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SDR Products that Meet Mil-Spec Requirements in Radar, GPS, and SIGINT

Parent Category: 2022 HFE

By Brendon McHugh

 

Introduction

Critical applications of radiofrequency (RF) devices used in military and defense settings require strict compliance with a list of predefined standards regarding safety, performance, and robustness. In particular, RF military standards describe the minimum performance in terms of service life, performance under tough environmental conditions, electromagnetic compatibility (EMC) and interference (EMI), and failure/maintenance standard practices. Furthermore, military specs vary with the country, the application of the RF equipment, and figuring out the required performance for a certain product may take a considerable amount of time during the design stage.


In this article, we discuss how software-defined radio (SDR) can be designed to meet the military environmental and service standards, including the MIL-STD-810 (environmental design standards), the MIL-STD416E (EMI standards), and the mean-time between failure (MTBF)/maintainability design techniques (DOD-HDBK-791). We also cover how the use of high-end commercial off-the-shelf (COTS) SDRs with ruggedized chassis can be used to meet the military standards at a relatively low cost, while also providing easy maintenance, repair, and upgradability due to its modular design. Finally, we discuss how SDRs meeting the aforementioned standards and specifications can be used in military applications, such as radar receivers and satellite communication systems (GPS/GNSS).


What is an SDR?


The term SDR designates any radio device that performs most of the signal processing and application-specific functions in the digital domain, in contrast with traditional radio systems mostly based on analog electronics. The general architecture of an SDR can be divided into two stages: the radio front-end (RFE) and the digital backend. The RFE is composed of several receive (Rx) and transmit (Tx) channels, with each channel capable of operating in a wide range of frequencies, going from DC up to tens of gigahertz. The highest-bandwidth COTS SDR on the market provides 3GHz of instantaneous bandwidth per channel. The RFE also performs amplification and filtering of signals, interfacing with the digital backend through fast and precise ADCs and DACs. In high-end SDRs, each Tx and Rx channel works independently, allowing complex MIMO operations that are synchronized by precision time boards.


The digital backend is the heart of the SDR, where essentially every major function is performed. It consists of a high-performance field-programmable gate array (FPGA) with digital signal processing (DSP) capability. The FPGA architecture provides a high level of flexibility and upgradability so that commercial off-the-shelf (COTS) SDRs can be constantly improved and repurposed without any change in the hardware. The digital backend performs basic radio functions, such as modulation, demodulation, up/down converting, CORDIC mixing, and data packetization over Ethernet. Moreover, the FPGA can be programmed to perform application-specific functions, such as communication protocols, beamforming/beam steering, artificial intelligence (AI), and machine learning (ML). Besides flexibility, the FPGA also provides fast parallel processing for huge amounts of data, which combined with on-board SFP+/qSFP+ (10-100 Gbps) interfaces, ensure high data throughput. Finally, the FPGA implementation allows the SDR to be easily integrated into virtually any system, including legacy equipment. Figure 1 shows how SDRs can be integrated into a radar.

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Figure 1 • Generic SDR block diagram integrated into a radar system.


Military Specifications


The critical nature of military missions demands that the SDRs, and overall electronics, implemented can reliably operate in a variety of adverse environments, including natural events (dust, precipitation, humidity, temperature) and manmade adversities (EMI, jamming). In this context, military standards are used to enforce quality control and tracking over different commercial devices, providing the minimum requirements for both electrical and mechanical components for a particular condition or application. Currently, the most popular standard system is the one developed by the USA military, which is the reference around the world for warfare and defense technology, and here we discuss the most important standards for SDR applications.


The MIL standard for EMC is the number 461, with the latest version being the MIL-STD-461G. This standard contains approximately 20 test parameters, which strictly define the minimum requirements for electromagnetic behavior. The nomenclature of the tests is also standardized: ‘E’ designates emission, while ‘S’ designates Susceptibility – which is the opposite of Immunity. For example, RE and RS mean Radiated Emission and Radiated Susceptibility, respectively. SDR-based devices need to comply with the EMI susceptibility standards over a wide range of tuning frequencies, which may force the SDR manufacturer to narrow down the frequency range, or the product developer to certify the whole device at the system level.


Another crucial standard is the MIL-STD-810, which describes tailoring the environmental design of the device and testing maximal conditions that the equipment will experience during its service life. It is divided into three parts. Part 1 describes the general guidelines for engineering, management, and technical specifications of environmental design and test tailoring. It focuses on design and test requirements for particular conditions of military equipment. Part 2 describes the laboratory tests necessary to enforce the guidelines presented in Part 1. This section encompasses a huge amount of tests, related to various effects including pressure, temperature/radiation, shock, vibration, precipitation, fluid contamination, and other natural issues that arise in harsh environments. 

Part 3 of the MIL-STD-810 standard deals with the world climatic data, providing planning guidance for device implementation depending on the climatic conditions of the application site. As a whole, the MIL-STD-810 is rather flexible, allowing the adjustment of test methods according to the application intended.

In the military field, maintainability is crucial to ensure the proper functioning of the device after damaging events or adversarial conditions. Most design guidelines for maintainability are described in the DOD-HDBK-791 handbook, which presents techniques for maintainability design. Maintainability is a particular figure of merit in military design, as it greatly affects the maintenance requirements of the device in terms of budget, material, and man-hours expenditure. If maintenance activities are required too often, the availability of the device decreases significantly, which may result in critical situations in military missions. The handbook guidelines cover technique standardization and design interchangeability, modularization, accessibility, and labeling, as well as test methods and diagnostic protocols. Preventive maintenance is also thoroughly discussed, in the context of human resources and environmental factors.

