The Threat of Passive Intermodulation Products (PIMP) to RF Coaxial Connectors

1. Introduction

This paper will provide a preliminary analysis and discussion on an important issue in the design of RF coaxial connectors, namely, the threat of passive intermodulation products (PIMP) to RF coaxial connectors. The nonlinearity present in passive networks (such as metals, metal oxides, metal contacts, ferrite magnetic materials, etc.) can generate intermodulation products (IMP) within the receiver's frequency band of the entire communication system. Designers often overlook the PIMP generated in components such as RF coaxial connectors. Therefore, it is important to address this issue and take additional precautions.

2. Factors Influencing the Generation of PIMP

The factors that influence the generation of PIMP include the presence of surface contamination (such as smoke, oil, sweat, oxides, and cleaning agents); current density; dimensional changes due to thermal variations in conductors and dielectrics; and humidity (including moisture retention and possible corrosion). The inherent non-permanent contact in the practical application of RF coaxial connectors increases the likelihood of PIMP generation. Therefore, when designing and selecting these components, all factors that influence PIMP generation between metal contacts must be considered. Designers must consider contact surface pressure (i.e., insertion and extraction force), the composition of contact materials, the geometric dimensions of the contacts and their final state after mating, and potential humidity. Additionally, RF coaxial connectors used in space environments must be designed to prevent the occurrence of multipactor effect (secondary electron multiplication effects).

Using hard metals for contacts and ensuring that the mating interface has significant positive pressure before deformation increases the penetration of surface oxides. However, the resulting surface contact area is larger than that of soft metals, resulting in a larger contact area, lower current density, and fewer passive intermodulation products. Therefore, our RF coaxial connectors should be made from high-hardness, elastic copper-beryllium alloy or phosphor bronze alloy materials. In designing RF coaxial connectors, materials prone to oxidation, such as pure copper and aluminum, should be avoided. Instead, materials such as gold, silver, beryllium, brass, and beryllium copper alloys are suitable for preventing PIMP. These metals are soft and are only plated onto parts made from other hard metals, providing adequate pressure and elasticity. Gold-plated brass connectors achieve the lowest PIMP levels when mated, but the plating must be uniform and smooth.

Sealed devices containing Kovar alloy should be avoided whenever possible. Non-metallic gaskets generate less PIMP than Kovar alloy. Ferromagnetic materials and stainless steel should be excluded from connector design. Thus, in the practical application of connectors in satellites and missiles, the greater the pressure between contacts during mating, the larger the contact area formed, the lower the current density, and consequently, the lower the PIMP level. Therefore, in the design, the bevel angles of a pair of connectors' mating ends should match each other, and elasticity should be maximized. Simultaneously, the curling and slotting of connector parts can result in unstable high levels of PIMP, so all parts of the connector must maintain high coaxiality, parallelism, and symmetry.

Designers must note that parts that can be formed in a single processing operation should be completed in one machining process to avoid separate processing and subsequent assembly. For example, precision N-type RF coaxial connectors are a standard choice due to their precision. They are widely used because standard connectors are specifically designed with outer contacts that do not have slots and are integrally formed with the outer conductor in a single machining process, preventing deformation and creating significant pressure at the contact surface to ensure good contact.

In space systems, semi-rigid cables are preferred as transmission lines. However, using semi-rigid cables in space environments poses a series of challenges. Cosmic rays and intense thermal radiation can lead to dielectric expansion and thermal deformation of contacts, increasing connector stress or causing movement of the center contact, resulting in significant PIMP generation. In severe cases, dielectric expansion can lead to insulation breakdown, destroying the coaxial transmission line.

When using flexible cables, it is essential to consider that PIMP increases with cable length. Secondly, the composition of the braid is crucial, as aluminum, stainless steel, and nickel-plated copper generate strong PIMP signals. Bare copper wires with sufficient inner braid tension can produce slight PIMP. It is best to use thin tin-plated, silver-plated, or enameled copper wire. Third, removing the cable sheath can increase PIMP by reducing the pressure between the connector and the cable, so minimizing sheath removal during cable connection is advisable. I recommend folding the braid over the sheath and crimping it to increase pressure and reduce PIMP. Fourth, the excitation of RF signals on the dielectric of flexible cables causes changes in energy density within the dielectric, leading to cable stretching or contraction, i.e., volume changes. This causes a second-order term in the dielectric constant that varies with the electric field strength, modulating the fundamental and harmonic waves of the input frequency with the original field and leading to PIMP generation. According to relevant information, eliminating PIMP caused by this factor depends on the designer's experience. It is also noted that the magnitude of volume changes is proportional to the elastic coefficient of the dielectric material.

3. How to Reduce PIMP

The following points summarize methods to reduce PIMP in the design of RF coaxial connectors:

1. Minimize Current Density at Expected PIMP Generation Sites
• Maintain high positive pressure on metal contact surfaces to allow      current to pass through larger contact areas, reducing current density.      Simultaneously, significant pressure in contact areas causes surface      oxides to break down, improving nonlinear current conduction.
• Use conductors with large cross-sections to reduce current density      when ferromagnetic resonance is absent, but note that the volume of RF      connectors should also be controlled within a reasonable standard range.
• Adjust VSWR and transmission line length so that the maximum      standing wave current does not occur at the critical points of contacts.      However, note that PIMP increases with cable length, so cable length      should be controlled within a reasonable range.
2. Reduce Collisions Between Metal Parts During Mating or Matching
• Replace flexible cables with semi-rigid coaxial cables.
• Connectors must have good shielding; therefore, it is recommended      to add shielding devices at the installation points of microstrip      connectors.
• Ferromagnetic alloys should not be used in areas of the connector      that require welding, and all welding flux should be volatile and      low-corrosive. To avoid oxidation at welding points, argon welding is      recommended. However, in practical applications, considering production      costs and equipment conditions, this is generally difficult to achieve.
• The insertion and extraction force of connectors should be as large      as possible.
• After forming the equipment, the user should cover paired      connectors with encapsulation material.
3. Avoid Using Ferromagnetic Materials in the Design, Processing, and     Assembly of Connectors
• Connectors should be designed to avoid sealing (i.e., avoid using      Kovar alloy), and during assembly, avoid using paints, epoxy resins, and      some structural irregular fillers. All screws, pins, and rivets used in      connectors should not be ferromagnetic.
• Non-magnetic tools should be used during assembly, including      screwdrivers and pliers.
• Keep workbenches free of metal and magnetic dust. The cutting      processing area should be isolated from the assembly, testing, and      inspection areas, including personnel isolation.
4. Protect Connector Components from Corrosion
• Minimize handling of parts, and assembly workers must wear clean      gloves.
• If coaxial cables need to be connected, the end of the cable should      be cleaned within a 10 cm range before connection.
• Before final assembly, connector parts should be cleaned but not      immersed in cleaning fluid; ultrasonic cleaning in a rinsing state is      recommended.
• Maintain continuity at high-field-strength locations, and avoid      irregularities in the inner walls of the outer conductor and the outer      walls of the inner conductor of the connector as much as possible. The      dielectric should also maintain uniformity.
5. Implement Effective Testing and Inspection Methods
• Change the frequency of the signal source during testing to observe      changes in the frequency of parasitic signals.
• Add an attenuator between the signal source and the connector      during testing to observe the effect on the input level as the attenuator      changes.

• Apply an external magnetic field to observe the connector's impact      to identify PIMP signals caused by ferromagnetic materials in the      connector.

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The End.