24 Apr 2012

Locating Passive Intermodulation, PIM Faults

Paul Holes, RF and Microwave Field Applications Engineer, Anritsu looks reducing the cost of repairing passive intermodulation, PIM faults in cellular base stations

Today’s cellular communications systems use an advanced architecture for transmission and reception which is more sensitive than ever before, and therefore more vulnerable to interference.

This requirement for sensitivity – and the dangers of interference consequent on it – will only grow as wide-bandwidth 3G and 4G networks are built out. In particular, when high level RF signals are applied to non-linear junctions they can become active and produce spurious emissions such as harmonics, intermodulation products and broad-band noise.

These spurious emissions can interfere with receiver noise floors. Corroded, dirty or mechanically compromised connections or components will cause two high-power signals to mix and produce passive intermodulation, PIM mixing product. Unfortunately, when running test routines to measure PIM interference, the frequencies of these products can be calculated, but the source is not so easily found.

This causes a problem for the network operator: how to repair a fault that cannot be located? The current practice is to connect and disconnect in turn suspected sources of PIM within the system until the correct one is found, or even to completely re-construct a site. This is both time-consuming and expensive to execute, and potentially contributes additional PIM products as connections and components are overworked during the test process.

The nature of PIM

Intermodulation is the mixing of two or more signals of different frequencies, which in turn produces additional signals on other frequencies. These additional signals are mathematically related to the original signals (see Figure 1). Passive intermodulation can form wherever two or more signals encounter a non-linearity, such as a faulty connector. Most cellular transmit antennas carry at least two signals, and any such base station is therefore at risk of generating PIM.

PIM Table PIM Spectrum

Figure 1: Formula for calculating the frequency of PIM products, and a graph showing an example for a base station transmitting two signals at 925MHz and 960MHz

The generation of spurious emissions through PIM effects can be harmless if they do not interfere with other local transmissions. Practical problems arise, however, when these spurious emissions – generated by high-power transmit signals – fall at a frequency used by the system’s receiver. And this is exactly what is happening today in European 3G cellular telephone systems.

In older 2G systems, network operators have great flexibility to choose widely spaced transmit and receive frequencies; using the formulae in Figure 1, the operator can select combinations of Tx and Rx frequencies to ensure that higher-order PIM products will not be at an Rx frequency.

European carriers are now, however, installing 3G systems at base stations to operate at the 900MHz band. Regulatory restrictions mean that carriers only have a narrow range of frequencies in which to squeeze both Tx and Rx channels. As a result, there is no way for carriers to prevent the PIM products generated by these 3G systems from interfering with, and often completely overwhelming, the low-power incoming signals from users’ handsets and other terminal equipment.

The potential damage to the mobile carriers’ business is large: to the user the problem is both baffling and infuriating, as their handset might show they have a strong (incoming) signal, and yet their handset suffers from poor audio quality, a slow data rate or dropped or blocked calls. The proximate effect is the loss of revenue each time a call is dropped or a data session fails. But more worryingly, users tend to report bad experiences to family, friends and acquaintances, and the effect of the consequent bad reputation locally can lead to a large loss of market share to any rival carrier which does not suffer from PIM problems.

In any communications system in which the signals are modulated with a frequency or phase modulation component, the width of the modulation spreads on the intermodulation products, increasing the likelihood of interference in other receive bands (see Figure 2).

PIM Spectrum

Fig. 2: the frequency spread affected by intermodulation products widens in systems using frequency modulation or phase modulation

A high level of noise (both in bandwidth and amplitude) can be generated when more than one spread-spectrum signal passes through a non-linear connection.

  • A base station may show an Rx/Main Diversity alarm or high RSSI

  • Handsets are forced to raise their Tx power in an effort to exceed a high noise floor

  • Coverage area shrinks, causing early handoff which then overloads neighbouring cell sites

  • If dead zones appear as a result of the receding coverage, calls are dropped

In summary, passive intermodulation is of particular concern when:

  • Passive intermodulation, PIM products fall in the Rx band

  • Two or more transmitter channels share a common antenna

  • Tx signal levels are high

  • Rx sensitivity is high

  • Tx and Rx are diplexed

The causes of PIM

Loose connections are the most common cause of a PIM failure, and those loose connections come about from hurrying the installation, using the wrong tools or installing incorrect or defective parts (such as impaired connectors).

