Block Upconverter Design Considerations for Airborne Applications

by  Mehdi Ardavan, Norsat

The recent breakthrough in satellite communication services has provided limitless communication options for airborne applications. With the right equipment and worldwide services, one can provide secure, consistent, two-way broadband communications to various types of airborne missions. This includes mobile Command & Control services (C2); tracking of VIPs and dignitaries; live Intelligence, Surveillance, Reconnaissance (ISR) or any type of in-flight emergency communications. Since airborne applications are unique, there need to be special considerations in the design of block upconverters to truly provide complete control beyond the horizon and seamless “eye-in-the-sky” support.

Communication devices such as transmitters and receivers are required to maintain a satisfactory performance while in operation. For regular applications, ensuring that a block upconverter (BUC), is placed in a ventilated area and on a fixed and steady rack, with small gain variations within the operational temperature range, is not a difficult task as the operational temperature range is very limited and other environmental conditions are benign. However, a BUC installed on an airplane will experience vibrations and temperature variations that are considered harsh and unusual compared to regular applications. If installed on the exterior of the aircraft body where there is no isolation from vibration, the communication device will have to meet even stricter requirements. Therefore, a manufacturer of communication devices for airborne applications needs to have an advanced skillset to design products that have sufficient heat dissipation, gain control, vibration tolerance in the electronic parts, vibration isolation in the mechanical components, and more. Prevalence of airborne applications, both in civil or military sectors, has led to a growing demand for communication products that can perform well in challenging conditions. Often these demands are accompanied by stringent requirements for size, weight, and power (SWaP) which further complicates the situation.

Crucial aircraft communication and navigation systems must be robust enough to withstand the harsh environmental conditions they may be subject to during the course of flight. To standardize the production and testing of these sensitive components, the Radio Technical Commission for Aeronautics (RTCA) published DO-160, Environmental Conditions and Test Procedures for Airborne Equipment. This is a set of best practices which has been adopted by all major aerospace and avionics manufacturers.

The DO-160 standard sets out requirements for temperature, pressure, vibration, shock and other conditions under which an airborne device is expected to demonstrate an acceptable performance. However, designing to comply with the DO-160 standard or other airborne requirements may lead to products which are larger or heavier. The SWaP characteristics of a BUC are among its most important characteristics and cannot be entirely sacrificed to comply with DO-160 or other requirements. Norsat’s ATOM line of BUCs offers industry leading SWaP parameters and the capability to operate within the challenging conditions of airborne applications.

The mean time between failures (MTBF) is another characteristic of communication devices such as block upconverters which, if not taken into account during the design and development process, is usually adversely affected when compliance with DO-160 or similar standards and with SWaP requirements become the only priorities. Reducing the size of a high-power BUC, which is designed and built to tolerate high temperature, altitude and extreme vibration and shock conditions, will lead to a situation where the average lifetime of the components might decrease.

In this paper, we review the requirements in the DO-160 and some similar standards. We also discuss that in some applications, the end-user may require products that perform well in conditions harsher than those explained in DO-160. We then summarize some of the design strategies that Norsat has been using to overcome these challenges while achieving extremely competitive values for SWaP and MTBF.

Standards

“Environmental Engineering Consideration and Laboratory Tests,” MIL STD 810G presents a set of conditions ranging from low pressure (altitude) and temperature range to humidity and vibrations. This standard is mainly focused on environmental or mechanical conditions, whereas MIL STD 461F focuses on the requirements for the control of electromagnetic interference characteristics of subsystems and equipment, and MIL STD 704 focuses on the characteristics of electric power on an aircraft. DO-160, the de facto standard for airborne applications, encompasses all of the above aspects. This standard consists of some requirements in the following areas:

• Temperature
• Altitude
• Temperature variation
• Humidity
• Operational shocks and crash safety
• Vibration
• Explosion proofness
• Waterproofness
• Fluids susceptibility
• Sand and dust
• Fungus resistance
• Salt spray
• Magnetic effect
• Power input
• Voltage spike conducted
• Audio frequency conducted susceptibility
• Induced signed susceptibility
• RF susceptibility (radiated and conducted)
• Emissions of RF energy
• Lightning induced transient susceptibility
• Lightning direct effects
• Icing
• Electro static discharge
• Flammability

Considering the list above — temperature, altitude, temperature variations, humidity, shock and crash, vibrations and icing are discussed in this paper. In DO-160, the temperature and altitude requirements vary based on where the equipment will be installed or used. There are twenty categories from A1 to F3 mainly defined by the altitude at which the equipment will be used, whether or not the location is pressure controlled  or temperature controlled. The range of requirements for the above-mentioned conditions are listed in Table 1.

