What is Passive Intermodulation?
Passive Intermodulation (PIM) creates a challenge for those developing components and maintaining networks in today’s mobile communication systems. Passive intermodulation arises due to the signal degradation effects of passive elements within the transmission system. An incorrectly torqued connection, poorly maintained equipment, minor damage to an antenna or even a rusty bolt can result in two or more frequencies clashing. PIM can affect any RF-based communications system but as mobile transmissions rates rise to 100MB/s and beyond it is now accepted that passive intermodulation is an industry benchmark in determining the health of a cell site.
PIM vs Return loss
PIM vs Time
What Causes Passive Intermodulation?
Ferromagnetic materials, when in the current path, exhibit a non-linear voltage to current ratio. This non-linear effect is accentuated at higher power levels because of increased current density. Looking at Ohm’s law from the perspective of “Power” helps clarify the fact that the squaring effect of current results in a higher magnetic flux, which makes metals with high bulk resistivity, such as, iron, steel and nickel exhibit a magnet like memory effect. This effect is better known as magnetic hysteresis. Metals that exhibit this asymmetrical magnetic flux are often the main contributor of PIM energy.
Poor metal to metal contact junctions can create additional nonlinearities resulting in PIM. Such nonlinearities can come from under torqued male to female DIN 7/16 mates, as well as irregular contact surfaces such as poorly manufactured connectors and surface metal oxidation. Oxidation (corrosion) creates tiny air gaps, which promote voltage potential barriers, which in turn result in a nonlinear voltage to current ratio, sometimes referred to as the diode effect. High power passive intermodulation measurements are therefore essential in order to reveal network problems.