Connectors are not the only source of PIM products, however. Other components such as cables, antennas, lightning protectors, diplexers, current injectors and tower-mounted amplifiers can also produce PIM. Particular attention needs to be paid to cables, ensuring that they are not scratched or scored, or bent near a connector.

Interestingly, even objects beyond the antenna can cause PIM products. Cellular base stations have been known to experience PIM failures attributable to:

  • the rusting of a metal fence surrounding the base station

  • reflections from a nearby building erected since the original installation of the system. (Base station installers know to position antennas to avoid reflections from buildings, but new construction can interfere with an existing base station’s transmissions.)

How is PIM measured?

PIM Test Setup

Fig. 3: system diagram of a typical PIM test set-up

The basic elements of a PIM test instrument are shown in Figure 3. Since PIM products are only generated when there are two (or more) signals present on a transmission line, a PIM test instrument needs two signal generators. The signal generator outputs must be amplified to the levels found in a base station. The signals are then combined, filtered, and fed into the device under test (DUT), such as a base station cable.

At this point, the test instrument may measure the PIM signal either as a reflection or a transmission, since the PIM product in the DUT is propagated in both the forward and reverse directions. The advantage of the transmitted method is that the measured value will be relatively flat over the frequency range, so sweeping is not necessary. By contrast, reflections phase in and out over a relatively long cable assembly. This necessitates sweeping in order to confirm that the instrument is measuring the maximum PIM in the test frequency range.

The disadvantage of the transmitted method is that it requires a second duplexer (which adds cost) and is prone to reflecting harmonics of the incident signal, since it is not matched at the higher frequencies at which the harmonics are generated.

After the reflected or transmitted PIM signal is captured, it is fed through a bandpass filter into a low noise amplifier (LNA) and measured on a spectrum analyser.

A typical such test set-up can measure with good accuracy the amplitude of PIM products, but it cannot give any indication of the source (location) of the PIM. Other drawbacks of typical PIM test set-ups include:

  • PIM generated in an external source, such as a rusty fence, appears as PIM in the system under test

  • PIM from a mechanical source on the tower appears as PIM in the system under test

  • interpreting uplink noise from outdoor BDAs (cell repeaters) as PIM

An improved method for testing and locating PIM sources

Anritsu’s new PIM Master tester has been introduced to overcome the drawbacks of the current generation of PIM testers. The PIM Master is a two-tone carrier-wave signal generator with a high-power (40W) output for use in testing tall masts. High-Q filters and duplexers provide better selectivity, reducing false positives, and the use of a high sensitivity spectrum analyser improves accuracy by avoiding false positives due to interference and noise in the uplink.

Crucially, the instrument offers patented Distance-To-PIM technology which reveals the source of PIM products (see Figures 4 and 5). The PIM Master measures the time difference between transmitting the test signal and receiving the incoming PIM product. By using known data about the cable’s characteristics, the tester can calculate precisely the distance between the tester and the source of the PIM products.

PIM Spectrum

Fig. 4: plot showing a cable with acceptable PIM at 1.45ft from the PIM Master tester

PIM Spectrum

Fig. 5: plot showing excessive PIM at 39.64ft from the PIM Master tester

The PIM Master’s STOP distance should be set at a length equal to the actual cable length plus 50ft. The instrument can measure past the antenna to detect PIM problems on the tower or rooftop and beyond. Setting the STOP distance longer than the actual cable enables it to test the antenna (which adds several feet to the cable length) and the surrounding area.

Conclusion

Distance-To-PIM measurement enables network operators to fix base station malfunctions much more quickly and at much lower cost. Where before, technicians had to slowly check each cable and component in a base station in turn and re-test until they found the problem, using the new PIM Master from Anritsu with Distance-To-PIM technology they can go straight to the source of the PIM product and fix it quickly and efficiently.

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About the author

Paul Holes is an RF and Microwave Field Applications Engineer, Anritsu. He has just completed 20 years service at Anritsu. Before that he was an apprentice at Marconi Instruments and went on to gain an HND in Telecommunications at University of Hertfordshire. He lives in Dorset, with his wife and 4 children.

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