Testing Low PIM Installations and Components
Low PIM components are a key factor when building telecommunication networks with lowest possible passive intermodulation interferences. However, 60 % of all PIM issues are not caused by faulty components but are man made and happen during installation. This particularly because RF-cables are usually assembled in the field.
Unintended scratches in the plating, chipping, entrapped dust are just a few of many issues that can occur. The only way to ensure that base station installations operate at the expected low pim levels is to conduct thorough PIM tests of both, individual RF branches and the complete installation. Three simple PIM tests have gained general acceptance and serve as excellent reference for both, installation and component testing. The tests are described below. They will detect virtually all sources of PIM in cables, connectors and components.
Static PIM Test
The static test analyzes both, components and cables. PIM analyzers that deliver continuous 2 x 20W measurement signals are connected to the Device Under Test (DUT). PIM measurements are performed for at least 30 seconds, ensuring to fully energize the system and also apply thermal stress to the tested components, similar to a live telecommunications signal. This test detects bad materials, scratched surfaces and alien bodies like dust or metal chips in the RF path. Measurement values of static tests need to be below the required limit, but they should also be stable. The signal should not alter too much during the measurement. Changes of 2-4 dB are acceptable, higher swings – even if they are within the required limits – can be an indicator for (future) PIM problems.
1st Dynamic PIM Test – “Wiggle Test”
The first dynamic test, also called “wiggle test”, checks the quality of assemblies between cables and connectors. Tested cables are moved in a circular way (turn diameter about 10 cm). The test is to be conducted with at least 10 turns in each direction. Wiggle tests detect loose contacts and poor workmanship of cable assemblies. They find also bad soldering and shielding cracks. PIM measurements must be stable throughout the test. Correct cable-connector assemblies will endure this mechanical stress test easily.
2nd Dynamic PIM Test – “Tap Test”
The second dynamic test, is also known as “tap test”. It requires PIM analyzers to be set to “PIM versus time mode”. PIM readings are continuously shown over a time axis in this mode. A harder device, made of wood or plastic material but not metal, is used to tap 10 times at all connectors. Field technicians use often the handle of a screw driver for this test. It is hard but does not dent or scratch the connectors. PIM readings should stay stable during this test. Possible contamination like dust, metal chips or other alien bodies in the connectors will cause spikes in the reading whenever the connectors are tapped. Remedy is to open the connectors again, clean them with alcohol wipes and dry them with moisture free pressured air. Afterwards the tap test has to be repeated.