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IMD -Intermodulation Distorion basics,types and products

In this page we will cover intermodulation distortion (IMD) basics, types and IMD products. We will understand PIM (Passive Intermodulation) one of the IMD types and understand methods to minimize or mitigate PIM.

When two or more than two signals are mixed in nonlinear component whether active or passive, other signals get generated as a result of product of these original signals. This condition is referred as IMD or Intermodulation Distrotion. This will occur in amplifiers operating in nonlinear mode. Hence it is desirable to operate amplifier within certain limit set by the standard. Due to this amplifier operates below operating point and hence works in linear region without much of distortion. Check below other methods to minimize or mitigate IMD.

The frequencies which are generated due to intermodulation can be sum or difference of various IMD products. Based on IMD products which are generated there will be second order, third order, fourth order and so on. Let us assume for sake of understanding that there are two frequencies f1 and f2 which exists at the input of nonlinear operating amplifier. This will generate frequencies as shown in the figure-1.

IMD products Figure-1: IMD Products

As shown in the figure there are many products but the major consequencies will be due to 2f1+f2 and 2f1-f2 as they exists near to desired frequencies(f1,f2) in the frequency spectrum.

The table below mentions various intermodulation classes(with different orders and number of channels) and their outputs.

Intermodulation order
/channels
IMD products
2nd Order
2 channels(f1,f2)
f1+f2, f1-f2, 2f1, 2f2
3rd Order
2 channels(f1,f2)
2f1+f2, 2f1-f2, f1+2f2, 2f2-f1
3rd Order
3 channels(f1,f2,f3)
f1+f2-f3, f1+f3-f2, f2+f3-f1

Similarly fifth order, seventh order with either 2 input channels or 3 input channels will generate IMD products.

What is PIM-Passive Intermodulation Distortion

As mentioned if we overdrive the amplifier we might land into few dB of more power but will result into these IMD problems which will generate the intermodulation interference. The intermodulation distortion have following three types namely receiver produced IMD, transmitter produced IMD and Passive IMD.

The IMD is generated in the receiver when two or more received signals are mixer in RF amplifier. The IMD is generated in the transmitter when two or more signals are mixed in nonlinear component of transmitter such as RF mixer, MMIC amplifier etc. These two types of IMD are generated mainly due to active components in transmitter and receiver systems. Let us understand what is PIM now.

PIM stands for Passive Intermodulation. This is generated due to passive components such as power splitters, cables etc. The main source of PIM based distortion is mainly connection or junction between different types of materials. Examples include connection between connector and cable, junction between two different types of cable made of different materials, connection between cable and antenna.

How to minimize or mitigate PIM

Following techniques or methods are adopted to minimize or reduce or mitigate Passive Intermodulation(PIM) distortion.

• Avoid bad RF connections using connectors. Make sure wherever connectors are used, they are securely tightened.

• Cable absorbers are used as needed on end points.

• Low PIM cables are preferred from manufacturers with fully soldered connectors.

• Passive components with minimum specifications of -155dBc @ 2 x 20 Watt should be used.

• It is preferable to use connectors, cables and tools from the same manufacturer. This is because technical specifications vary manufacturer to manufacturer and if avoided will lead to mismatch at junction/connection points.

• Ideal installation should be maintained without any loose connections and without any extra stress than needed.

• Parts should be cleaned before connections or soldering.

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