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Precautions for designing high-power RF and microwave passive components

Views:6     Author:Site Editor     Publish Time: 2021-07-15      Origin:Internet

RF and microwave passive components bear the burden of many design constraints and performance indicators. According to the power requirements of the application, the requirements for materials and design performance can be significantly improved. For example, in high-power telecommunications and military radar/jamming applications, high performance levels and extremely high power levels are required. Many materials and technologies cannot withstand the power levels required for these applications, so specialized components, materials, and technologies must be used to meet these extreme application requirements.


High levels of radio frequency and microwave power are invisible, difficult to detect, and can generate incredible heat in a small area. Generally, overpower pressure can only be detected after component failure or complete system failure. This situation is often encountered in telecommunications and aerospace/defense applications, because high power levels of use and exposure are necessary to meet the performance requirements of these applications.


High enough RF and microwave power levels can damage components in the signal path, which may be the product of poor design, material aging/fatigue, or even strategic electronic attacks. Any critical system that may encounter high-power RF and microwave energy must be carefully designed and supported by components designated for the maximum potential power level. Other problems, such as RF leakage, passive intermodulation distortion, and harmonic distortion, are exacerbated at high power levels because more consideration must be given to the quality of the components.


Any interconnection or component with insertion loss may absorb enough RF and microwave energy to cause damage. This is why all RF and microwave components have maximum power ratings. Generally, since RF energy has several different operating modes, the rated power will be specified for continuous wave (CW) or pulsed power. In addition, since the various materials constituting the RF component can change the behavior of different power, temperature, voltage, current, and age, these parameters are usually specified. As always, some manufacturers are more generous with the specified functions of their components, so it is recommended to test specific components under actual operating conditions to avoid field failures. This is a particular concern for RF and microwave components, because cascading failures are common.



Coaxial or waveguide interconnection


Depending on frequency, power level and physical requirements, coaxial or waveguide interconnects are used for high-power RF and microwave applications. The size of these two technologies varies with frequency, requiring higher precision materials and manufacturing to handle higher power levels. Generally, as a product of the way RF energy passes through a waveguide with an air dielectric, waveguides tend to be able to handle higher power levels than comparable coaxial technologies. On the other hand, waveguides are usually more expensive than coaxial technology, custom installations and narrowband solutions.


This means that for applications that require lower cost, more flexible installation, higher signal routing density, and medium power levels, coaxial technology may be the first choice. In addition, due to the reduced cost and size, there are more options for using coaxial interconnection components on the waveguide interconnection. Although broadband and generally more straightforward installations, waveguide technology often surpasses coaxial in terms of high performance, robustness, and reliability. Usually, these interconnection technologies are used in series, where possible, the highest power and fidelity signals are routed through the waveguide interconnection.


An important feature of coaxial technology that needs to be noted is that their power and voltage-related dielectric breakdown is much lower than that of waveguide interconnects of similar frequencies. If weight and cost are of high concern, this may be acceptable. However, the problems of material outgassing and material performance changes under high temperature and high pressure may reduce the feasibility of coaxial technology in aerospace applications.



Adapter and terminal


Since each adapter and terminal will introduce unnecessary insertion loss and reflection, careful selection of the correct components can prevent unnecessary signal degradation and may be used for sensitive electronic equipment. The adapter and terminal have many forms, usually coaxial or waveguide. , For high power applications. In addition, the adapter may be more complicated because the size and type of either end of the adapter may be different. In addition, the adapter itself may introduce turns or bends.


The power and frequency range of the adapter must be carefully checked, especially if the adapter is waveguide to coaxial conversion. The waveguide naturally can only transmit the bandwidth of the frequency range with high signal fidelity, and the coaxial technology only has a cut-off frequency. However, different types of coaxial connectors also have different power and frequency capabilities. If the adapter is a transition between two different coaxial connector types, the frequency, power handling, PIM, insertion loss and other parameters will be affected.


The terminal bears the brunt of exhausting the potential extreme RF energy in the device. Generally, terminals for high-power applications will have heat-dissipating metal bodies and may force air thermal management. The impedance matching and voltage standing wave ratio (VSWR) of the terminal are absolutely critical, because unpredictable reflections can cause overpower and overvoltage conditions in upstream electronic equipment. In the case of shunting a high power amplifier (HPA) to a terminal that does not meet sufficient VSWR specifications, this may be dangerous because it may permanently damage the HPA.



Attenuator


Like the terminator, the attenuator is designed to dissipate RF energy within the body of the device without any unwanted signal distortion or reflections. There are fixed and variable attenuators. For most very high power applications, fixed attenuators are more common. Like terminator, they can be waveguide or coaxial. In addition, the attenuator can also be a coaxial connector size adapter of different sizes, although this is rarely done with a waveguide connector.


