Passive Intermodulation

The phenomenon of 'passive intermodulation, or PIM (Passive Intermodulation), is a well-known problem that plagues plants RF transmission, but that is coming to the forefront because it threatens to make tremendously difficult coexistence between new LTE mobile networks generation with the other communications networks.
The passive intermodulation in itself a simple phenomenon, but it is difficult to identify and measure. Let's see how you can do.
A little 'refresher on basic concepts and terminology intermodulation.
The intermodulation is a phenomenon caused by the imperfect linearity of the circuits.
Recall that if a circuit is perfectly linear, not ever distort the frequency spectrum of the signal passing through it.
In a circuit perfectly linear, a frequency component of the input would be found, maybe attenuated or amplified, even in output. Furthermore, if a signal crossed a circuit perfectly linear, the output could never happen that there are frequency components which are not present at the entrance.
Summing up: linear circuits through the 'frequencies are not born' at most 'die' because they are strongly attenuated.

Spread spectrum transmitted due to passive intermodulation due to faulty cables and connectors

Instead, when a signal passes through a nonlinear circuit, even the purest of the sinusoids at its input is transformed into an output signal with different frequency components 'born within the non-linear circuit'.
The non-linear phenomena are not necessarily harmful, indeed, sometimes are right at the base of the operation of many electronic circuits; an example for all of the mixer, which just serves to translate the frequency spectrum, in a controlled manner.
The non-linear phenomena unexpected or uncontrolled are certainly harmful, because they can 'pollute' the signal by displaying signals on frequencies unexpected, which can potentially interfere with the useful signal of the system or systems operating nearby.
The intermodulation phenomenon is typically associated to the active circuits, such as amplifiers, mixers, and so on, precisely because they are rich in non-linear components (such as diodes and transistors) and because they have been designed to exploit in a controlled manner the phenomenon of non-linearity.
Instead, the intermodulation is a much more general phenomenon, which can also occur in the most simple passive components, such as a transmission cable or a connector.
And it is precisely in transmission systems where you use cables and connectors to connect the output of a power amplifier to the radiating system composed of one or more antennas that the phenomenon of 'passive intermodulation is likely to do damage.
For example, in the case of a radio base station working perfectly, from the exit of the power amplifier part of a coaxial cable that goes up to the antennas. Usually, the cable is also decomposed into more sections linked together by connectors, to facilitate installation and maintenance. Due to some imperfection in material or an assembly is not perfect, or simply due to vibration, corrosion or aging, the set composed of cables and connectors and any filters that may begin to exhibit a behavior no longer perfectly linear.
Let's see what can happen in a mobile broadcast system on a tower affected by passive intermodulation. The signal of a channel generates a copy unconsciously distorted and shifted in frequency outside of its operating band. Therefore, it generates an interfering signal that goes to disturb the operation of another channel of the same system or even of another communication system.

Risk of interference from passive intermodulation in mobile networks

Therefore, even in the presence of active equipment in perfect working according to design specifications, it is possible that the entire cell of a mobile system stops working even as the 'passive intermodulation makes' deaf' channels that disturb a l 'another.
The phenomenon has long been known, but the risks are now significantly increasing due to the imminent launch of next-generation networks such as LTE radio that very often, to reduce the economic and environmental impact of the installation of new base stations, will reuse sites and infrastructure of existing plants.
It can therefore happen that routing the power signal LTE on the same cable that carries already from the amplifier to the antennas the signals of existing mobile radio networks, the phenomena of intermodulation caused by passive components do arise new frequencies that go under reflux in the band of operation of the systems existing and vice versa.
Unfortunately, being the passive intermodulation caused by 'last link in the signal chain, can not be filtered upstream, therefore the only solution applicable is that of identifying the phenomenon and to eliminate it by acting on the original cause, most times simply a connector or cable is not fully installed or degraded by the wear of time.