What is Intermodulation?
Intermodulation(IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system.
Causes of bad intermodulation or intermodulation distortion
The electron tunneling effect and semiconductor behavior at the metal contact through the thin oxide layer of the separated conductor;
Micro discharge between micro slits and across voids in metal;
Nonlinearity related to dirt and metal particles on the metal surface;
High current density at the contact;
Nonlinear resistance coefficient of carbon fiber;
Nonlinear hysteresis effect in ferromagnetic materials
Poor installation process may cause loose connections.
In addition to linear factors, there are several reasons for passive intermodulation in filters and coaxial connectors.
(1) The electroplating solution that is not cleaned in the electroplating process of metal parts.
(2) The conductivity of the coating is not good and the thickness of the coating is not enough.
(3) The surface is rusted.
(4) Different metal materials of center contact.
(5) Magnetic wire material in the signal channel.
(6) Lower positive contact pressure.
(7) Large surface roughness.
(8) Debris and dust in the connector.
(9) Spiral signal channel.
Filter intermodulation maintenance methods
First, make sure that there is no problem with the system. The intermodulation value of the system itself must be greater than the value required by the product. (Note: the big here refers to the absolute value, because many values in passive products are negative). Only when the system has no problem can we judge that it is the problem of the product. However, the common maintenance methods of intermodulation are as follows:
1) Tighten the ant end screw by hand
2) Hand tighten the cover plate screws
3) Remove the coupling PCB board and judge whether it is affected (the coupling plate is pressed too Deep?)
4) Remove the lightning protection PCB board and judge whether it has influence (the first level inductance is isolation sufficient?)
5) Disassemble the cavity, tighten the mounting screws of the resonance rod by hand and repair it with soldering iron solder joints, find and remove burrs in the cavity
6) Replace antenna head
7) Replace the low-pass valve
8) Disassemble the cavity and clean it with ultrasonic
Preventive measures for bad intermodulation:
In view of the above reasons for passive intermodulation, the following measures should be taken in the design and manufacture of filter and RF coaxial connectors.
1) From the perspective of electroplating, in order to reduce passive intermodulation and contact resistance, it is best to use a silver plating layer with a thickness of more than 6um. The coating shall be free of impurities and must be passivated with chromate. However, the problem that the silver coating is easy to change color should be carefully considered on some occasions. Gold plating is also a better choice, but the cost is high.
2) In terms of product structure design, impedance discontinuity shall be avoided as much as possible, consistent characteristic impedance shall be maintained as much as possible, and nonlinear factors shall be reduced.
3) Select materials with high conductivity, such as copper and its alloys, and avoid using stainless steel or other magnetic materials, even if the permeability should be less than 2.0.
4) To improve the surface processing quality, the general surface roughness should be less than 0.4um. And there shall be no pits, debris and other sundries.
5) The surface of the conductor shall be free of spots and rust.
6) Select good elastic materials, and carry out fine processing and vacuum thermal treatment to ensure that the contact has stable and reliable positive contact pressure and small contact resistance during 500 times of plugging.
7) Avoid the combination of different metals, especially incompatible metals that may produce electromotive force coupling with each other, so as to prevent electrochemical corrosion.