In the following section we discuss some of the design strategies and techniques to develop a product compliant with DO-160 and other customer requirements.

Design and Development Strategies

From early design steps to later assembly instructions, a manufacturer of airborne communication devices must consider the requirements listed above.

Temperature

To accommodate operational temperature requirements, two parameters are more important than others: heat dissipation and limiting gain variations. Norsat uses special mechanical design for the structures of its ATOM line of SSPA and BUC products to facilitate air flow. Electrical and mechanical engineers work together to locate the optimal position for the final stage transistors or MMICs. Fans are used, and thermal simulations are conducted to optimize heat dissipation. In some products, instead of fans, thermal interfaces such as heat sink plates are used which can further complicate the design process.

To limit the RF gain variations, electrical and RF engineers at Norsat choose the components with the least gain sensitivity to temperature variations. Gain or attenuation levels of designated components are controlled to compensate for gain variations of other amplifiers. Reducing gain variations in BUCs using GaN devices is more challenging because these devices have higher gain variation over temperature. In Norsat ATOM BUCs, GaN devices are mostly used in the final-stage amplifier only in order to limit such adverse effects. Many systems require strict control of gain over temperature and frequency. Also, the gain must be almost identical between units at small signal and rated power levels. Keeping all this in mind, Norsat ATOM BUCs are designed to meet these requirements.

Even with good thermal management, the internal temperature of the BUC will be relatively higher than the ambient temperature. For extreme temperature conditions, it is necessary to pay closer attention to the operational temperature of each component, such as amplifiers and capacitors because in combination they operate at a high temperature and power. A derating analysis may be necessary to either troubleshoot or ensure that the product will work within the expectations. It may be necessary to use only screened parts in the final product.

On the other hand, operating in extreme low temperatures may lead to additional gain. Although this additional gain is compensated through the controllable attenuators, it may lead to some unexpected problems such as spurious signal if the additional gain in the local oscillator circuitry is more than usual.

The dependency of the output power of most amplifiers on the operational temperature is another issue that the designer must pay attention to. The saturation power or the 1-dB compression points in most devices, especially in the GaN technology, will vary with temperature. Usually there is a design requirement for a minimum output power level. If the requirement holds for the entire operational temperature range, the designer may have to choose a device with higher output power in the final stage to meet the requirements. This may add to the cost and bring more challenges to meet the SWaP requirements.

Vibration

To control the effects of vibration, the most sensitive board, the synthesizer, is developed on a separate PCB which can be isolated further. To mount the synthesizer board, the correct screw hole pattern must be used on the board to avoid having a natural frequency range, which can cause phase noise problems. Based on the size and weight of the board and the frequency and intensity of the vibration, the right vibration isolators are used to screw the board in its place. Attention is paid to any cable connection to the synthesizer board in order to avoid passing any vibration effects through hard connections. For harsher vibration environments, external vibration isolators are identified and provided. As a result, the 100W Ku-band Norsat ATOM BUC is compliant with DO160-Section 8-Category S.

Humidity

A few electronic components inside a BUC are not designed to tolerate extreme humidity. These components are within a housing which separates them from the outside environment. In the mechanical design of the housing walls, grooves are used to place gaskets. Other components like fans cannot be placed in the sealed area and need to be rated to tolerate the operating environmental conditions. Heat pipes may need to run from the sealed section to where the fans are located. Attention must be paid to seal holes or gaps through which the heat pipes are routed. Membrane ventilators are also used to equalize pressure and hence prevent humidity from entering the device. This is how the 100W Ku-band Norsat ATOM BUC passes the requirements of Category B of Section 6 of DO160G.

Altitude

To accommodate low-pressure conditions, hermetic sealing is widely used across the industry, but relatively large enclosures make this option impractical. Instead, wherever cables must be brought into the sealed housing, membrane ventilators are used in order to prevent air flow in the outward direction, hence maintaining the inside pressure. At higher altitudes, the pressure of the atmosphere and the coefficient of thermal convection between housing of the BUC and air is reduced. This situation has a potential to reduce the total heat dissipation at higher altitudes, leading to a significant increase in the temperature of the components. 