Reliability is also essential in military applications. Design standards and guidelines are presented in the MIL-HDBK-217 Military Handbook, which covers the reliability prediction of electronic equipment. To ensure reliable operation, all components of radio electronics, including integrated circuits, cables, and discrete elements, must be assessed for failures. Uncertainty analysis can be used to evaluate the mean time between failures (MTBF), which corresponds to the average time between system breakdowns. MTBF is an important figure of merit to measure device safety and performance, particularly in complex radio systems.

Military SDRs: Applications and Standards

It is impossible to discuss military RF technology without mentioning radars. Radar receivers are widely implemented in electronic warfare (EW) and defense, in techniques that range from Moving Target Indicator (MTI) radars to naval fire-control systems. Due to the critical nature of these devices, and their fundamental role in military missions, the SDR systems involved must comply with several MIL-SPECS and standards. In terms of EMC (MIL-STD-461), the receiver is often tested using two-tone and spurious response receiver tests, which are described in CS03, CS04, CS05, and CS08 specifications of the MIL-STD-461 standard, which can evaluate the spurious-free dynamic range of the receiver device under test (DUT). Test CS08 verifies performance under a single undesired interference signal, CS04 tests response under both desired and undesired interference signals, CS03 checks for intermodulation, and CS05 tests cross-modulation.


All tests consist of applying strong signals to the radar receiver and evaluating its response, performance, and operation. Figure 2 shows a two-tone test setup of a radar receiver DUT. If the antenna of the receiver can be removed, two-directional couplers are used: one at the output to send the signal to spectrum analysis, to verify the purity of the test signals, and another one to couple both frequencies of the two-tone tests (F1 and F2). Frequency F1 is the CW signal of the radar, which is applied in the coupling arm (port B) of the directional coupler. F2 is the pulse signal, which is applied to the straightforward path of the coupler (port C). The signal of port A, therefore, contains both F1 and F2 signals, which are then sent to the DUT for testing. If the antenna cannot be removed, the setup requires the use of antennas in an anechoic chamber, as seen in Figure 2 (b).

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Figure 2 • Set up for MIL-STD-461 tests on radar receiver. Setup (a) is used when the antenna can be removed, whereas setup (b) is when the test must be performed with the active antenna.


Modern military technology is highly dependent on satellite-based radio systems. In these systems, SDRs are used both onboard satellites and on-base stations on the ground. Satellite base stations are frequently located in remote regions with adversarial conditions, including humidity, dust, and precipitation. Because base stations are absolutely necessary for GPS-guided weapons, aerial and naval navigation, and command and control (C2) communications, they require a high level of reliability and enough robustness to reduce the need for maintenance activities. The MIL-STD-810 standard discussed previously, which deals with the device performance under harsh environmental conditions, is highly important for SDRs implemented in remote base stations. The standard describes ruggedization techniques to improve device robustness, including climate-controlled enclosures, design of custom chassis, and conformal coating for PCBs (Figure 3). Conformal coating specification is described in the MIL-I-46058C standard, which evaluates the coating performance in terms of several properties, including thickness, appearance, insulation, flexibility, as well as resistance to fungus, electricity, flames, temperature, and humidity. SDRs in harsh remote ground stations also need to be tested under the rain and combined environments methods of the MIL-STD-810, which encompasses tests 506.6 and 520.5. They also must comply with the same EMC tests designated for radar receivers (MIL-STD-461-CS03, -CS04, -CS05, -CS08). Moreover, due to the high precision level needed for timing purposes - satellite signals must be in sync for proper evaluation of position, velocity, and communication – reliability and maintainability standards are crucial for the implemented clock components, including crystals and oscillators. High-end SDRs are known to provide very stable and precise time boards that meet the MIL-SPECS requirements.

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Figure 3 • Appearance of a PCB with transparent conformal coating. (Source: https://resources.pcb.cadence.com/blog/2021-conformal-coating-process)

Conclusion

Modern military technology is highly dependent on RF devices, and SDRs and software-based networks have become the main approach for precise, flexible, and robust radio systems. To meet the military demands for quality control, SDR designers and manufacturers must develop and test their products in accordance with the well-established military specifications and standards. The most important standards and guidelines for RF devices are the MIL-STD-810, which describes EMC specifications, the MIL-STD-461, which focus on designing and testing for particular environmental conditions and situations, and the maintainability and reliability guidelines presented in the DOD-HDBK-791 and the MIL-HDBK-217 military handbooks. The level of compliance with these specifications varies for the intended application: for instance, SDRs for radar receivers must be extensively tested in terms of EMC and reliability, while devices applied in remote base stations must provide good levels of robustness and low need for maintenance, to endure the harsh environment of where the SDR is located. Nevertheless, knowing the standards and tests required for your application and considering them early on in the design chain is fundamental to properly designing and/or selecting the right SDR component for your military equipment.

About the Author

Brendon McHugh is a field application engineer and technical writer at Per Vices. Brendon is responsible for assisting current and prospective clients in configuring the right SDR solutions for their unique needs. He possesses a degree in theoretical and mathematical physics from the University of Toronto. Per Vices has extensive experience in designing, developing, building, and integrating high-performance SDRs in many RF applications, including satellite communications, EW/SIGINT, medical, radar, low latency links, spectrum monitoring and recording, test & measurement, and mobile communications. Contact solutions@pervices.com today to see how we can help you with your SDR needs.

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