Parent Category: 2015 HFE
By Mark Blackwood
RF and Microwave passive components bear a burden of many design constraints and performance metrics. Depending upon the power requirements of the application, the demands on the material and design performance can significantly increase. For example, in high power telecommunications and military radar/jamming applications, high performance levels are desired alongside extremely high power levels. Many materials and technologies cannot withstand the power levels these applications demand, so specialized components, materials, and techniques must be used to meet these extreme application requirements.
High levels of RF and Microwave power are invisible, challenging to detect, and capable of producing incredible amounts of heat in a small area. Often, over power stress is only detected after a component failure, or complete system failure. Such a scenario is commonly encountered in the telecommunications and aerospace/defense applications, as the use and exposure of high power levels are necessary to meet these applications performance demands.
Figure 1 • For either weather or military radar, high power amplifiers often generate hundreds to thousands of watts of RF energy to the radar antennas or antenna arrays. Source: http://www.erh.noaa.gov/iln/research/ThunderstormProject/TSP.php.
RF and Microwave power levels high enough to damage the components in the signal path can be a product of poor design, material aging/fatigue, or even strategic electronic attack. Any critical system that may encounter high power RF and Microwave energy must be carefully designed and bolstered by components specified for the maximum potential power levels. Other concerns, such as RF leakage, passive intermodulation distortion, and harmonic distortion are exacerbated at high power levels where greater consideration must be placed on the quality of components.
Any interconnect or component with insertion loss has the potential to absorb enough RF and Microwave energy to risk damage. This is why all RF and Microwave components have a maximum power rating. Often, as there are several different operating modes for RF energy, the power ratings will be specified for either continuous wave (CW) or pulsed power. Additionally, as the various materials that make up RF components may change behavior at different power, temperature, voltage, current, and ages, these parameters are also often specified. As always, some manufacturers are more generous with their components’ specified capabilities, so testing of a particular component under actual operating conditions is recommended to avoid in-field failures. Which is of particular concern with RF and Microwave components, as cascade failures are common.
Figure 2 • Waveguides can be tapped with either a magnetic loop or electric field probe to convert the TE or TM waveguide mode to a TEM coaxial transmission mode. Source: http://www.microwaves101.com/encyclopedias/waveguide-to-coax-transitions.
Coax or Waveguide Interconnect?
Depending upon the frequencies, power levels, and physical requirements, either coaxial or waveguide interconnects are used for high power RF and Microwave applications. Both technologies vary in size as a function of frequency and require higher-precision materials and manufacture to handle higher power levels. Generally, as a product of the way the RF energy is carried 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 a more expensive, custom installed, and a narrower-band solution than coaxial technologies.
This being said, for applications that require lower cost, higher flexibility installation, greater signal routing density, and medium power levels, coaxial technologies may be the preferred choice. Additionally, there are a greater selection of components that use coaxial interconnect over waveguide interconnect due to reduced cost and size. Though broadband and generally a more straightforward installation, where high performance, ruggedness, and reliability are concerned, waveguide technologies tend to exceed coaxial. Often, these interconnect technologies are used in tandem, with the highest power and fidelity signals routed through waveguide interconnect, when possible.
Figure 3 • After an attenuator the coaxial connector type may be able to be reduced in size and cost as the signal power levels after attenuation may be low enough to avoid damaging a smaller coaxial connector. Source: http://www.pasternack.com/50db-fixed-sma-male-7-16-female-25-watts-attenuator-pe7355-50-p.aspx
An important feature to note with coaxial technologies is that their power and voltage related dielectric breakdown is much lower than waveguide interconnect at similar frequencies. This may be acceptable if weight and cost are high concerns. Though, issues with material outgassing and material performance changes at high temperatures and pressures may reduce coaxial technologies viability in aerospace applications.
Adapters and Terminations
As every adapter and termination can introduce unwanted insertion loss and reflections, carefully choosing the right component can prevent unwanted signal degradation and potential damage to sensitive electronics. Adapters and terminations come in many forms, generally coaxial or waveguide for high power applications. Additionally, adapters can be more complex as the sizes and types of either end of an adapter may be different. Moreover, the adapter itself may introduce turns or bends.
The power and frequency range of an adapter must be scrutinized, especially if the adapters are waveguide to coaxial transitions. Waveguides naturally only enable a bandpass like range of frequencies to be carried with high signal fidelity, where coaxial technologies only have a cut-off frequency. However, the different coaxial connector types also have varying power and frequency capacities. If an adapter is a transition between two different coaxial connector types, the frequency, power handling, PIM, insertion loss, and other parameters will be affected.
Figure 4 • Modern simulators now include EM and thermal simulations to predict the thermal behavior and stresses seen within a filter, or other passive component device. Source: https://www.cst.com/Applications/Article/Thermal-Analysis-Of-A-Two-Cavity-Dual-Mode-Bandpass-Filter.
Terminations bear the brunt of dissipating potentially extreme amounts of RF energy within the device. Generally, terminations for high power applications will have a heat-sinking metal body and possibly forced air thermal management. The impedance match and voltage-standing-wave-ratio (VSWR) of a termination are absolutely critical, as unpredicted reflections could lead to overpower and overvoltage conditions in the upstream electronics. This could be hazardous in the case of shunting a high power amplifier (HPA) to a termination that doesn’t meet adequate VSWR specs, as it could permanently damage the HPA.
Attenuators
Attenuators, like terminators, are designed to dissipate RF energy within the body of the device without producing any unwanted signal distortion or reflections. There are fixed and variable attenuators. For most extremely high power applications, fixed attenuators are more common. Like terminators, they can be either waveguide or coaxial. Additionally, an attenuator can also be an adapter to different sized coaxial connector size, though this is rarely done with waveguide connectors.
