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Types of Interference

Interference is at least an occasional problem with most types of radio equipment, including wireless microphones. The effects of interference range from being a minor annoyance to making the wireless system completely unusable. Serious interference is not as common as is sometimes assumed, especially when some simple precautions are taken. However, when it does occur, it can be highly frustrating.

There are three basic types of interference: radio frequency interference (RFI), electrical interference and intermodulation. RFI is caused by radio and TV transmitters, communications equipment, cable television systems and other types of equipment that generate radio frequency energy as part of their operation. Electrical interference is caused by computers and digital equipment, heavy electrical equipment, lighting systems, faulty electrical devices, etc. Intermodulation is a type of interference caused by the internal combination of strong radio signals in wireless receivers.

A basic understanding of the symptoms and causes of the three types of interference is key to dealing effectively with any problems that might arise. One reason interference problems can be frustrating is that it is often difficult to know where to start. Sometimes it seems that everything affects the problem, but nothing solves it. Simply knowing which type of interference is present helps avoid wasting time on unproductive approaches and greatly simplifies the process of finding the real source of the problem.

Radio Frequency Interference
This type of interference is caused by radio frequency (RF) signals on or near the frequency of the affected wireless receiver. The interfering signals might have been transmitted intentionally, or unintentionally as the result of some defect or undesired characteristic of the source. It is not necessary that the interfering signal be exactly on the same frequency as the wireless system to be troublesome. Strong RF signals that are near the wireless frequency can affect the operation of the wireless receiver, causing audio and reception problems.

It is important to realize that what might be interference to a wireless user could be entertainment or essential communications to others. All wireless microphone systems operate on frequency ranges that are shared with television broadcasting or various types of radio communications. Because of this, wireless microphone users must remember that they do not have exclusive rights to the frequencies they are utilizing.

Many wireless microphone systems operate in the frequency bands used for TV broadcasting. TV transmitters are generally quite powerful and can interfere with wireless receivers at very considerable distances. Wireless systems using operating frequencies on or near any frequencies used for TV broadcasting can expect moderate to severe interference even in locations beyond the distance where good TV reception is possible.

Other wireless microphone transmitters are a frequent source of RFI problems. In some instances, a wireless system includes both a body-pack and a handheld transmitter. Unless considerable care is taken, it is not unusual to accidentally have both transmitters turned on at the same time. In this situation, the receiver will produce an extremely loud audio tone and the system will be completely unusable.

Similarly, if two wireless systems happen to use the same frequency, only one transmitter can be used at a time. Having two systems on the same frequency occurs more often than might be expected, especially in situations where wireless users come and go with their own systems. Synthesized equipment can also be troublesome in this regard, since a given model usually shares a particular small set of frequencies. If the frequency of a transmitter is changed, the chances are relatively high that there will be a conflict with another system of the same type.

The existence of other nearby wireless systems is a possibility that must be considered. Wireless systems can interfere with each other at distances of 2000 feet (600 meters) or more. It is not highly unusual to have interference from another wireless system in a different area of a building or facility, at a house of worship down the street, or at a club around the corner. Television and radio news crews also use wireless microphones extensively, so temporary interference may occur near the scene of newsworthy events.

Another source of interference is harmonics from FM radio stations and communications transmitters. Powerful FM station transmitters usually have a small amount of output at twice their operating frequency (the "second harmonic"), and this can be a source of interference to wireless systems operating in the VHF TV band. Unless the FM transmitter power is high and the transmitter nearby, interference from this source is rare. Still, it is best to avoid wireless frequencies that are near harmonics of local FM radio stations.

