Diagnosing Passive Intermodulation

Executive Summary

Passive Intermodulation (PIM) is an important consideration in modern cellular networks. As networks use more frequency bands and increasingly sensitive receivers, unwanted intermodulation products generated within passive infrastructure can have a direct effect on network performance.

When a PIM test produces a high reading, it is common to suspect the cable or jumper under test. However, a high reading shows that there is a source of intermodulation somewhere within the measured path. It does not, by itself, identify which component is responsible.

This paper describes a systematic approach to PIM troubleshooting, covering verification of the test system, inspection of connections, controlled substitution of components, installation and mechanical effects, environmental sources and the importance of recording and reproducing test results.

The aim is to identify the source of PIM before a component is replaced, reducing unnecessary returns and avoiding repeated site visits caused by an incorrect diagnosis.

1. Introduction

Passive Intermodulation occurs when two or more RF signals interact at a non-linear point within a passive system and produce additional frequencies. Where these products fall within a receive band, they can increase the apparent noise level seen by the receiver and reduce system performance.

Potential sources of PIM include connectors, cable assemblies, antennas, filters and other passive components. Poor electrical contact, contamination, corrosion, mechanical damage and dissimilar metallic interfaces can also contribute.

The difficulty when troubleshooting PIM is not normally establishing that a high result exists. The more difficult task is identifying where the intermodulation is being generated.

A PIM test measures the complete path between the test equipment and the termination. The result is therefore affected by everything included in that path, as well as conditions around the installation.

This can make replacing the most obvious component an unreliable method of fault finding. If the replacement does not address the actual source, the original problem remains. Even where the PIM reading improves, changing several parts of the system at the same time can make it difficult to establish which change produced the improvement.

A more useful approach is to establish a reliable test baseline and then change the system in a controlled manner, recording the result at each stage.

2. Understanding the PIM Measurement

A PIM analyser applies two or more RF carriers to the system under test and measures the resulting intermodulation products. The products are normally expressed in dBc, relative to the carrier power.

The measured value is determined by the complete test arrangement. This includes the analyser, test leads, adapters, loads, connectors and device under test. When testing an installed system, the cable routing, mechanical installation and surrounding metalwork also form part of the test environment.

This is important when interpreting a PIM result. A high reading demonstrates that intermodulation is present under the test conditions. It does not establish the physical location of the source.

For example, a jumper may produce a high result when connected to one test arrangement but a substantially lower result when connected to another. This does not necessarily mean that the jumper itself has changed. The test configuration, connections or surrounding conditions may also have changed.

For this reason, PIM measurements should be made using a known and repeatable test arrangement wherever possible.

3. Establishing a Test-System Baseline

The test system should be checked before investigating the assembly under test. If the analyser, leads, adapters or termination are producing PIM, the result obtained from the assembly cannot be interpreted with confidence.

A verified low-PIM load can be used to measure the residual PIM of the test system. The analyser, test leads, adapters and termination should be checked as part of this process and the result compared with the required measurement limit.

The baseline should be within the required PIM limit and stable during repeated measurements. If the residual PIM is higher than expected or changes significantly between measurements, the test system should be investigated before the assembly under test is assessed.

4. Inspecting Connections and Interfaces

Once the test system has been verified, the connections within the test path should be inspected.

Connector interfaces are a common area of investigation because their electrical performance depends on both the condition of the mating surfaces and the mechanical assembly of the connector.

The presence of a particular material combination does not, by itself, demonstrate that it is a source of PIM. It should instead be treated as one of the conditions to be considered when investigating an unexpected result.

5. Controlled Substitution

If the test system and connections have been checked and the PIM result remains high, individual components can be substituted to identify changes in the measured result.

The process can then be repeated for other components within the test path.

For example, if a jumper is replaced and an adapter is simultaneously reseated, the cable routing is changed and the connector torque is altered, an improvement in the PIM result cannot be attributed to the jumper alone.

The more variables that are changed at the same time, the more difficult it becomes to establish the source of the original result.

6. Installation and Mechanical Effects

PIM performance can also be affected by the way a cable assembly is installed. An assembly that performs correctly during a controlled test may produce a different result when subjected to mechanical stress in an installation.

Controlled Mechanical Testing

Where appropriate, controlled mechanical movement can also be used as part of the investigation. A defined section of cable or a connection can be lightly flexed or tapped while monitoring the PIM result.

If the PIM reading changes when a particular section is moved, the test can be repeated to determine whether the change is consistent.

A repeatable change may indicate a problem associated with the cable, connector, termination or mechanical installation. The result should then be investigated further rather than treating the movement itself as proof of a particular fault.

7. Environmental and Alien PIM

The source of PIM does not always form part of the electrical circuit being tested.

Metallic objects in the surrounding environment can interact with RF energy and may contribute to the measured result. This is sometimes referred to as alien PIM.

When investigating an unexpected or intermittent result, the surrounding installation should therefore be considered as well as the RF components themselves.

This can be particularly relevant when a result changes with the position or movement of the installation, or when a result obtained on site cannot be reproduced under laboratory conditions.

The surrounding environment should not be assumed to be the source of the PIM, but it should be included in the investigation where the measured behaviour indicates that it may be relevant.

8. Test Records and Reproducibility

PIM troubleshooting depends on being able to compare measurements. Test conditions should therefore be recorded in sufficient detail to allow the result to be reproduced.

The sequence of changes is particularly important. Recording the initial result, the change made and the subsequent result provides a useful record of the investigation.

Where a component is identified as a potential source of PIM, it should be retained for further testing wherever practical. Cleaning, re-terminating, repairing or disposing of the component before the investigation is complete can remove evidence that may be needed to establish the cause.

9. A Practical PIM Troubleshooting Procedure

When a PIM result is higher than expected, the following sequence provides a practical starting point for the investigation.

10. Conclusion

A high PIM result does not automatically indicate a defective cable or jumper. The measurement is made across the complete test path, and the test equipment, connections, installation and surrounding environment can all influence the result.

For this reason, replacing the most obvious component is not always the most effective way to identify the source of PIM.

A reliable investigation starts by establishing a known test-system baseline. The connections can then be inspected and individual components substituted while recording the effect on the measured result. Where necessary, the investigation should also consider mechanical conditions and sources of PIM within the surrounding environment.

Recording the test conditions and repeating the result are equally important. Without a reproducible measurement, it can be difficult to determine whether a change has actually identified the source of the problem.

For network operators, a systematic approach can help reduce unnecessary component replacement, avoid repeat site visits and provide greater confidence in the corrective action taken.

11. References