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Passive intermodulation in communication systems


Technics
Assembly and functional description

Intermodulation products are generated when two or more frequencies are applied to a non-linear transmission element. A non-linear device is produced when the signal applied does not have a proportional ratio between current and voltage at the same time.The frequencies of these intermodulation products are in accordance to the following equation: 

ƒIM = n * ƒ1 ± m * ƒ2 ± p * ƒ3 ±

with
ƒIM 


intermodulation frequencies

ƒ1,2,.. 

carrier frequencies

n,m,p,...

integral multiplicators

These mixed frequencies can cause disruption in communication systems or measuring equipment if several carrier frequencies with high intensity have the same transmission path as weak receiving signals, i.e. mobile phones.

Intermodulation in passive components is caused by very small non-linearities of the conducting parts. PIM critical elements are:

  • Connectors
  • Cables
  • Filters
  • Antennas

The reasons for non-linearities in passive devices are:

  • Unsuitable contact geometry
  • Use of magnetic materials (i.e. steel, nickel) in the conducting path
  • Contaminated or oxidized contacts
  • Dissimilar contact base materials
  • Microscopic field discharging effects

The PIM-products of a system with several IM-sources i.e. cable assemblies, filters, antennas etc. can be observed. This however, is often difficult to determine because of the additive or subtractive interferences of these components. This means that the resulting IM-intensity can vary at different frequencies because of the change of the phase relations. If there is only one single component within a system with a high pin level, the performance of the entire system is affected. Due to this reason all the components in a transmission path have to be selected to suit the system requirements.

Connector series:
It is recommended in PIM critical applications to choose the largest conducting surfaces possible in order to avoid high current densities. The design of the connector should incorporate a defined mechanical stop for the outer conductor. The use of a threaded coupling interface ensures that the interface is correctly engaged. The 7/16 and N ranges are particularly suitable for low IM applications and also the TNC and SMA type interfaces could be considered.

Surfaces:
The use of non magnetic materials is a very important factor for low IM products. Therefore materials such as nickel and steel should be avoided. Silver and gold are recommended platings for center contacts because of their high conductivity and their contact characteristics. However, the gold layer should not have a nickel undercoating. The plating of the outer conductor is dependent on the demands of the application (i.e. corrosion properties, number of mating cycles, solderability, price). Other options are also available such as silver, white bronze or a combination of both.

Termination technique:
To ensure the long term electrical and mechanical stability of the assembly it is essential that this operation is carried out correctly. Cable or panel mount connectors should have a full 360 degree electrical contact of the outer conductor, preference is for the solder/clamp termination method which offers an improved PIM stability over the crimp termination variants.

Cables:
The selection of a suitable cable is as important as the type of the connector or the termination technique used. For high power applications with high IM requirements, corrugated copper tubes or semi-rigid wires without a steel center conductor are recommended. In the majority of cases flexible coaxial cables are considered unsuitable for these applications due to their braided screening. In these cases the cable manufacturer’s specification should be referred to.

Handling:
In order to prevent the deterioration of the contact quality direct contamination with dust, salt or grease should be avoided before and after the assembling of the connector. Furthermore it is essential to protect cables and connectors from any mechanical damage.

Regarding the selection and handling of low IM connectors for different applications, IMS Connector Systems is always available to assist you with your special requirements.


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