Fortunately, the air temperature is low at higher altitudes and the total heat transfer is a function of both the convective coefficient and the delta temperature.  Despite the reduced convective coefficient, the total heat transfer might not be significantly affected. The 100W Ku-band Norsat ATOM BUC passes the requirements of DO160-G Section 4 Category D2.

Conducted and Radiated Emissions

Through careful design of PCB boards, power modules, enclosures, and housing and by using conductive gaskets and reject filters, both conducted and radiated emissions have been controlled.

Electrostatic Discharge

Norsat protects its products against electrostatic discharge. For example, in the ATOMBKU100EF, adding surge protection diodes to the connectors on the interface board helps pass the requirements in Section 25 Category A of DO-160G where the peak voltage is ±15 kV. In standards for ground applications such as IEC EN 61000-4-2, the peak voltage varies from ±2 kV to ±8 kV for contact discharge and from ±2 kV to ±15 kV for air discharge.

Lightning

Protection against lightning effects is another criterion in DO-160. To prevent damage caused by lightning effects, Norsat has added TVS diodes to the connectors on the interface board, meeting the requirements of DO-160G Section 22 Category A3 J3 L3.

Icing

Although the majority of the components in a BUC are inside the sealed housing, fans remain exposed to harsh environmental conditions. When cold temperature and high humidity are accompanied by quick temperature variations, icing around the fan blades may occur. The fan may stop working, which will lead to overheating of the BUC. This problem can be mitigated by building a mechanism which issues a warning and eventually turns off the RF power of the BUC.

Even if high-quality fans are chosen carefully, they may not stop rotating with icing on the blades. Instead, the fan attempts to continue running but due to the high load on the blades, starts drawing a current much larger than it is supposed to. The DC power supply will be overloaded and suddenly shut down, leading to the controllers being turned off as well. Ultimately, the BUC will not transmit power or communicate through its M&C interface, meaning that the user will have a small chance of understanding the problem or cause of failure. Hence, Norsat takes care to ensure fans only operate within humidity requirements and are protected with the necessary circuitry to avoid drawing more current that they are supposed to. 

Mean Time Before Failure

The MTBF predictions can be determined using MIL HDBK 217F. The MTBF is dependent on the operational temperature, environment and the quality of the part. If a product is used in airborne applications, its MTBF is significantly reduced compared to standard commercial ground applications. For example, assuming a commercial product has 100,000 hours MTBF for a ground benign application at 30 deg Celsius, its MTBF is dropped to about 7,407 hours if used in an inhabited fighter at 80 deg Celsius. If the product is rebuilt with a part quality of “full military,” the MTBF is increased to 30,370 hours, still well below the ground application MTBF. Norsat conducts MTBF calculation for its products using the MTBF information of all the components inside a product, operational temperature, application and the quality category of the application. 

Summary

Equipment used in airborne applications, including communication products such as BUCs, are exposed to environmental conditions which are more challenging than ground applications. Requirements related to the operational temperature range, humidity, vibration, altitude, lightning, ESD and many other criteria are summarized in some standards such as DO-160. This standard sets out a series of requirements which represent the conditions in airborne applications. To meet these requirements while being competitive in the SWaP criteria is a challenge. Norsat overcomes this challenge by considering all the requirements in the design process. Electrical and mechanical aspects must be considered together in order to meet all the requirements without having to significantly increase the size and weight of the product.

MIL-STD-810G, Department of Defense Test Method Standard for “Environmental Engineering Considerations and Laboratory Tests”

Reference

• RTCA/DO-160 G, RTCA, INC., December 8, 2010.

• MIL-STD-810G, Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests.

• MIL-STD-461G, MILITARY STANDARD: ELECTROMAGNETIC INTERFERENCE CHARACTERISTICS REQUIREMENTS FOR EQUIPMENT (2015).

• IEC 60068-2-27 Environmental Testing – Part 2-27: Tests – Test Ea and Guidance: Shock.

• IEC 60068-2-64:2008  Environmental Testing – Part 2-64: Tests – Test Fh: Vibration, Broadband Random and Guidance.

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