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| Significance of PIM studies |
PIM is an inter-modulation distortion generated by non-linearity in passive circuits. This occurs basically in communication systems having frequency-separation between up-link and down-link channels. Also this is serious especially in the system using devises shared with the transmission and the reception such as an antenna. For instance, when the inter-modulation signal f3 generated by the transmit frequencies f1, and f2 interferes the up-link channel f4, that is, f3 is equal to f4, undesired f3 cannot be eliminated by any duplexing devices. This is "the case that PIM falls into the receive band". Damage due to direct falling of PIM into reception band reduces as the order of IMD become higher because the higher order of IMD has basically smaller amplitude. If there exists a supplier requiring the complete check of 5th, 7th, 9th and 11th order PIM falling into the receive band, its manufacturing system would have some problems.
On the other hand, there is another approach to PIM. We shall consider that PIM is an effect reacting with the electromagnetic non-linearity, so that it can be standard of either "a defect in materials" or "a defect due to assembly". The former is seen from the viewpoint of "material choice", of which significance depends on the product specification or link budget. Hence, once material choice is followed the specification enough well, PIM would not be a problem. It does not matter whether the employed materials are low-PIM silver or high-PIM Nickel. Meanwhile in the latter, PIM would be a useful tool to evaluate assembling qualities, which should be carried out even though PIM does not fall into the receiving band.
As an example of PIM due to assembling process, problems such as "rough contact" and "rusty bolt" are well known. Conventional documents usually describe that "rough contacts should be reduced to reduce PIM". However, it seems to be incorrect because products should never be provided with rough contacts. It is also general that cable connections are regulated by specified screw-torque. Thus, the correct expression could be that "PIM-test should be introduced in order to detect rough contacts".
Besides the rough contact problem, PIM helps to detect an electrical connection failure due to oxidization, corrosion and so on. From this viewpoint, it would be an important parameter in terms of the product quality assurance if PIM does not fall into the receive band. In order to show whether the employed materials are satisfied with the required specification, it would be enough to make a discussion in a specific order of IMD. In order to assure the product quality in terms of assembling process, lower order IMD such as 3rd order PIM would be discussed because of its high sensitivity. In fact, the IEC standard recommends the two-tone test with 3rd order PIM (43dBm/tone).
Discussion only with higher order PIM is not sufficient and need not dare to be done. The 3rd order PIM test is the best for quality check, especially when the transmitting frequencies are much closer. The 2nd PIM would be better when the transmitting frequencies are much separated. PIM is a powerful tool to detect the small changes in a device as a nonlinearity phenomenon. It is necessary to use it effectively by understanding the advantage of PIM-test well.
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