Passive components were once taken for granted in a system design, with their contributions at the system level usually considered predictable. Even at their best, passive components like power dividers or couplers could only diminish the magnitude of a signal (in terms of insertion loss) or add to impedance mismatches .
Of course, with the growing demands on wireless communications systems for more information and higher data rates, and the need to implement complex digital modulation formats to achieve said data rates, passive components can no longer be taken for granted. The letters P, I, and M (standing for passive intermodulation) have become notoriously tied to any and all passive components used in modern wireless communications systemsespecially those operating with complex digital modulation schemes.
PIM is simple in concept, yet difficult to eliminate. It is essentially the nonlinear mixing of multiple signals, such as the fundamental frequency of a first signal with the second or third harmonic of the second signal. Almost any kind of signal discontinuity in a communications signal path can serve as the mixer for PIM, like metal-to-metal junctions within antennas or connectors. problems occur in the system when the PIM signal products resulting from this mixing are at high enough levels to act as interference to the system's receivers. And with the high power levels of transmitted signals in cellular and wireless base stations, it is not uncommon to produce PIM signal levels of -80 dBc or higher which can effectively "block" a receiver from functioning properly.
PIM has also been something of a test for many manufacturers of RF/microwave passive components. Many passive component designs have been well defined over the years, incorporating precision machined parts, perhaps designed with the aid of a commercial electromagnetic (EM) software simulator, and tested to verify that performance is repeatably at the highest levels. Imagine the collective groan of the industry upon learning that the performance of many of these passive components was inadequate to the needs of modern wireless communications towers and infrastructure equipment.
By and large, however, the industry has responded. Numerous suppliers of passive components have addressed the needs for low-PIM components by taking second and third looks at the construction of their components and how sharp edges can be smoothed and metal-to-metal junctions can be refined. Even manufacturers of one of the most notorious generators of PIM products, the coaxial connector, have spent countless hours revisiting their designs and connector interfaces.
In short, faced with the specter of PIM, passive-component designers have responded aggressively. the components themselves may be passive, but the people who design and fabricate them are anything but.