Passive Intermodulation, PIM Distortion Basics

- passive intermdodultion, PIM arises when non-linear passive components or other general elements like rusty bolts on antenna towers, etc, enable two or more signals to mix or multiply to give intermodulation distortion.

Passive intermodulation, PIM is a form of intermodulation distortion that can occur even when no active components are present.

PIM can occur in a variety of areas from coaxial connectors to cables, even rusty bolts or any joint where dissimilar metals occur.

Passive intermodulation, PIM can produce interference, and this can sometimes hide the wanted signal. Accordingly it is often necessary to try to remove the element that generates the passive intermodulation so that interference generated can be removed.

What is passive intermodulation, PIM?

Passive intermodulation occurs when two or more signals are present in a passive non-linear device or element. The signals will mix or multiply with each other to generate other signals that are related to the first ones.

The nature of passive intermodulation, PIM is that it occurs in elements that would otherwise be expected to operate in a linear fashion. Typically any 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 is normally as a result of the spurious generation of non-linearities - typically it may occur in connectors, switches, isolators of the like. Here oxidation or 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.

If the two signals involved are f1 and f2, then the intermodulation products can occur at frequencies of ± M ⋅ f1   ± N ⋅f2.

Diagram showing the frequencies of the different signals that can be produced by passive intermodulation distortion
Frequencies generated by intermodulation distortion

When referring to intermodulation products, the order of the products is often mentioned. This is the number of times a product within the output is multiplied, i.e. one that is generated by 2 x F1 - 1 x F2 would be a third order product and 3 x F1 + 2 x F2 is a fifth order product and so forth.

Typical PIM levels & relevance

Passive intermodulation is almost always a small effect. This arises from the fact that it is often caused by small spurious non-linearities. 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

A diagram showing how passive intermodulation can generate frequencies that affect the receiver in a duplex system
How passive intermodulation affects transmitter receiver systems

Even if the passive intermodulation products are 100 or even 120 down on the transmitted signal, these levels are still enough to cause the receiver performance to be impaired if the products fall within the receive band.

Examples of systems where transmitter and receiver use the same feeder and antenna include cellular base stations, satellite systems, and other radio or wireless systems where duplex transmission, i.e. transmission in both directions is required.

Causes of passive intermodulation, PIM products

Passive intermodulation effects occur as the result of a non-linearity being experienced by two or more signals.

A whole variety of instances can give rise to a non-linearity that can cause PIM effects. Some of the more common ones include:

  • Coaxial connectors - these connectors have joints between dissimilar metals and areas where oxidation can occur. Connectors that are assembled well using high quality components and water proofed well if they are to be used externally should last, but any connector will deteriorate with time. Connections made with poor connectors, or of the connectors have been assembled poorly will generate higher levels of PIM faster.
  • Feeder lines - feeders such as coaxial feeders generate levels of passive intermodulation. The braid in the outer conductor provides many metallic interfaces in which passive intermodulation, PIM can be generated. Even if foil is used, there are interfaces between the layered construction of the foil wound round the dielectric. For applications where passive intermodulation is important coax with a good PIM performance can be bought. Some manufacturers may guarantee levels to below a given level, e.g. -140dBc, although at a cost.

  • 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, on a macroscopic or even a microscopic scale, can cause an inconsistent flow of current and generate inhomogeneous electromagnetic fields.
  • General anodic effects
  • The use of ferromagnetic metals, like iron, nickel and steel can give rise to passive intermodulation. These metals show magnetic hysteresis effects when energy is applied. This means that the resulting signal level is dependent, even to a small degree on input level. Hence this is a non-linear effect.
  • Spark discharges can give rise to passive intermodulation. The spark will cause craters to be formed and some oxidation that will generate a diode effect. Sparks can result from poor connections or even the hot connection or disconnection of a connector.

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 oxidised, then they are prime candidates for being the source of any passive intermodulation products that are being experienced.

One of the difficulties when trying to locate passive intermodulation is that the nonlinearity that gives rise to them does not manifest itself at low input signal levels. This may result from a number of reasons including:

  • PIM levels are normally low and at low power levels the passive intermodulation products may fall below the thermal noise level
  • Not enough stress may be placed in any material involved. If the passive intermodulation arises from a hysteresis effect, then the activating signal may not be large enough to drive the material round its hysteresis loop.

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 for passive intermodulation signal, measuring them and also locating sources 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.

By Ian Poole

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