Depending on the amount of power dissipated by the attenuator design, metal radiators usually surround the body, and even forced cooling is an option. The higher the frequency, power handling and attenuation, the RF energy will be converted into heat. When installing the attenuator, it is important to ensure that the attenuator is adequately ventilated and not installed close to other heat-dissipating electronic devices.



filter


Since the filter can be used as a band-selective attenuator or a reflector for out-of-band signals, it is necessary to consider the type of upstream electronic equipment and the signal entering the filter. The absorption filter will absorb the RF energy from the out-of-band signal and convert it into heat. Among them, the reflection filter redirects the RF energy back to the source. This type of filter may damage sensitive upstream electronic equipment due to overpower or overvoltage. According to the filter technology and structure, the power handling capability of the filter is usually highly dependent on the frequency.


Like most RF and microwave components, higher frequency components have lower power thresholds than their lower power components. The relative size and material of the filter will have a significant impact on the power and frequency limitations. The passband of the filter naturally attenuates the signal slightly, so the passband characteristics are as important as the out-of-band filter characteristics in terms of RF energy absorption or reflection.



Directional coupler and power splitter/combiner


The directional coupler has many of the same concerns and constraints as the adapter, increasing the complexity of the built-in termination or forward/reverse coupling signal path. Moreover, the coupled signal path of the directional coupler is a few hundred, thousands or tens of thousands of times smaller than the RF energy passing through the main propagation line. Since the power level on the coupling line is significantly reduced, even for high-power waveguide couplers, the coupling line is usually a coaxial connector. This is obviously not the case for hybrid couplers or 3dB 90° hybrid couplers, which distribute the power of the signal evenly in two equal RF signal paths.


Generally, directional couplers are designed to have very low insertion loss and reflection. At high power levels, if not precisely designed, the coupling method can introduce significant insertion loss and reflections. Another factor to consider is the loading of the coupling line. Although at low power levels, a simple termination may be sufficient. However, at higher power levels, any mismatch or reflection may cause a large amount of power to be fed into the main signal path. Moreover, depending on the coupling strength, the terminal of the directional coupler may require higher power handling than its low-power counterpart.


Much like a directional coupler, a power splitter separates RF signal energy along multiple paths. Among them, the power combiner feeds the RF signal energy into a main path. The problems of insertion loss and reflection are roughly the same as power splitters/combiners because they are the same as directional couplers. The main difference is that power splitters/combiners are usually at roughly equal power levels, but not in phase. As a product of this, any impedance or VSWR mismatch in the connection or feeder line may cause undesirable signal degradation, phase deviation and reflections. Some power splitters/combiners have an input or output as a waveguide or coaxial connection, and the input and output use different connector sizes or technologies.



Passive intermodulation distortion in high-power passive devices


PIM has a significant impact on wireless network performance, especially for high-power radio frequency electronic equipment. Since PIM is usually difficult to determine in a complete passive device system, if PIM is a design issue, passive components with high precision and low PIM may be the first step to ensure a lower PIM threshold. Any non-linearity in the material or environmentally induced non-linearity can lead to high levels of PIM.


Whether it is surface defects, micro-cracks or connections of different materials, high power levels usually exacerbate the non-linear effects that lead to PIM. Since high-power applications are usually also associated with more extreme environments, temperature changes, vibration, and material aging can also cause non-linearity in PIM. To reduce the PIM response, each individual connection and component can be verified to operate with a reduced third-order intercept point, thereby reducing distortion. Through rigorous post-assembly test, PIM response can be confirmed after installation.



Thermal management challenges, life and material degradation


High power levels at high frequencies tend to cause RF energy dissipation in non-ideal surfaces and materials. Dissipation of RF energy to most surfaces causes heating. RF heating may cause material changes in peak power operation or material degradation within a few cycles of use.


It is understandable that the temperature and RF power level specifications of the equipment should be kept reasonable within a reasonable range. Since many manufacturers are very optimistic about the performance of their products, there are reasons to allow as much power and heat margin as possible under other design constraints. This is especially important in critical applications that cannot withstand downtime, because thermal stress can cause thermal runaway, which can lead to rapid equipment failure.


Other environmental factors, such as moisture ingress and shock/vibration, can also temporarily reduce the power and heat treatment capabilities of the components. Thorough testing of high-power components in salt spray, temperature and mechanical stress test benches is often used to verify component designs in extreme cases for certain applications.


Reference materials:

http://www.odyseus.nildram.co.uk/Systems_And_Devices_Files/Component%20Reliability%20Tutorial.pdf

http://www.asc-i.com/pdf/Thermal_Management_for_Power_Electronics.pdf

https://mp.weixin.qq.com/s?src=11&timestamp=1626318298&ver=3191&signature=wtrTtzf*3jsQO64zqJwG1s0NDqkHnJChSALhdS4sdULW87Rrj*VoMDPSvxv76OWCeKRRTO-vc9SyTpgv76OWCeKRRTO-vc9SyTpgv76OWCeKRRTO-vc3XAeJc9UQF


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