Passive Intermodulation (PIM) occurs when two or more high power RF signals encounters PIM sources or materials in RF Path. These PIM sources behave like a mixer causing new signals to be generated at mathematical combinations of the original RF inputs. In the figure 1, f1 and f2 represent two transmitter frequencies present at typical mobile site and ±mf1±nf2 are the PIM signals generated by transmitter frequencies.
When the PIM signal fall into the Rx band of mobile phone site, the noise floor rises causing increased dropped calls, reduced data rate transmissions and inferior mobile network capacity. The PIM causes poor mobile network quality in modern mobile phone systems such as LTE, LTE Advanced, UMTS and CDMA. But, what is PIM sources?
The primary sources of PIM at cell sites are inconsistent metal to metal contacts in high current density regions such as inside transmission lines, inside RF components or outside the system but in the main beam of antenna. Clean metal surface with high contact pressure generally behave like a linear manner and do not generate PIM. When there is loose contact between metal surface, a nonlinear relation develops between applied voltage and resulting current flow across the junction and PIM to be generated. But as mobile sites are installed in filed, PIM can cause via multiple sources,
- Contaminated surfaces and contacts due to dirt, dust and moisture, oxidation
- Loose mechanical junctions due to improper installations, poor alignment or poorly prepared contact surfaces
- Loose mechanical junctions caused by transportation shock and vibration
- Poorly prepared RF connections
- Trapped dielectric materials
- Cracks or distortion at the end of coaxial cables caused by tightening the back-nut installation
- Hollow inner conductors
- Nearby metallic body in the main beam and side lobe of the transmit antenna such as roof flashing, vent pipes, additional nearby wires
PIM Test equipment works by transmitting two high power signals into the line or device under test. If the test signals encounter a nonlinear junction mixing occurs causing PIM frequencies to be generated. The PIM signals travels in all directions from the point of generation. In a coaxial system, this means they travel out toward the antenna and back towards the PIM test equipment. The PIM signal generated is displayed at the screen and shared with the mobile operator.
The third order product IM3 is primarily used to characterize PIM performance. The IM3 signal generated by a PIM source is usually higher the magnitude than the other PIM products enabling the higher measurement accuracy. The higher order products IM5, IM7, IM9 typically drop in magnitude by 5 to 10 dB for each successive PIM products. By controlling IM3 to a specified level, the higher order products are held below the levels.
The magnitude of the PIM signal generated by a given defect is highly dependent on the power level of the two RF signals interacting with that defect. A 3rd order PIM product, for example, will change between 2.2 and 3.0 dB in magnitude for every 1 dB change in test power. As shown in the figure below, this rate of change, or PIM slope, remains fairly constant across a wide range of applied Tx power levels.
Because the magnitude of PIM generated by a defect changes depending on the applied test power, it is important that all specifications clearly state what power level to use when performing the test. IEC 62037 recommends that +43dBm (20W) test tones be used when performing PIM tests on communications systems. Using a common test power across the industry both in the factory and in the field, provides a consistent method to characterize and compare PIM performance to a common specification.
Low power, battery operated test sets are available for testing in locations where AC power is not available or where transporting a 20W test set is not feasible. These low power systems are useful for finding gross PIM failures in components and are useful for troubleshooting sites to determine the location of PIM problems. Absolute results may vary, however, when compared to 20W test results due to differences in measurement accuracy (S/N ratio) between low power testing and 20W testing.
PIM Measurement Frequency
PIM test equipment is used to find and eliminate PIM sources at the cell site. The specific test frequencies used to identify these defects are not critical as long as the following criteria are met:
- All RF components in the path (cables, antennas etc) must be able to pass the two set of frequencies , f1 and f2 and able to pass the IM frequency you are measuring.
- The two test frequencies must be within the operator’s licensed spectrum or be guard band frequencies between licensed spectrum blocks. This applies to all system level tests where the test frequencies will be broadcast through the antenna
- The two test frequencies need to be selected so that they will produce the specified IM product within the receive band for that system. When IM3 is specified this will typically require test tones with wider frequency spacing than can be achieved within the licensed frequency block for a given market. For this reason, guard band frequencies will typically be selected.
- Some test equipment provides the ability to sweep the two transmit frequencies across a range of frequencies during the test. Swept frequency modes will transmit frequencies outside of the operator’s licensed spectrum. For this reason, swept frequency modes should only be used to test systems terminated into a low PIM load.
Dynamic Vs Static PIM Testing
During a PIM test, all components and interconnections on the line should be subjected to light mechanical stress or “dynamic” test conditions. If the component or RF connector has loose internal connections or internal debris that can result in increased PIM (as identified during dynamic testing) there is a high probability that the condition will present itself in the future and invariably at the most inconvenient time.
Tap Test Guidelines
- Tap RF components such as TMA’s, Filters and Antennas using a hard plastic or rubberized metal object to prevent nicking or damaging protective finishes
- Lightly tap the nut and/or back shell of RF connectors using a hard plastic or metal object. Do not tap the coaxial cable itself as this could cause dents in the line
- Tap with sufficient force to excite PIM problems if they exist but do not tap with excessive force. A good rule of thumb is that if you tapped your unprotected palm with the same force, it should not hurt
- Tap before weatherproofing is installed on RF interconnections. If weatherproofing is in place, substitute a “flex test” to apply stress to the interconnection
Flex Test Guidelines
- For stiff cables, rock the RF connector back & forth in two orthogonal directions while holding the cable rigid
- For flexible cables, hold the RF connector rigid and flex the cable back & forth in two orthogonal directions. Hold the cable approximately 12 inches (300mm) away from the connector and flex the cable ±1 inch (25mm) in each direction
Antenna PIM Measurement
- Do not PIM test antennas indoors. (Unless an anechoic chamber is available designed to absorb the RF energy)
- Place the antenna on non-metallic supports at least 1 foot (300mm) off the ground with the front of the antenna pointing to the sky
- Position the test equipment to the top or bottom of the antenna (not to the sides.) Typical sector antennas have minimum radiation at their top and bottom ends
- Position the antenna so that no metallic objects obstruct the view of the sky within the antenna’s half-power beam widths (both azimuth and elevation)
The iPA from Kaelus 1800 MHz Series Passive Intermodulation (PIM) analyzer is the first battery powered PIM Test Analyzer versatile enough to support multiple test scenarios such as testing at the top of the tower, base of tower, roof top and in-building for DAS systems. This IEC compliant 20W, rugged, battery operated design includes a tablet computer in a ruggedized case for remote control. This allows hands-free dynamic testing that is safe and convenient. Add the optional Range to Fault (RTF) module to quickly identify the location of PIM and Return Loss sources. Evolved from a design legacy of field proven analysers, this PIM Analyzer enables network operators to improve site performance by finding and eliminating sources of passive intermodulation at the cell site. To learn more, please visit the web page: https://www.microlease.com/in/products/Kaelus/Cable-Antenna–PIM-Analyzers/IPA?BaseModelId=97423
– Communications Today Bureau