Views:2 Author:Site Editor Publish Time: 2021-03-26 Origin:Site
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. However, when affected by the high RF power found in cellular systems, these devices can generate intermodulation signals of -80dBm or higher.
Figure 1. Carrier f1, and use 3 F2 times to 7 times.
Intermodulation signals are generated late in the signal path, they cannot be filtered out, and may cause greater harm than stronger but filtered IM products from active components.
On-site PIM testing is a comprehensive measure of linearity and construction quality.
PIM is shown as a set of unwanted signals produced by mixing two or more strong RF signals in a non-linear device, such as loose or corroded connectors or nearby rust. Other names for PIM include the diode effect and the rusty bolt effect.
Figure 2. PIM bandwidth increases with the order of products.
This pair of formulas can predict the PIM frequency of the two carriers:
F1 and F2 are carrier frequencies, and the constants n and m are positive integers.
When referring to PIM products, the sum of n + m is called a product order, so if m is 2 and n is 1, the result is called a third-order product (Figure 1). Under normal circumstances, the third-order products are the strongest and cause the most harm, followed by the fifth-order and seventh-order products. As the PIM amplitude becomes lower when orders increase, high-end products are usually not enough to cause direct frequency problems, but they usually help increase the adjacent noise floor (Figure 2).
Tier 3 products are unlikely to fall directly into the designed cellular receiving frequency band. It is likely that energy from other external transmissions will be mixed in the nonlinear transmission line, causing many smaller PIM levels to be repeatedly mixed, resulting in an increased noise floor for broadband, which usually spans all operator-licensed spectrum. Once this elevated noise floor enters the Rx band, it opens a door (sometimes obtained through the LNA) and enters the BTS.
IM from modulated signal
Intermodulation products from continuous wave (CW) signals (such as may be produced by a PIM tester) appear as single-frequency CW products. When discovering the PIM generated from the modulated carrier, you may see the kind of failure that appears in the real-time signal. It is important to know that the intermodulation generated by the modulated signal requires more bandwidth than the basic signal. For example, if both fundamental waves are 1 MHz wide, the third-order product will have 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 that causes the receiver to lose meaning at 1710 MHz
Figure 4. PIM causing the receiver to lose meaning at 910 MHz
By superimposing the spread spectrum signal into the current site infrastructure, it would be disastrous to mix 3-channel UMTS transmission with 10 MHz LTE (assuming 10 MHz instead of 20 MHz!) due to the linearity of the transmission system. In theory, this can create a third-order product with a bandwidth of more than 30 MHz, which does not include any effects introduced by the fifth and seventh stages. It will be an interesting experiment to record 100 MHz + noise problem definitely exists.
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 belongs to the AWS-1 base station receiving frequency band. If the 1940 and 2130 MHz carrier sources are physically close to each other, or even share the same antenna, any corrosion or other nonlinear effects will produce 1710 MHz third-order passive intermodulation products, which may cause reception failure or blocking. It is worth mentioning that PIM products do not need to fall directly on the uplink channel to cause problems. They only need to be within the receiver's pre-filter, which is usually as wide as the network operator's licensed bandwidth.
The widely used PIM example for the 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 located in the base station receiving frequency band (Figure 4).
The calculations so far have assumed that only two carriers exist. This is not always the case in the real world. At the base station, not only the carrier in the antenna system needs to be considered, but also the stronger signal from nearby transmitters. The signal can be fed back to the antenna system, find non-linear devices, mix with other carriers, and create PIM. This problem quickly becomes complicated when using highly complex modulation platforms; even when using relatively narrow bandwidths, things are already very obvious in the cellular field.
When three or more carriers are involved, the calculation quickly becomes complicated. There are programs and spreadsheets available online to help accomplish this task. If possible, a quick alternative is to turn off one transmitter at a time to find out which carriers and antenna operations contribute to PIM. This can greatly simplify calculation and troubleshooting tasks.
PIM-like effects can also be caused by 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 interference caused by this mechanism is broadband and bursty in nature, with a frequency from infrequent to two to three times per second. This effect is caused by micro-arc or sintering and can be found by PIM testing.
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Superlink(SLK) produces as many as 100,000 interconnection products, including: RF connectors, adapters, high-speed cable assemblies, coaxial cables, transient electromagnetic pulse protectors, test probes, industrial and military circular connectors, custom wiring harnesses, etc.
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