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Passive intermodulation and base station system performance
Summitek Instruments’ Sales & Applications Engineer, James A. Pierson Jr. and Andy Singer, RFS’s Director Marketing & Technical Services for the Americas, explore the challenge of measuring passive intermodulation.

 

In today’s competitive wireless telecommunications industry, network providers constantly strive to achieve high levels of customer satisfaction and system capacity. Poor cell site performance can equate toreduced capacity and poor call quality, which means a loss of income.
One potential cause of poor network performance can be high Passive Intermodulation (IM) power levels at the base station. Providers can help improve network performance by carefully selecting components, such as base station antennas, with low IM responses.

What is passive IM?
Passive IM is similar to Active IM, except that it occurs exclusively in passive devices. Every passive RF device generates passive IM products when two or more frequencies are simultaneously present. When these signals encounter a non-linear device, junction or material, they combine to produce harmonics of the source frequencies—the passive IM products.
Typically, it is the odd-ordered IM products that prove most problematic—for example, the third order IM product (IM3). Should these IM products fall within the base station receive band, they appear to the receiver as interference. Once the passive IM power level rises above the noise floor of the receiver, the system carrier to interference ratio (C/I) becomes adversely impacted.
As passive IM products typically increase significantly with average transmit power level, the impact of passive IM on a base station may only become apparent as the base station approaches its fully loaded state. Just when the most capacity is needed, the passive IM level can rise up and interfere with normal base station operation. The result can be a receiver desensitisation that is independent of the receiver’s noise floor.
Although most wireless transmit and receive frequency bands are carefully selected to avoid landing the largest IM products within the receive band, higher order IM products—the fifth, seventh and ninth order—do land within some communication bands. More frequently, IM products from a nearby or co-located competitor’s site can become troublesome sources of interference.

What causes passive IM?
There are several component-based sources of passive IM and unfortunately their effect can be additive to the system’s intrinsic passive IM. Non-linear devices or materials present an ideal passive IM generation source. These can include: ferrous metals in RF paths; poorly connected or aligned parts; poor mechanical junctions; dissimilar metals in direct contact; poor quality or contaminated component plating and bad solder joints.
Component-level passive IM generation can be minimised during the component design and production stage by ensuring proper plating, good materials and avoiding ferrous content materials. Other factors such as the quality of connector alignment and torquing, cable connector assembly, and mechanical connections can be controlled during base station installation and maintenance. Despite the best attempts to minimise these effects, the deployment of poor quality components in the outside environment can lead to deterioration of the system’s passive IM performance.