Think Deeply About PIM

High peak powers and new digital-modulation methods increase in speed as demand rises. As a result, passive-intermodulation distortion (PIM) becomes a more significant concern. Due to expanding channel density and data rates, nonlinear distortions may dramatically reduce loading speeds and even block calls. The reliability and capacity of modern telecommunication arrays can be damaged by even moderate changes in PIM level, such as those corresponding to aging equipment, co-locating new carriers, or installing new equipment. Anritsu offers a white paper to guide engineers into understanding and testing for the PIM phenomenon, “Understanding PIM.”  

Traditional transmission-line quality tests, such as impedance tests, measure the linear quality of a system. PIM testing, in contrast, measures the nonlinear quality of a transmission line. A combination of linear and nonlinear quality tests are recommended for effective testing, as this approach grants a more complete picture of the transmission line’s health. For example, return loss or voltage-standing-wave-ratio (VSWR) testing could reveal bent cabling where PIM testing would not. The days in which channels could be selected to avoid PIM production have passed. Today, the growth of spread-spectrum modulation techniques has led to a channel-dense system that often generates significant levels of nonlinear distortion.

In a system, common generators of PIM include: metallic contacts, oxide build-up, ferromagnetism, surface abrasion, damaged cables, antenna faults, time-domain events, and connector design faults. While testing PIM systems, a common figure of 2 × 20 W is applied when the component undergoes dynamic stressing. This method has also been adopted in the field, as there is no standard for PIM testing. To perform testing that advances beyond the use of two fixed power frequencies, one of the frequencies can be swept to potentially reveal more PIM generators. This method requires a low-PIM termination to prevent high-power-level PIM broadcasting.

Generally, the limits for older systems that are not designed for low PIM are approximately -80 dBm/123 dBc. New antenna systems with PIM considerations in the design generally go by a guideline of a maximum of -107 dBm/-150 dBc. Traditionally, manipulating the suspect PIM-generating component, visual inspection, or repetitive part replacement have been the common methods of PIM identification and correction. Anritsu offers another option in a distance-to-PIM (DTP) technology with in-field PIM detectors, which is detailed in the white paper.

Anritsu, 490 Jarvis Dr., Morgan Hill, CA 95037-2809, (408) 778-2000.