In the previous paragraph to help a colleague complete the leak detection project of the headquarters company, an interesting thing happened. When the functional requirements of the entire module and the requirements of the conventional RF indicators are met, the European headquarters has also proposed an indicator that passive intermodulation is better than -110 dBm.
Silly. There is no concept at all. The term was used before my master's second grade, and it was just an impression! Check it, first of all two paragraphs of text, of course, these are selected from Baidu Encyclopedia.
"In wireless communication systems, passive intermodulation becomes an important factor in limiting system capacity as the voice and data information that needs to pass through a fixed bandwidth increases. As in active devices, two or more The mixing of frequencies in nonlinear devices produces spurious signals—passive intermodulation. When the spurious intermodulation signal falls within the base station's receive band, the receiver's sensitivity decreases, resulting in call quality or system The reduction of the carrier-to-interference ratio (C/I) and the reduction in the capacity of the communication system.
Positive Intermodulation Measurement Diagram
Reverse intermodulation measurement diagram
Passive intermodulation is caused by a number of factors, including: poor mechanical contact, magnetic conductors contained in the RF channel, and contamination of the RF conduction surface. Although it is difficult to accurately predict the passive intermodulation level of a device, we can use the measured data to characterize the device. Because the performance of passive intermodulation changes with minor changes in the structure, manufacturers should perform 100% inspection of the RF devices used in the base station to ensure that the passive intermodulation of the device is always within the acceptable range. "- Baidu "passive intermodulation"
"Passive Inter Modulation (PIM) effect is a type of HPM (High Power Microwave) effect. Under HPM conditions, HPM enters electronic systems through different coupling paths. Due to its high power characteristics, Making traditional passive linear devices generate strong nonlinear effects, the nonlinear characteristics of components and systems will become more apparent, leading to more serious PIM problems, and then affecting the performance of the entire system. This makes the PIM effect Analysis and research are particularly important.
Reverse intermodulation measurement of passive devices
Measurement of PIM is an important research direction in the PIM problem. On the one hand, the measured data can be used to predict high-order PIM. On the other hand, experimental data is provided for studying the PIM mechanism of passive components.
The characteristics of the measurement system: In principle, the measurement method of PIM is similar to that of the active component, but due to the special nature of PIM, the structure of the measurement system is more complex and the requirements are higher. In general, PIM measurement systems should have the following characteristics:
(1) High-power signal source: PIM measurement is a high-power measurement problem. Generally, it needs to measure at a power 2 to 4 times higher than the working power level, and the microwave power can be as high as hundreds of watts or even several kilowatts.
(2) High-sensitivity receiver Since the power level of PIM is generally very low, the sensitivity of the measurement system is very high.
(3) Low PIM components: The components of the PIM test system must be high performance and low PIM. The PIM level generated by a dedicated synthesizer, directional coupler, filter, etc. must be controlled below the -6dB level of the PIM level of the device under test. Even the matching load must use a special load that does not generate PIM to ensure that the entire test system can normal work.
(4) PIMP (Passive Inter Modulation Product, passive intermodulation product) is related to the ambient temperature and changes over time, so a long-term temperature cycling test is required.
(5) In addition, the correlation between the PIM measurement system and the frequency and bandwidth is very strong, and the measurement system is difficult to use universally. Generally, it needs to be specially produced according to the purpose of testing. At the same time, it is not only necessary to measure PIM products of passive components but also to measure antennas and whole stars. Therefore, how to design a low PIM measurement system is the first problem that must be solved for PIM measurement. "- Baidu "PIM"
It's long, but it's clear. Its causes and solutions. And it can be seen that there are similarities with the three- and five-order interactive tuning of active devices.
This kind of thing, of course, can be detailed to check the reasons for the third-order. The mathematical expression produced by the third order describes where the two frequencies of f' and f' appear to be third-order and where testing is required. The test suggestions and methods that the old engineer gave me were the same as the test power amplifiers. They were inevitably inaccurate. At present, there are no good methods.
Leave some current experience:
1. The PCB level should be grounded on both sides of the microstrip line of the RF board. It is best not to simply select a single line instead of a top layer. The test result will improve PIM at the top level.
2. The grounding of the bottom layer is good during installation, and the SMA must be firmly and firmly fixed.
3. In the board, the microstrip line needs the electric capacity, try to use the small Q value, its frequency effect is slightly better, can't think of the colleague and I say, Murata's LLL series broadband electric capacity is not very satisfactory in use.
4. The temperature and test environment have a great influence on the test of PIM. The jump of PIM is very severe. Therefore, the worst point can only be taken as a record point.
DVB Antenna,DVB-T1 Antenna,DVB T2 Antenna,DVB T2 Antenna Outdoor
Yetnorson Antenna Co., Ltd. , https://www.xhlantenna.com