RF Interference and Intermodulation FAQs

• What is Intermodulation Interference?
• What is Transmitter Harmonic Interference?
• What are Transmitter Spurious Output Interference?
• What is Receiver Desensitization Interference?
• What is Transmitter Noise Interference?
• What will LBA's IM analysis service provide me?
• Where does RF interference come from?
• What problems are caused by RF interference?
• What are the benefits of RF shielding?
• How is shielding installed?
• What RF levels will impact equipment operations?
• What are some typical manifestations of shielding problems in communications systems?

What is Intermodulation Interference?
There are three basic categories of Intermodulation (IM) interference.  They are receiver produced, transmitter produced, and "other" radiated IM.  Transmitter produced IM is the result of one or more transmitters impressing a signal in the non-linear final output stage circuitry of another transmitter, usually via antenna coupling.  The IM product frequency is then re-radiated from the transmitter's antenna.  Receiver produced IM is the result of two or more transmitter signals mixing in a receiver RF amplifier or mixer stage when operating in a non-linear range.

"Other" radiated IM is the result of transmitter signals mixing in other non-linear junctions.  These junctions are usually metallic, such as rusty bolts on a tower, dissimilar metallic junctions, or other non-linear metallic junctions in the area.  IM products can also be caused by non-linearity in the transmission system such as antenna, transmission line, or connectors.
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What is Transmitter Harmonic Interference?
Transmitter harmonic interference is due to non-linear characteristics in a transmitter.  The harmonics are always fundamental frequency multiples and the non-linear design of the final output stage of the transmitter.  If the harmonic signal falls within the passband of a nearby receiver and the signal level is of sufficient amplitude, it can degrade the performance of the receiver.
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What is Transmitter Spurious Output Interference?
Transmitter spurious output interference can be attributed to many different factors in a transmitter.  The generation of spurious frequencies could be due to non-linear characteristics in a transmitter or possibly the physical placement of components and unwanted coupling. If a spurious signal falls within the passband of a nearby receiver and the signal level is of sufficient amplitude, it can degrade the performance of the receiver.
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What is Receiver Desensitization Interference?
Receiver desensitization interference occurs when an undesired signal from a nearby "off-frequency" transmitter is sufficiently close to a receiver's operating frequency.  The signal may get through the RF selectivity of the receiver.  If this undesired signal is of sufficient amplitude, the receiver's critical voltage and current levels are altered and the performance of the receiver is degraded at its operating frequency.  The gain of the receiver is reduced, thereby reducing the performance of the receiver. A high-power transmitter can be operating several megahertz away from the receiver frequency and/or its antenna can be located several thousand feet from the receiver's antenna and still cause "desense" interference.
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What is Transmitter Noise Interference?
Transmitter noise interference occurs because a transmitter radiates energy on its operating frequency as well as frequencies above and below the assigned frequency.  The energy that is radiated above and below the assigned frequency is known as sideband noise energy and extends for several megahertz on either side of the operating frequency.  This undesired noise energy can fall within the passband of a nearby receiver even if the receiver's operating frequency is several megahertz away.  The transmitter noise appears as "on-channel" noise interference and cannot be filtered out at the receiver.  It is on the receiver's operating frequency and competes with the desired signal, which in effect, degrades the operational performance.
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What will LBA's IM analysis service provide me?
The analysis calculates all possible IM product frequencies that could potentially interfere with receivers at the communications site based on each receiver’s individual bandwidth.  It then predicts each IM signal level present at the input of each affected receiver.  For each IM frequency, the analysis considers all possible sources of IM generation in the transmitters.  The analysis takes into account the transmitter’s power output, modulation bandwidth, and conversion losses.

The analysis also takes into account the transmitter’s harmonic characteristics and output level, predicts each transmitter’s signal level and each transmitter’s noise signal level present at the input of each receiver.

It also takes into account the transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and other RF devices that are present in each system.  Additionally, the analysis considers the antenna separation space loss, horizontal and vertical gain components of the antennas as well as how they are mounted on the structure.  The gain components are derived from antenna pattern data published by each manufacturer.

Additionally, the analysis determines how much isolation, if any, is required to prevent receiver performance degradation for each IM interference signal that occurs, any harmonics that fall within a receiver’s passband, receiver desensitization and transmitter noise interference.
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Where does RF interference come from?
Radio frequency interference (RFI) can originate from a wide number of sources—from natural phenomena such as lightning and sun spots to a host of man-made generators. Typically, those trying to guard from interfering signals are concerned more with man-made sources. These sources include high power broadcast systems like AM/FM radio and TV transmitters and a multitude of other communications systems, including 2-way radio, paging, mobile telephone, and emergency and public safety communications systems. There are also unintentional emitters that are sometimes a source of RFI. Power lines, transformers, medical equipment, electro-mechanical switches and many others—produce RF energy as they perform their functions.
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What problems are caused by RF interference?

The impact of RFI depends on what you are doing. Radio frequency emissions are very carefully guarded around explosives, for instance, because it can cause detonation! In common communications systems, RFI degrades or completely disrupts signal quality, overall system performance, connectivity between two or more stations, and system carrying capacity. Simply put, RFI can make a RF based system totally non-functional, it can cause an intrusion of music, conversation or noise, or it can create problems in system performance that are extremely difficult to trace back to the RFI source.
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What are the benefits of RF shielding?
When properly designed and installed in accordance with the specific requirement at hand, facility RF shielding can effectively eliminate the impact of RFI on systems that are contained within the shield. The proper shield serves as an impermeable barrier to the incoming radiated radio frequency that would otherwise create interference. It allows for the safe and proper operation of otherwise sensitive equipment in high RF environments. In some cases, the RF is high enough to be considered a health hazard, in which case a RF shield would not only protect the equipment but also the people required to be at that location, as well.
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How is shielding installed?
The shielding materials and components (a shielding system) must be installed in accordance with the procedures that are particular to the facility being shielded, the degree of shielding effectiveness to be achieved, the special materials being employed, and the environmental conditions in which the system will be operating. In short, the installation should be performed by a competent and experienced shielding professional with the necessary tools and RF measuring equipment available to assure a thorough and long-lasting shield.
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What RF levels will impact equipment operations?
This is truly a case-by-case issue. There are no set regulations defining a single RF level that will cause RFI. Among other things, it depends on the sensitivity of the equipment under consideration, or how low an interfering signal can be in the presence of the equipment and cause problems. Also, equipment can be particularly sensitive to very low signal levels of one frequency and yet quite immune to high signal levels of another frequency—so frequency is an important factor. Some electronic system components are internally shielded and have a very high immunity to interference. But generally, most equipment has not been so engineered.

Electronic equipment is often tested to RFI sensitivity by the manufacturer, but the results are not always easily obtainable. At Lawrence Behr Associates, communications facilities are often studied for the potential for RFI conditions, and unless there are specific threshold limits that are known beforehand, we use 1 Volt/meter as a point of concern. We do realize, however, that some systems are rendered incapacitated by RFI levels of more than one hundred times smaller values.
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What are some typical manifestations of shielding problems in communications systems?
In a wireless system Electromagnetic Interference (EMI) often does not cause problems in providing quality reception but it does cause problems in providing quality service. Typically a wireless operator will raise or lower tower height, add ratios or change system configurations in an attempt to eliminate service problems. These approaches do not work when EMI strikes. As the following three instances demonstrate, well-designed shielding systems provided the cost-effective and lasting solutions.
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