSC. Any power-line-related RFI source will exhibit this same upward movement in standard NTSC video. Other RFI can cause “herringbone” patterns of various sorts.

There are two basic strategies to control RFI. The first is to stop the offending source from generating it in the first place. This usually takes the form of putting filters or arc snubbers at the source, shutting off or relocating equipment, replacing faulty fluorescent lamps, repairing loose connections, etc. The second strategy is to prevent pickup or coupling into the audio or video system. This usually involves repositioning or shortening cables, using signal path ground isolators, adding shielding or ferrite RF chokes to cables, or installing low-pass filters at equipment inputs or outputs. Some guidelines for solving RFI problems:

Locate and treat the offending source. This applies primarily to unintentional power-line-related sources. Since these sources tend to generate both conducted and radiated wideband RFI, a portable AM radio tuned to a quiet frequency can be very 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 can be installed.

Keep cables short. Make them as short as possible and pay attention to routing. A long cable not only increases power line common-impedance coupling (for unbalanced cables) but also makes the cable a better antenna. Routing cables close to “ground planes” such 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 little improvement unless they're 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 which changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold plated connectors.

Don't 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 of #10 wire has an impedance of 22 z at 1 MHz (AM broadcast band). Using #0000 wire (about 1/2-inch diameter) only reduces it to 18z. Of course, you should never disconnect a safety ground or lightning protection ground to solve a problem — it is both illegal and very dangerous. The RF does not just follow the green “ground” wire back to the earth ground rod and magically disappear!

Use ground isolators in problem signal paths. Ground isolators, whether transformer or optical types, couple signals while completely breaking electrical connections. This 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 degrade signal quality.

Install RFI filters in the signal path. If the offending RFI is over about 20 MHz, ferrite “clamshells,” which are easily installed over the outside of a cable, can be very effective. In most cases, they work best when placed on the cable at the receiving end. If the offending frequency is lower (such as AM radio), you can add an RFI filter on the signal line. Schematics for unbalanced and balanced filters are shown in Figure 2. For microphone line applications, L should be a miniature toroid to prevent hum pickup from nearby magnetic fields. 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 very short leads. For severe AM radio interference, C may have to be increased up to about 1,000 pF maximum.



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