The PIM appears as transmit power appears. Its spectrum and input frequency meet the mf1+/-nf2 relationship. It occurs most probably in antenna-feeder system, especially at the connector of feeder and antenna.
For connectors to feeders, a low PIM concerns the following six aspects:
z Contact design
z Combining interface of connectors
z Internal connection of connectors
z Cable clamp device
z Materials and electroplate
While designing the structure of cable clamper of RF connector, increase the contact square and pressure as possible. Keep the mechanism stability of the contact interface between internal and external conducts of connectors and that of cables. Prevent the slight displacement of contact parts caused by curved cable or mechanical vibration from severe PIM.
Usually the jointing external conducts are ideal and on sale, but the majority is flared mechanical clamper. Connectors are usually assembled by operators on site, so the quality of the components is not under the quality control of connector manufacturer.
Therefore consider the assembling conditions upon design, draw accurate cable layout and dimension table. This helps to assemble the connectors and reduces assembling errors.
Avoid the following pollution source during assembling connectors:
z Dust
z Sweat
z Grease
z Metal clast
z Scratch on the conductor surface
The causes to the previous pollution are nonstandard operations during assembling, package, transport, and installation. Therefore ensure to prevent pollution from existing inside components, especially the sweat, grease, the metal clast, and scratch on the conductor surface. Avoid scratch, fish tail, and impress on the surface, especially the contact surface.
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8.4.1 Connection of DIN connectors
Check the connection surface and internal conductor for foreign bodies, scratches, or problematic screw thread.
I. Checking Connection
1) Insert the connector forcibly and then connect
2) Rotate the bolt while the connector and the cable are relatively fixed.
3) Check the connection for loose components.
II. Important Checklist for Manufacturing Connectors Check the connectors whether:
z The stripper is accurate
z The size is proper
z The tools for manufacturing connectors is nonmagnetic
z The clearing is comprehensive
z The plating layer is complete
z Process adjustment is performed according to plating layer
z The connectors are in good condition
8.4.2 Occurrence of Antenna PIM
I. Two Types of Nonlinear Phenomena Two types of nonlinear phenomena include:
1) Nonlinear contact occurs upon contact of metal with nonlinear electricity and voltage.
In details, it includes looseness, being oxygenated. Nonlinear materials refer to alloys, such as magnet, carbon fiber, iron, cobalt, nickel, and aluminum, has the nonlinear V-I.
2) The antenna PIM occurs closely related to the following factors:
z Metal is used in antenna-feeder system to enhance the structure strength and MIM leads to occurrence of PIM.
z Slight cracks, crazing, and hollow inside the metal cause PIM.
z Nonlinear carbon fiber and planning layer.
z Electronic channel effect and semi-conductor effect at the contact point with metal.
z Impact of uneven material structure on conductivity and micro change of some particles due to unbalanced heat (energy).
z The edge processing procedure of antenna and some parts, rivet connection techniques lead to occurrence of PIM.
z Oxygenation, rustiness, and erosion at connection points.
8.4.3 Controlling Antenna PIM
Manufacturing controlling board of reflectors is important to occurrence of PIM. The rivet connection aluminum controlling board is most frequently used. Every rivet might cause PIM. Solve this problem by using gluing technology during manufacturing.
PIM occurs probably where the high-voltage electricity intensively exists. Designing feeder channel is key. In principle the whole-set feeder is used, but it is possible actually.
The metal-to-metal contact must not be used in the feeder design. The contact electric potential difference caused by different metal contact or uneven expansion must be eliminated in antenna design. For manufacturing single chips not for electric spark erosion at feeder part, the PCB should be nonmetal board when power splitter is used.
The antenna must not be mounted in an environment where the temperature changes quickly.
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8.4.4 Features of Antenna PIM
1) When metal clasts exist in the feeder point, the antenna PIM features that the intermodulation outcome increases by over 20 dB and that intermodulation outcome fluctuates.
2) Metal clasts on the surface mounting components have less impact than the feeder point.
3) The reliability of grounding metal around the antenna unit affects the fluctuation of system PIM. The metal partition board between the antenna units must be grounded by multiple jointing points, not by large-square grounding.
4) Nonmagnetic materials must be used for manufacturing antennas.
5) The oxygenation layer formed by brand iron in jointing is a major cause to occurrence of PIM.
6) Do not touch the components. Clear the connector every time after using them, especially after test.
7) Joint the connectors to the supporting structure if the connectors will not be moved.
8) Keep the connector and cable relatively fixed during connection.
9) The connection of power splitter and ground impact PIM as much as over 20 dB, so the reliability of connection between power splitter and ground, as well as cable, must be guaranteed.
10) Clean the exposed parts of an antenna after it is made.
8.4.5 Relationship between PIM and NodeB Alarms
1) If alarms, such as VSWR alarms and TMA alarms, occur on the antenna and feeder with power, PIM occurs.
2) PIM occurs unrelated to previous alarms. It might occur with or without alarms. Do not determine the quality of connection of antenna-feeder by testing resistance and VSWR. Normal resistances and normal VSWR is necessary to good antenna-feeder, but is far from sufficiency.
3) The gain alarms on the Rx channel are usually impossible. Thought PIM causes irrelevant main and diversity RTWP, that the NodeB receives channel alarms is less probable, usually impossible.
4) Locate the antenna-feeder problems not by alarms, but by analyzing RTWP for basic features
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