Basic Concepts of PIM
The generation mechanism of Passive Intermodulation (PIM) caused by contact nonlinearity includes the following seven aspects:
(1) Mechanical effects caused by point contact on the joint surface;
(2) Electronic effect caused by point electron contact;
(3) Thermal effect caused by point electron contact and local large current;
(4) Strong dc current causes ion electromigration in a metal conductor;
(5) Relative movement, vibration, and wear of contact surface;
(6) The thermal cycle is caused by the contact of devices with different thermal expansion coefficients;
(7) Loosening and sliding of metal contact and formation of the oxide layer or pollutant.
The main mechanisms of PIM caused by material nonlinearity are:
(1) Tunnel penetration of dielectric thin layer: electrons pass through the dielectric thin layer with a thickness of less than 10 nm and directly pass through the tunnel from one conductor to another, such as the electron tunnel effect between metals separated by oxide layer;
(2) Ferromagnetic effect: ferromagnetic materials (such as iron, cobalt, and nickel) have great permeability, which changes nonlinearly with the magnetic field and shows hysteresis characteristics. Ferromagnetic materials can cause strong PIM products
(3) Contact capacitance: capacitance caused by contact surface thin layer and pollution layer;
(4) Electrostriction: the electric field causes the change of linearity. Electrostriction in pure non-polar dielectric contributes to the PIM products produced in coaxial cable;
(5) Magnetoresistance: the change of resistance of metal conductor caused by a magnetic field;
(6) Hysteresis effect: the electric dipoles of materials have the trend of self arrangement;
(7) Magnetostriction: the change of linearity caused by the magnetic field is produced in ferromagnetic materials;
(8) Micro discharge: secondary electron multiplication discharge caused by ionic gas generated by strong electric field in vacuum environment, which is generated between micro slits and sand holes in metal;
(9) Dielectric breakdown: nondestructive solid dielectric breakdown caused by strong electric field. The possible mechanisms are thermal breakdown and avalanche;
(10) Space charging: charging carriers enter the insulator or semiconductor at the contact point. This effect arises from a non-uniform internal electric field. In semiconductors, there are electrons and holes at the same time, which can produce a strong nonlinear current voltage relationship;
(11) Ionic conductivity: a conductive phenomenon caused by ions (such as holes). It is a nonlinear effect in strong electric field. In RF band and microwave band, this effect is shown only when the DC component is large;
(12) Thermionic emission effect: the effect of electrons passing through the potential barrier caused by the statistical distribution of heat energy, which can be produced on the oxide film of conductor;
(13) Field emission: quantum mechanical tunneling of electrons through potential barriers. In the case of strong electric field, the current density changes nonlinearly with the field strength. The dependence of this effect on temperature is not as strong as that of thermionic emission, and it occurs at low temperature;
(14) Internal thermionic emission effect: similar to thermionic emission effect, it originates from traps filled in insulators or semiconductor materials;
(15) Internal field emission: quantum mechanical tunneling effect of charge from bound state to conduction band. In the case of strong electric field, this effect is stronger than the thermal ion emission effect.
To reduce the impact of passive intermodulation, attention should be paid to:
(1) Try not to use non-linear materials. If it is necessary, it must be coated with silver or copper plate with a certain thickness, and do not put the non-linear materials near the current channel or current channel;
(2) Minimize the use of nonlinear devices (such as lumped virtual loads, circulators, isolators and some semiconductor devices);
(3) Nonlinear components shall not be used in areas sensitive to PIM or places prone to PIM problems;
(4) The current density on the conductive channel should be kept low, such as large contact area and conductor block;
(5) Minimize the number of metal contacts, provide sufficient current channels, and keep all mechanical connections clean and tight;
(6) Improve the connection process of linear materials to ensure a reliable connection, no gap, no pollution, and no corrosion;
(7) Avoid using tuning screws or metal to metal moving parts on the current channel as far as possible. If they have to be used, they should be placed in the area of low current density;
(8) Keep the minimum thermal cycle, because the expansion and compression of metals and materials can produce nonlinear contact;
(9) Use the filter and physical separation method to separate the high-power transmitted signal from the low-power received signal as far as possible;
(10) Generally speaking, the cable length should be the minimum, and it is necessary to use high-quality low passive intermodulation cable.
Our Conclusion:
【1】 Passive intermodulation is mainly composed of connection nonlinearity and material nonlinearity;
【2】 Passive intermodulation is easy to occur at the place with the highest current density;
【3】 Passive intermodulation is easy to occur when loose connections are in contact with small areas;
【4】 Passive intermodulation is closely related to the contact force, which is easy to occur if the contact force is too small or too large;
【5】 The generation and level of PIM are related to the working frequency. The higher the basic signal frequency is, the higher the object level is;
【6】 Passive intermodulation products cannot remain stable in time. They are extremely sensitive to physical motion or temperature cycle processes or temperature changes. The change of extreme products is extremely sensitive to the outside world, and it is easy to produce and disappear;
【7】 Passive intermodulation products change with time;
【8】 Passive intermodulation products have a threshold effect;
【9】 Passive intermodulation products are related to the transmission power level, but often show unpredictability relative to the power level.