Communications transmitters also have harmonic outputs that can potentially interfere with wireless systems. Generally, in the rare instances when interference from communications transmitters is experienced, it is a result of intermodulation or defective equipment. Misadjusted or poorly maintained communications transmitters can have excessive harmonic or spurious output. Interference from this source is likely only when the communications transmitter is nearby. Antennas for communications transmitters are often located on top of buildings. If the transmitter is powerful and the antenna nearby, wireless interference becomes a possibility. The same can be true when a vehicle with a powerful two-way transmitter drives by a location where wireless systems are in use. In this situation, spurious signals from a misadjusted or defective transmitter can cause interference. In some cases, the strong transmitter signal can overload the wireless receiver and cause interference due to intermodulation.

Spurious outputs from various kinds of RF equipment is an occasional source of interference. Cable television systems, communications receivers, cordless phones, garage door openers and even home TV and FM receivers can cause interference in rare instances. When the obvious sources of interference have been eliminated, it is always wise to suspect any electronic device capable of transmitting or receiving an RF signal, or which uses RF in its operation.

AM radio stations are sometimes a source of interference. However, in the vast majority of cases, this problem is not due to direct interference by the AM station. Wireless systems operate at frequencies far above that of AM transmitters and it is extremely unlikely that a transmitter harmonic or spurious output will affect a wireless receiver. AM radio transmitters sometimes interfere with many types of audio equipment, including mixers, power amplifiers, processors and other non-RF devices.

Normally, interference by AM radio stations is only a problem when the transmitter is close by. The high RF energy levels present directly affect sensitive audio circuitry, usually introducing the AM program into the audio system. In general, the interference is picked up by audio cables or comes in through the power lines, and may still be present even when the wireless receiver is turned off. The solutions to this problem involve filtering and shielding of audio and power cables - quite different from the techniques required to eliminate direct RF interference.

Solving RFI problems almost always involves either eliminating the source of the interfering signal or changing the frequency of the wireless system. Many interference problems of this type can be avoided entirely by simply selecting frequencies other than those already in use by local TV stations and other nearby wireless systems. As a practical matter, convincing someone else to change their frequency or forcing them to repair defective transmitters is not often feasible, so changing the frequency of the wireless system may be the only realistic option.

Electrical Interference
Electrical interference does not benefit anyone and is almost never intentional. With few exceptions, the equipment causing electrical interference problems was not intended to be a source of RF energy. Often, the interference is the result of a defect, failure or maintenance problem that can be readily corrected. Some types of electronic equipment simply tend to generate interference in the normal course of operation.

Generally, manufacturers are required to design and manufacture their products so that they do not cause harmful interference. In many cases, this is a legal requirement because the government has imposed regulations that place strict limits on the unintentional generation of RF interference. Manufacturers do their best to comply with these regulations because there can be severe penalties for the sale of devices not meeting the standards.

However, certain types of equipment, such as lighting equipment and industrial electrical gear, are not covered by these regulations. In addition, electronic equipment which does conform to government requirements can still interfere with wireless systems if certain precautions are not taken. Age, wear and inadequate maintenance can also take their toll on equipment and cause it to eventually become a source of interference. In some instances, equipment was manufactured before the government regulations were in place, and there may have been no effort made to reduce interference.

There are three basic types of electrical interference: noise from electrical equipment, noise generated by electronic devices such as computers, and interference from natural sources such as lightning. Perhaps surprisingly, all forms of electrical interference are relatively rare and account for only a small percentage of all wireless interference problems. To a large extent this is because reduction of harmful interference has become a priority of both government and industry in recent years. Improvements in the design of wireless equipment have also contributed.

In those cases where electrical interference does become a problem, the most commonly encountered type is RF noise generated by computers and digital equipment. Digital interference can usually be identified by its characteristic raspy whining or buzzing sound. The loudness and character of the sound will usually change as the computer performs different types of operations, or when the control settings on a digital device are changed. Sometimes, a particular device will cause interference when actively processing, but not when idle.

