How to perform on-site PIM testing on RF connectors?

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How to perform on-site PIM testing on RF connectors? [Copy link]

PIM in RF connectors is a form of intermodulation distortion that occurs in components that are generally considered linear, such as cables, connectors, and antennas. Field PIM testing is a comprehensive measure of linearity and construction quality.

When subjected to the high RF powers found in cellular systems, these devices can generate intermodulation signals of -80dBm or higher.

Figure 1. Carrier f1 and F2 used 3 to 7 times.

Intermodulation signals are generated later in the signal path, they cannot be filtered out, and can cause more harm than stronger but filtered IM products from active components.

On-site PIM testing is a comprehensive measure of linearity and construction quality.

PIM appears as a set of unwanted signals resulting from the mixing of two or more strong RF signals in a nonlinear device, such as a loose or corroded connector or nearby rust. Other names for PIM include the diode effect and the rusty bolt effect.

Figure 2. PIM bandwidth increases with product order.

This pair of formulas can predict the PIM frequencies of the two carriers:

F1 and F2 are carrier frequencies, and constants n and m are positive integers.

When referring to PIM products, the sum of n + m is called the product order, so if m is 2 and n is 1, the result is called a third-order product (Figure 1). Typically, third-order products are the strongest and most damaging, followed by fifth- and seventh-order products. Because PIM amplitudes decrease as orders increase, higher-order products are usually not large enough to cause direct frequency problems, but they often contribute to raising the adjacent noise floor (Figure 2).

It is unlikely that 3rd order products will fall directly into the designed cellular receive band. It is likely that energy from other external transmissions will mix within the nonlinear transmission lines, resulting in many smaller PIM levels that mix repeatedly, resulting in a broadband elevated noise floor, typically spanning all of the operator's licensed spectrum. Once this elevated noise floor enters the Rx band, it opens a door (sometimes obtained via an LNA) into the BTS.

IM from the modulating signal

Intermodulation products from continuous wave (CW) signals, such as might be produced by a PIM tester, appear as single frequency CW products. When discovering PIM produced from a modulated carrier, the kind of failure you might see appearing on a live signal, it is important to know that intermodulation produced by a modulated signal requires more bandwidth than the fundamental signal. For example, if both fundamentals are 1 MHz wide, the third order product will have a 3 MHz bandwidth, the fifth order product, 5 MHz bandwidth, and so on. PIM products can be very broadband, covering a wide frequency band.

Figure 3. PIM causing receiver to lose meaning at 1710 MHz

Figure 4. PIM causing receiver loss of meaning at 910 MHz.

By overlaying spread spectrum signals into the current site infrastructure, mixing a 3 channel UMTS transmission with a 10 MHz LTE (assuming 10 MHz instead of 20 MHz!) would be disastrous due to transmission system linearity issues. In theory, this could create a 3rd order product with a bandwidth of over 30 MHz, and that does not include any effects introduced by the 5th and 7th orders. It would be an interesting experiment to document the 100 MHz+ noise issues that are definitely there.

PIM Calculation Example

Here are two PIM examples; one from the 850 MHz band and one from the 1900 MHz band. In the first example, 1750 MHz is one of the third-order products and falls within the AWS-1 base station receive band. If the 1940 and 2130 MHz carrier sources were physically close to each other, or even sharing the same antenna, any corrosion or other nonlinear effects would create a third-order passive intermodulation product at 1710 MHz that could cause reception failure or blocking. It’s worth mentioning that PIM products don’t need to fall directly on the uplink channel to cause problems. They just need to be within the receiver’s pre-filter, which is typically as wide as the network operator’s licensed bandwidth.

An example of PIM in the widely used 900 MHz band assumes two GSM carriers, one at 935 MHz and the other at 960 MHz. In this case, the 910 MHz third-order product is in the base station receive band (Figure 4).

The calculations so far have assumed that only two carriers are present. This is not always the case in the real world. At the base station, not only do the carriers within the antenna system need to be considered, but also stronger signals from nearby transmitters. The signal can feed back into the antenna system, find nonlinear devices, mix with other carriers, and create PIM. This problem is rapidly compounded when highly complex modulation platforms are used; something that is already very apparent in the cellular realm even when using relatively narrow bandwidths.

When three or more carriers are involved, the calculations quickly become complex. There are programs and spreadsheets available online to help with this task. If possible, a quick alternative is to shut down one transmitter at a time to find out which carriers and antenna operations are contributing to the PIM. This can greatly simplify the calculation and troubleshooting tasks.

A PIM-like effect can also be caused by the periodic breakdown of the insulating film between the mating surfaces of the connector. Corrosion or foreign deposits and their effects can cause this insulating material to appear over time. The disturbances caused by this mechanism are broadband and bursty in nature, with occurrence rates ranging from infrequent to two or three times per second. This effect is caused by micro-arcing or sintering and can be found through PIM testing.