Minimizing passive sources of intermodulation distortion requires care in the selection of communications-system passive components, including antennas, cables and connectors, and electromagnetic switches.
| These Type N connectors have been engineered with bronze-plated brass bodies and gold-plated contacts for low PIM performance. (Photo courtesy of Pasternack Enterprises) |
Intermodulation (IM) occurs when multiple signals in a transmission path act on each other. In this age of cramming as much information into a signal path as possible, multitone carriers are commonplace, and the components that transport those signals must do so with minimal distortion. IM was once associated only with active components, but at higher signal levels, passive components can be just as guilty of generating unwanted signals and amplitude modulation (AM), producing signal side-effects known as passive intermodulation (PIM).
Two or more signals can mix in a wireless receiver, transmitter, or in the antennas between them to form unwanted signals due to the nonlinear behavior of the various components in the system. The unwanted signals are related by the spacing of the desired signals and their power levels. By way of example: two signals (F1 and F1) being handled by one system (such as 830 and 840 MHz in an LTE system) will create additional signals that are at harmonic multiples of both signals, as well as signals that are equal to the sum and difference frequencies of the original signals and multiples of those sum and difference frequenciesfor instance, 2F1 F2 and 2F2 F1. The lower orders of PIM distortion include difference frequencies, such as the third-order products 2F1 F2 and 2F2 F1 , as well as the fifth-order products 3F2 - 2F1 and 3F1 2F2 . But these are lower-order PIM products, and higher-order products are also generated. This makes the calculation of PIM performance by means of commercial computer-aided-engineering (CAE) tools, such as electromagnetic (EM) simulators, quite difficult and time consuming.
The PIM performance of a component (like a connector) is generally specified in power level (dBm) of the generated tone relative to the power level of the test tones. If two +40 dBm test tones produce a PIM signal of -115 dBm, the PIM level is said to equal -155 dBc. In a case where two or more test tones are at unequal power levels, such as +40 and +38 dBm, the PIM level is referenced to the larger or largest of the test tones.
System-level PIM can be minimized, but this requires careful component selection. Even the choice of materials in those components can impact the level of generated PIM. For example, antennas are critical components in a wireless communications system because they are found at receive and transmit ends of the system. Especially in the case of the transmit antennas, which are subject to higher power levels, the presence of ferromagnetic materials (such as nickel) can increase the level of PIM. Low-quality metal plating and poor mechanical junctions can also contribute to high levels of PIM.
Manufacturers of low-PIM connectors generally advise their customers to make sure that connector mating surfaces are without damage and that connectors are properly torqued with a torque wrench, rather than tightened by hand. In addition, there should be no strain on a connector/cable interface. PIM can also degrade over time, as oxidation buildup and corrosion on connector surfaces can result in less-than-optimum electrical contact for the mating surfaces of the connectors. For connectors, cables, and other components in the system, the source and load impedances presented to each component should be tightly controlled to the system impedance (typically 50 Ω).
What is considered acceptable PIM performance? That answer will depend on the requirements of a particular system. As an example, cables and connectors are among the most PIM-prone components in many systems. the LMR-SW cables from Times Microwave have been developed for applications where PIM must be minimized. they feature a thin-wall aluminum outer conductor and can achieve better than -170 dBc PIM performance.
A line of type N connectors (see figure) developed by Pasternack Enterprises was designed to terminate these cables with minimal additional PIM. The connectors, usable to about 11 GHz, are interchangeable with any type N connector meeting MIL-C- 39012 specifications.