Passive Intermodulation

All PIM-specified HUBER+SUHNER lightning protectors and their piece parts are designed according to the latest knowledge of PIM theory and practice. This is a continuous, progressive process.

Generation of Passive Intermodulation Products (PIM)

• non-linear behavior of elements in a signal path used with more than one carrier generates IM.
• the occurring spectral lines of the IMP (Intermodulation Product) can be described as:
• fIMx  = mf1  + mf2  + ... + y fm
• where  f1...fm  are the used carrier frequencies
• m...y are positive or negative integers
• fImx  = frequency of one generated IMP

• absolute linearity exists only as a mathematical idealization – passive elements are all weakly non-linear
• problem with PIM only occurs by:
  - high transmit levels
  - high receiver sensitivity
  - several transmit channels
  - where only one antenna for transmission and receive path is used
• once in receive band, PIM cannot be reduced by filtering
• in passive elements there are some dominant contributors of non-linearity:
  - similar or dissimilar metal-to-metal joints
  - plasma effects (local high fields causing corona)
  - magnetic non-linear effects
  - high-current density
• for cable and connectors the metal-to-metal joints are the most significant PIM contributors
• gold, silver, copper, brass and copper-beryllium joints generate low PIM; steel, aluminium, stainless-steel-joints generate higher PIM

In practice

• the IM level generated over the whole signal path is a result of many IM sources. The value of the resulting IM level depends on the phase relation of all these sources (constructive or destructive interference). This phase relation varies with the frequency.
• IMP's of different order have different frequencies, and hence the resulting product does not have a constant amplitude.
• PIM's of different measurement setups are not exactly comparable (because of the different phase relations).

• 3rd order IMP's have a higher value and normally are used to describe the IM behavior of the device under test (DUT).
• in theory the IM level increases 3 dB per 1 dB power increase of the carriers. So, it is important when comparing different measured IM levels to consider the input power level. A standard value for input power is 2 x 25 W (47 dBm).
• all elements in the measurement setup generate IM. This ground level limits the measurement range (-120 dBm, related to carrier power:
• -120 dBm -47 dBm = -167 dBc).
• it is not possible to measure a single connector. Only assemblies can be measured.
• the measured level can vary up to 40 dB by vibration or bending of the cable. So we have to know if the application of the assembly is mechanically static or dynamic.
• it is difficult to give a typical value for a connector. It depends on the method of mounting (remove cable isolation, crimping, clamping, soldering and contamination).