Digital delays, digital effects processors and other equipment containing DSPs (digital signal processors) are more likely to cause wireless interference than computers. This is because such devices are often mounted in the same cabinet or equipment rack as the wireless receivers. Generally, this type of interference is only a problem when the receiver is close to the digital device. For example, a wireless receiver that is experiencing severe interference when mounted directly over a digital device might be trouble-free when moved as little as 12 inches (30 cm) away from the digital unit.

A digital device can generate interference that travels back through the AC power wiring or audio cables, eventually reaching the wireless receiver. At higher frequencies, the interference can sometimes travel on the outside of cables to the receiver. Physically separating both the devices and their power and audio cables will usually minimize the problem. In a few instances, using a separate power source for the digital device or installing a line filter might prove necessary.

Manufacturers of computers and digital equipment are required to control the amount of interference they generate and to obtain government approval of each model or type that they sell. However, under the rules, digital devices are still allowed to leak a very small amount of interference, sometimes enough to disturb a sensitive wireless receiver mounted next to the device. Loose or missing case screws can also greatly increase the amount of interference leaking from an approved digital device. In addition, older equipment might not have been designed to minimize interference because in the past the regulations sometimes were not taken very seriously.

Natural sources, mostly lightning, account for only a very small percentage of electrical interference problems. Improved designs have made receivers less vulnerable to RF noise bursts from lightning. However, noise bursts on the incoming AC power line can still cause problems with wireless receivers and other sensitive audio gear, especially when lightning strikes power lines. In areas where lightning is common, surge protectors with high-performance AC line filters can be a wise investment.

Electrical machinery and lighting systems are sources of electrical interference. In most cases, the interference results from sparking, arcing and electrical discharges. In a few instances, the interference is caused by electrical control devices such as motor speed controls, temperature controllers and lighting dimmers. High-voltage equipment, especially neon signs, is also a known source of interference.

Sparking is common in electric motors with brushes, particularly older motors, certain kinds of large industrial motors and those with speed controls. Many home appliances and small tools such as vacuum cleaners, blenders and hand drills also use motors with brushes. Filters can be added to this type of motor to greatly reduce the interference caused by sparking. However, this was not common practice in the past, and filters may be left out of present-day motors to reduce cost. Older, worn, dirty and poorly-maintained motors are more likely to cause interference than new units.

Arcing is not only a source of interference, it often also presents a serious fire and safety risk. Aside from arc welding, it is usually caused by defective wiring and equipment, loose connections and failed insulators. Although high-voltage circuits are more prone to arcing, it often occurs in lower-voltage circuits when loose wires rub against each other or ground. Loose electrical contacts can also arc when disturbed by vibration or shock. Arcing frequently tends to be intermittent, since a continuous arc soon burns away the conductors, or triggers a shutdown or complete failure. Intermittent occurrences make finding the exact source of the problem more difficult.

Because of the very high voltages present, neon signs are particularly prone to arcing. This generally occurs near the tube supports or at connection points. Often, wires in neon systems are merely twisted together rather than being soldered. This does not usually greatly affect the operation of the tube, but can create serious interference problems for wireless systems. Tubes can induce high voltages into metal objects close to them, causing secondary arcs in mounting frames and supports.

The high voltages in neon systems can also cause leakage discharges, known as corona, that create electrical noise. Other devices that use high voltages, such as television sets and x-ray machines, are also prone to corona and can cause wireless interference. The discharge in the neon tubes themselves generates surprisingly little interference under normal circumstances. However, if the tubes are dimmed by lowering the applied voltage, there is a point where they will generate huge amounts of radio interference. Dimming of neon lights should be avoided if wireless microphones are being used.

Electrical controllers, especially lamp dimmers and motor speed controllers, can be significant interference sources. Older theatrical lighting controllers are particularly prone to causing interference, especially those used with neon systems. Where dimmed fluorescent lights are used, both the dimmers and the lamps themselves are potential interference sources. Modern dimmers and controllers are much less likely to cause interference problems, but it is still good practice to keep the wireless receiving antennas away from such devices.

