Passive Intermodulation (PIM) is becoming more and more important as wireless users demand data, with high speeds. It affects network speed and performance, therefore, needs to be monitored, making PIM testing a basic requirement especially today.
What is PIM?
PIM occurs when two or more high power RF signals encounter non-linear electrical junctions or materials in an RF path. These non-linear junctions behave like a mixer causing new signals to be generated at mathematical combinations of the original RF inputs. When these PIM signals fall in the Rx band of the cell site the noise floor rises causing increased dropped calls, reduced data transmission rates and decreased system capacity. The impact of PIM on the network performance can be severe, especially for systems such as CDMA, UMTS or LTE. In the field, according to Kaelus, causes of non-linear junctions include:
• Contaminated surfaces or contacts due to dirt, dust, moisture or oxidation.
• Loose mechanical junctions due to inadequate torque, poor alignment or poorly prepared contact surfaces.
• Loose mechanical junctions caused transportation shock or vibration .
• Metal flakes or shavings inside RF connections.
• Poorly prepared RF connections
– Trapped dielectric materials (adhesives, foam, etc.)
– Cracks or distortions at the end of the outer conductor of coaxial cables caused by over tightening the back nut during
installation.
– Solid inner conductors distorted in the preparation process causing these to be out of round or tapered over the mating
length.
– Hollow inner conductors excessively enlarged or made oval during the preparation process.
• Nearby metallic objects in the main beam and side lobes of the transmit antenna including roof flashing, vent pipes, guy wires, etc. Because of all these scenarios and opportunities for service to be affected, its important for PIM to be tested. [1]
How to test PIM
Because today’s mobile handset users expect nothing less than the best when it comes to consistent high throughput from their devices, and because therefore, current networks are pushed to the limit, PIM testing has become increasingly important and will continue to do so. Because fourth generation (4G)/Long Term Evolution (LTE) networks feature an increased mobile data rate of 100 Mb/s, this higher transmission rate will expose PIM vulnerabilities in today’s networks on a much bigger scale. Fourth generation networks require superior network transmission fidelity, higher than previous generations. Network operators also face the challenge of maintaining customer loyalty in an unforgiving competitive arena. As such, good network PIM performance and PIM testing are now imperative.
PIM signals exist as a result of the combined transmission of multiple carrier frequencies within a transmission line path. The goal is to make sure that these levels occur at an amplitude which is below the base stations receiver sensitivity. The amplitude of these undesired signals is directly influenced by the fidelity of the transmission line path, including all components and junctions that can introduce a non-linear effect to the signals passing through them.
High Power PIM Testing
To mimic real-traffic network conditions, PIM testing measurements must be made at the base station radio power level or higher. PIM tests performed at low power can mask the presence of passive intermodulation in the network. “The ability to exceed the current 20W specification gives operators much more measurement confidence and with UMTS and LTE carrier power levels likely to be specified at 40W the ability for PIM testing at these levels will become mandatory.” [2]
PIM Testing Equipment
PIM testing equipment enables network operators to find and eliminate non-linear junctions at the cell site in order to improve site performance. The testing equipment transmits two high power signals into the line or device under test. If the test signals run into a non-linear junction, mixing occurs causing the PIM frequencies to be generated. The PIM signals travel in all directions from the point of generation. “In a coaxial system this means they travel out toward the antenna as well as back in the direction of the PIM test equipment. The PIM test equipment measures the magnitude of the PIM signal generated by the test signals and displays this information to the test operator.”
A product, known as the 3rd order product (IM3), is used to characterize PIM performance both in the factory and in the field. The IM3 signal generated by a non-linear junction is usually higher magnitude than the other PIM products enabling higher measurement accuracy. The higher order products (IM5, IM7, IM9, etc.) typically drop in magnitude by 5 to 10 dB for each successive PIM product. By controlling IM3 to a specified level, the higher order products are held well below that level, often by 10’s of dB’s. [1]
[1] www.kaelus.com, Passive Intermodulation (PIM) Testing Guidelines, http://www.kaelus.com/Kaelus/media/Site/QR%20Code%20Files/PIM-Testing-Guidelines_Brochure.pdf
[2] Electro Rent Europe, What is PIM Testing? http://www.electrorent-europe.com/pim-test/what-is-pim-testing.aspx
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