Intro4u2u

Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source. [1] The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.

EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.

EMI/RFI types

EMI or RFI may be broadly categorized into two types; narrowband and broadband.

Narrowband interference usually arises from intentional transmissions such as radio and TV stations, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency emitters. These include electric power transmission lines, electric motors, thermostats, bug zappers, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated, or varied, by the creating device in some way. Included in this category are computers and other digital equipment as well as televisions. The rich harmonic content of these devices means that they can interfere over a very broad spectrum. Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver chain. [2][3]
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[edit] Power line noise

Virtually all power-line noise, originating from utility company equipment, is caused by a spark or arcing across some power-line related hardware. A breakdown and ionization of air occurs, and current flows between two conductors in a gap. The gap may be caused by broken or loose hardware such as a cracked insulator. Typical culprits include insufficient and inadequate hardware spacing such as a gap between a ground wire and a staple. Once an ionized path is established in the gap, current flows at all parts of the cycle where the voltage is higher than the breakdown voltage of the gap. This typically occurs only at the positive and negative voltage peaks — the times of highest instantaneous voltage throughout the cycle.

As an example for a 60Hz system (i.e.power-lines carrying 60 Hz AC, such as in the US), the voltage on them passes through two peaks each cycle (one positive and one negative) and pass through zero twice each cycle. This gives 120 peaks and 120 zero crossings in each second (50Hz: 100 peaks and crossings correspondingly). Power-line noise follows this pattern, generally occurring in bursts at a rate of 120 bursts per second. This gives power-line noise a characteristic sound that is often described as a harsh and raspy hum or buzz. Because the peaks occur twice per cycle, true power-line noise has a strong 120-Hz modulation on the signal (50Hz system: 100Hz).[5]

[edit] Mitigation

Main article: Electromagnetic compatibility

On integrated circuits, the most important means of reducing EMI are: the use of bypass or “decoupling” capacitors on each active device (connected across the power supply, as close to the device as possible), risetime control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like.

The efficiency of the radiation depends on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.

At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.

Switching power supplies can be a source of EMI, but have become less of a problem as design techniques have improved.

Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).

[edit] Susceptibilities of different radio technologies

Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.

The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.

[edit] Interference to consumer devices

Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn’t exist in earlier, simpler decades. In the US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.[6]

Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest controls (bug zappers), heating pads, and touch controlled lamps.[7]

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.