Interference from auto ignition systems was once a serious problem with wireless, but this is no longer true. Improvements in the design of wireless receivers and the use of resistive ignition cable and resistor spark plugs in cars have almost eliminated this problem. Only in very rare instances will an auto with defective spark plug wiring cause noticeable ignition interference. However, the problem can still occur with antique cars, boat engines, industrial gas engines, lawn mowers and other machinery without proper ignition noise suppression.

Intermodulation or "intermod" is a type of interference sometimes encountered in wireless microphone systems. Intermodulation differs from other forms of interference in that it is created in the wireless system itself, not directly by some external source. Other types of interference are caused by other transmitters on the wireless operating frequency, TV station carriers, the harmonic output of transmitters at lower frequencies, spurious emissions from various kinds of electronic equipment, and similar external sources. In each case, the interfering signal is on a frequency very near that of the wireless system.

Interference due to intermodulation is caused by strong signals which are generally not near that of the wireless frequency. Instead, these strong signals overload some circuit in the wireless receiver, causing the circuit to internally generate harmonics of the strong signals. These harmonics then combine, or mix, in the receiver to create a new frequency that was not present at the receiver input. The newly-created frequency, called an "intermodulation product," then interferes with the wireless system in much the same way as other sources of interference.

For example, assume that there are wireless systems operating on frequencies of 200.050 and 210.450 MHz at a particular location. Assume further that there is a TV channel 12 in the area; its picture carrier will be on 205.250 MHz. Because TV transmitters are very powerful, the wireless receiver on 200.050 MHz will be slightly overloaded by the strong picture carrier of channel 12, creating a harmonic at 410.500 MHz in the receiver RF amplifier. This harmonic then mixes with the signal from the wireless transmitter on 210.450 MHz, as follows:

410.500 MHz

The second harmonic of the channel 12 picture carrier

-210.450 MHz

Another wireless transmitter

200.050 MHz

The frequency of the first wireless system

In this situation, the wireless system on 200.050 MHz can be expected to be more or less unusable whenever the transmitter for the other wireless system is operating. The reverse is also true, since 410.500 - 200.050 = 210.450 MHz. Therefore, it is likely that one or the other of the wireless systems can be used in this location, but not both at the same time. If there were no TV channel 12 in the area, both wireless systems should be able to operate simultaneously without difficulty.

Signals from nearby wireless transmitters are often stronger than the signals from local TV stations and frequently cause their own set of intermodulation problems. The third and fourth harmonics of strong input signals can also mix in various combinations to create additional types of intermodulation products, as can combinations of three input signals. Collectively, these various troublesome combinations are commonly referred to as "intermod." Reliable and trouble-free wireless operation depends upon avoiding frequencies that are vulnerable to intermod.

It is important to note that in the example above, the 410.500 MHz harmonic of the TV carrier was not actually present at the receiver input; it was generated within the receiver itself. However, the point at which a receiver will overload and generate intermodulation is almost entirely dependent upon its circuit design. Fewer intermodulation products will be generated in receivers with overload-resistant circuitry, and the ones that are generated will be lower in level and less harmful.

The quality of the RF filtering in the receiver also has a major effect, since reducing the level of an interfering signal by even a small amount will lower the level of any intermodulation products generated by a much greater amount. Thus, relatively small improvements in a receiver's overload capabilities and filtering can greatly enhance its intermodulation performance. This is one of the reasons that higher-quality equipment is less likely to have interference problems than cheaply-made equipment.

RF preamplifiers or active splitters can also be sources of intermodulation problems. Once an intermodulation product has been generated by one of these devices, the damage is done and no receiver will be able to reject the interference. For this reason, it is important to keep wireless transmitters well away from the receiving antennas to avoid overloading the RF preamplifiers. This is also the reason that it is best to avoid RF preamplifiers and active splitters when possible by using high-performance antennas and low-loss coaxial cables.