Editing Intermodulation (section)

==Passive intermodulation (PIM)==<!-- This section is linked from [[PIM]]. Hence, do not change the title w/o fixing the link from the disambiguation page -->
{{Main article|Rusty bolt effect}}
As explained in [[intermodulation#Causes of intermodulation|a previous section]], intermodulation can only occur in non-linear systems. Non-linear systems are generally composed of ''active'' components, meaning that the components must be biased with an external power source which is not the input signal (i.e. the active components must be "turned on").

Passive intermodulation (PIM), however, occurs in passive devices (which may include cables, antennas etc.) that are subjected to two or more high power tones.<ref name="Lui_1990"/> The PIM product is the result of the two (or more) high power tones mixing at device nonlinearities such as junctions of dissimilar metals or metal-oxide junctions, such as loose corroded connectors. The higher the signal amplitudes, the more pronounced the effect of the nonlinearities, and the more prominent the intermodulation that occurs — even though upon initial inspection, the system would appear to be linear and unable to generate intermodulation.

The requirement for "two or more high power tones" need not be discrete tones. Passive intermodulation can also occur between different frequencies (i.e. different "tones") within a single broadband carrier. These PIMs would show up as [[sideband]]s in a telecommunication signal, which interfere with adjacent channels and impede reception.

Passive intermodulations are a major concern in modern communication systems in cases when a single antenna is used for both high power transmission signals as well as low power receive signals (or when a transmit antenna is in close proximity to a receive antenna). Although the power in the passive intermodulation signal is typically many orders of magnitude lower than the power of the transmit signal, the power in the passive intermodulation signal is often times on the same order of magnitude (and possibly higher) than the power of the receive signal. Therefore, if a passive intermodulation finds its way to receive path, it cannot be filtered or separated from the receive signal. The receive signal would therefore be clobbered by the passive intermodulation signal.<ref name="Eron_2014"/>

=== Sources of passive intermodulation ===
Ferromagnetic materials are the most common materials to avoid and include ferrites, nickel, (including nickel plating) and steels (including some stainless steels). These materials exhibit [[magnetic hysteresis|hysteresis]] when exposed to reversing magnetic fields, resulting in PIM generation.

Passive intermodulation can also be generated in components with manufacturing or workmanship defects, such as cold or cracked solder joints or poorly made mechanical contacts. If these defects are exposed to high radio frequency currents, passive intermodulation can be generated. As a result, radio frequency equipment manufacturers perform factory PIM tests on components, to eliminate passive intermodulation caused by these design and manufacturing defects.

Passive intermodulation can also be inherent in the design of a high power radio frequency component where radio frequency current is forced to narrow channels or restricted.

In the field, passive intermodulation can be caused by components that were damaged in transit to the cell site, installation workmanship issues and by external passive intermodulation sources. Some of these include:
 
* Contaminated surfaces or contacts due to dirt, dust, moisture or oxidation.
* Loose mechanical junctions due to inadequate torque, poor alignment or poorly prepared contact surfaces.
* Loose mechanical junctions caused during transportation, shock or vibration. 
* Metal flakes or shavings inside radio frequency connections.
* Inconsistent metal-to-metal contact between radio frequency connector surfaces caused by any of the following:
** Trapped dielectric materials (adhesives, foam, etc.), cracks or distortions at the end of the outer conductor of coaxial cables, often caused by overtightening the back nut during installation, solid inner conductors distorted in the preparation process, hollow inner conductors excessively enlarged or made oval during the preparation process.
* Passive intermodulation can also occur in connectors, or when conductors made of two [[galvanization|galvanically]] unmatched metals come in contact with each other.   
* Nearby metallic objects in the direct beam and side lobes of the transmit antenna including rusty bolts, roof flashing, vent pipes, guy wires, etc.

=== Passive intermodulation testing ===
{{see also|Two-tone testing}}
IEC&nbsp;62037 is the international standard for passive intermodulation testing and gives specific details as to passive intermodulation measurement setups. The standard specifies the use of two +43&nbsp;dBm (20&nbsp;W) tones for the test signals for passive intermodulation testing. This power level has been used by radio frequency equipment manufacturers for more than a decade to establish PASS&nbsp;/ FAIL specifications for radio frequency components.