What is Passive Intermodulation?
Posted 28 May 2014 by Mark
Passive Intermodulation (PIM) is an unwanted distortion in radio frequency waves generated by the non-linear mixing of two or more frequencies inpassive devices such as antennas, connectors, coaxial cables or in-building Distributed Antenna System (DAS). This mixing produces additional unwanted signals that can create interference and degradation in the quality of a wireless communication system.
Passive Intermodulation is a significant issue within the cellular industry and extremely difficult to troubleshoot. When an Intermodulation Product, particularly third order products (IM3), falls on a frequency of interest, it can deteriorate or overtake the preferred received signal in the receiving equipment. In a digital system, this Intermodulation product could result in degradation of the signal, resulting in an increased bit-error rate (BER) and unusable data or voice communications.
Let’s look first at PIM’s potential causes and then at the steps to minimize it.
Passive Intermodulation can occur for a variety of reasons.
1. Use of dissimilar metals in RF components
When dissimilar metals are used in a product there is a natural corrosion process that occurs between the two metals.This corrosion can contribute greatly to the generation of PIM distortion.
2. Manufacturing Defects or Damage
PIM distortion can be caused by workmanship or manufacturing defects, such as poorly made mechanical contacts or solder joints. Even in cases where parts are manufactured properly, physical damage caused by environmental effects (wind vibration, moisture breakdown) can also cause an increase in PIM products.
3. Components containing materials that exhibit hysteresis
Ferromagnetic materials such as nickel or steel should be avoided since they can add to the PIM distortion due to the hysteresis effect of these metals and their nonlinear voltage to current ratio.
4. Poor contact junctions
Loose mechanical junctions due to poor mechanical alignment, incorrect torque or poorly prepared surfaces can be mixing points for PIM. It may also become an issue due to the constant wear and tear of units in transportation and everyday operation.
5. Contaminated connections
Surfaces can become contaminated by dirt, moisture, oxidation or other forms of corrosion. This contamination can exacerbate the effects of PIM.
Passive Intermodulation Prevention
First, always specify and utilize quality RF components in your equipment. New equipment should be engineered to best-in-class practices and installed per manufacturer’s recommendation to mitigate PIM. Using the right parts is important, but proper installation plays a big part as well. Always use proper torque wrenches and depth gages when installing cables and connectors.
Next, initiate a proper maintenance program that regularly audits and replaces/repairs any equipment. For example, test the PIM performance of the system every year to see if any significant changes have occurred. Significant changes in PIM performance may be an early indicator of corrosion or deterioration of equipment, and will help you replace compromised components to minimize RF signal degradation and prevent unscheduled downtime.
Finally, the best way to mitigate PIM is to try and design to avoid interference hits. Since PIM is caused by a mixing of high level signals, avoid using receive frequencies that fall on a high level mix of two of the transmitters at the same site.
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