How Passive Intermodulation Affects Network Performance
Passive intermodulation (PIM) is a big concern for wireless providers because of the negative effect it can have on network performance. In active devices, PIM occurs when signals at two or more frequencies mix with each other in a non-linear manner to produce spurious signals. PIM is caused by a multitude of factors present in the RF signal path. These include poor mechanical contact, presence of ferrous contents in connectors and metals, and contact between two unmatched metals. PIM spurious signal, which falls in the up link band, can degrade call quality and reduce the capacity of a wireless system. [1]
The nature of passive intermodulation, PIM is that it occurs in elements that would otherwise be expected to operate in a linear fashion. Typically, mixing or multiplication in diode components in circuits is not termed PIM as the mixing is generally wanted and the diodes are expected to be in place. Instead PIM usually occurs in non-linearities. For the most part, it shows up in connectors, switches and isolators. Oxidation, or various other effects may cause the generation of a non-linearity.
The passive intermodulation products caused by the non-linearity follow exactly the same principles of those of wanted modulation products in a mixer. It is found that the various harmonics of the input frequencies mix together to form products that can remain within the required operational band.
Typical PIM levels
Most of the time, PIM is a small effect, because usually, it’s caused by small spurious non-linearities. According to Radio-Electronics.com, typical signal levels may be 100dB down on the levels of the signals that give rise to the PIM products.
“Active circuits such as amplifiers and the like generate much higher levels of intermodulation and other forms of distortion. However this can often be removed by filtering. In instances where passive intermodulation occurs this may not always be the case. One major area where passive intermodulation is a major issue is on systems where both transmitter, Tx and receiver, Rx use the same antenna, and utilise a common feeder. The transmitter signal can be filtered and reduced to a level where it does not affect the receiver. Also the signal combiner will prevent direct signal from the transmitter reaching the receiver. However any passive intermodulation occurring after the signal paths are combined, e.g. in the feeder or antenna will cause a major issue.”
Even if the PIM products are 100 or even 120 down on the transmitted signal, if that product enters the receive band, it’s still enough to do damage and affect performance.
PIM Causes
PIM effects are the result of a non-linearity being experienced by two or more signals.
So, what causes the non-linearity that, in turn, can cause PIM effects? Some of the more common ones include:
- Coaxial connectors
- Feeder lines
- Joints where dissimilar metals meet and oxidation, etc. occurs converting the joint into an area that exhibits some diode effects.
- Dirty connections
- Loose connections and irregular contact areas
- General anodic effects
- The use of ferromagnetic metals, like iron, nickel and steel can give rise to passive intermodulation.
- Spark discharges
“It can be imagined that one of the most common components where passive intermodulation is generated is within coaxial connectors. If they are not tightened sufficiently, or if the connections have become oxidized, then they are prime candidates for being the source of any passive intermodulation products that are being experienced.”
Although passive intermodulation products are low level signals when compared to the signals that give rise to them, the fact that transmitters are collocated with the receiver and often sharing the same feeders and antennas, means that even low level signals can cause significant problems.,/p>
Testing has become necessary to avoid PIM issues. PIM testing, however, can be a tough challenge for test equipment, but one that is becoming more important with the increasing number of systems that use duplex transmissions with collocated transmitters and receivers. [2]
[1] ZCG, Antenna Terminology, http://www.zcg.com.au/terminology.htm
[2] Radio-Electronics.com, Passive Intermodulation, http://www.radio-electronics.com/info/rf-technology-design/passive-intermodulation-pim/basics-tutorial.php
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