Figure 5 • Waveguide directional couplers may have coaxial outputs, as the power level of the coupled signal is low enough to be carried in a lower weight and cost coaxial transmission line. Source: http://www.megaind.com/customApplications.html.
Depending upon the amount of power the attenuator is designed to dissipate, metallic radiators will commonly surround the body and even forced cooling may be an option. The higher the frequency, power handling, and attenuation will dictate the amount of RF energy is converted into heat. When installing an attenuator it is critical to ensure that the attenuator gets adequate ventilation and isn’t mounted in close proximity to other heat dissipating electronics.
Filters
As filters can act as either frequency selective attenuators or reflectors for out-of-band signals, considering the types of upstream electronics and the signals entering a filter are necessary. Absorptive filters will absorb the RF energy from out-of-band signals and convert that into heat. Where, reflective filters will redirect the RF energy back to the source. This type of filter may risk damaging sensitive upstream electronics due to overpower or overvoltage. Depending upon the filter technology and construction, the power handling capability of a filter is often highly frequency dependent.
Figure 6 • There are a variety of power divider technologies, each with their own impedance and performance characteristics. Source: http://www.microwavejournal.com/articles/3042-the-basics-of-print-reciprocal-dividers-combiners.
As with most RF and Microwave components, higher frequency components have a lower power threshold than their low power cousins. The relative sizes and materials of a filter will have significant impact on the power and frequency limits. The passband of a filter naturally attenuates the signal slightly, so the passband characteristics are just as important as the out of band filter characteristics in terms of RF energy absorption or reflection.
Directional Couplers and Power Dividers/Combiners
Directional couplers have many of the same concerns and constraints as adapters, with the added complexity of having built in terminations or forward/reverse coupled signal paths. Moreover, a directional coupler’s coupled signal paths are hundreds, thousands, or tens of thousands of times less power than the RF energy passing through the main propagation line. As the power levels are dramatically lower on the coupled lines, even for high powered waveguide couplers, the coupled lines are often coaxial connectors. This is obviously not the case for hybrid couplers, or 3dB 90° hybrid couplers, which evenly split the power of a signal in two equal RF signal paths.
Figure 7 • Moisture ingress can cause device failure by changing the electrical characteristics and increasing the power dissipating in a connection, such as a rotary connector. Source: http://www.microwaves101.com/mortuary/993-microwave-mortuary-2011.
Generally, a directional coupler is designed to have very low insertion loss and reflection. At high power levels, the method of coupling can introduce significant insertion loss and reflection, if it’s not precision designed. Another factor to consider is the loading of the coupled lines. Though at low power levels a simple termination may be adequate. But, at higher power levels any mismatch or reflections could lead to significant power fed pack into the main signal path. Also, the terminations of a directional coupler may need to have higher power handling than their low power counterparts, depending upon the coupling strength.
Much like directional couplers, power dividers split the RF signal energy along multiple paths. Where, power combiners feed the RF signal energy into one main path. The concerns for insertion loss and reflections are much the same with power dividers/combiners as they are with direction couplers. The major difference is that power dividers/combiners are often at roughly equal power levels, though not phases. As a product of this, any impedance or VSWR mismatch in the connections or feed lines may induce undesired signal degradation, phase deviation, and reflections. Some power dividers/combiners have the inputs or outputs as waveguide or coaxial connections and the inputs and outputs use a different connector size or technology. The frequency and power handling capability for each connection must be accounted for in a final design to ensure acceptable response.
Passive Intermodulation Distortion in High Power Passives
PIM has significant impacts on wireless network performance, specifically with high power RF electronics. As PIM is often challenging to determine in a complete system of passive devices, if PIM is a design concern, having highly precision and low-PIM passive components may be a first step in ensuring lower PIM thresholds. Any nonlinearities in the materials or induced from the environment may lead to high levels of PIM.
Be it the presence of surface imperfections, microfractures, or dissimilar material junctions, high power levels often exacerbate the effects of the nonlinearities resulting in PIM. As high power applications are also generally associated with more extreme environments, temperature variations, vibrations, and material aging can also lead to nonlinearities causing PIM. In order to reduce the PIM response, each individual connection and component can be verified to operate with a reduced 3rd order intercept point, and thus lower distortion. The PIM response can also be confirmed after installation with rigorous post-assembly testing.
Thermal Management Challenges and Lifetime and Material Degradation
High power levels at high frequencies tend to induce RF energy dissipation in non-ideal surfaces and materials. The dissipation of RF energy into most surfaces induces heating. RF heating may cause material changes in peak power operation or material degradation over several use cycles.
Understandably, the temperature and RF power level specifications of a device should be maintained with a reasonable within a reasonable margin. As many manufacturers are very optimistic about the performance of their products, it stands to reason to allow for as much power and heat headroom as other design constraints enable. This is especially important in critical applications that cannot afford downtime, as thermal stresses can induce thermal runaway that rapidly leads to device failure.
Other environmental factors, such as moisture ingress and shock/vibration, can also temporarily reduce the power and thermal handling capability of a component. Thorough testing of high power components in salt fog, temperature, and mechanical stress test benches is commonly employed to verify a component design for the extremes of certain applications. Many manufacturers have detailed information of the testing done with their in-stock products and are willing to share that information with component buyers.
References & Resources
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.
About the Author
Mark Blackwood is Pasternack’s Product Manager for Passive RF Components. He has more than 20 years of engineering, program management, product marketing and product line management expertise in the RF and microwave industry. Prior to joining Pasternack he held management positions in engineering, program management, product marketing and business development with some of the industry’s most recognized corporations including TriQuint Semiconductor, Texas Instruments, Anritsu and others. Mr. Blackwood holds advanced degrees in electrical engineering and physics.