For the Defense agencies, specifying a rugged laptop’s electromagnetic interference (EMI) and its electromagnetic compatibility (EMC) is an important judge on how well the rugged laptop will operate within many “noisy” environments. MIL-STD 461 documents the EMI requirements for a wide range of applications, from trucks to ships to aircraft to fixed installations, not to mention the different requirements within an application—above deck and below deck on a Navy ship. There is also a trend to tailor the requirements to particular applications. Although the most modest EMC requirements are not much different from commercial requirements, most military applications are decidedly harsh.
MIL-STD-461 is a widely used test standard for military electronics suppliers, due largely to the fact that it covers inter-service operability issues. Approved on December 10, 2007, the newly released MIL-STD-461F supersedes MIL-STD-461E.
The military’s concern for EMI began as far back as the installation of the first radio in a vehicle before World War I. Because the application of radio technology and understanding its effects seem to parallel each other, it wasn’t until 1934 that the U.S. Army Signal Corps published its first EMI standard: SCL-49, “Electrical Shielding and Radio Power Supply in Vehicles.” This document “protected” radio receivers by requiring vehicle operations to not “disturb” radio reception through shielding the ignition system, regulator and generator.
From this simple beginning, military EMI evolved and changed as the complexity of the systems increased, frequencies jumped, and the threat from EMP (electromagnetic pulses) was documented and quantified. As the specifications evolved, each branch of the service defined requirements specifically for their departments or platforms. This encouraged rugged laptop manufactures to comply with significantly different specifications for each branch as well as different specifications for specific programs within each branch.
As a result, the Defense formed a working group to consolidate and replace approximately 20 requirements into the initial MIL-STD 461 (the requirements), MIL-STD 462 (the measurement methodology) and MIL-STD 463 (definitions and acronyms) that were published in 1967. As with any general standard, especially the initial standard, revisions were required resulting in MIL-STD 461A being issued in August 1968. Although mandated, many programs made revisions and exceptions to the standard (even as 461 went from revision A to C).
In 1993, the Tri-Service EMC Committee issued an updated MIL-STD 461 and MIL-STD 462, revision D. MIL-STD 463 was dropped and its definitions referenced to the American National Standards Institute (ANSI) C63.14 “Standard Dictionary for Technologies of Electromagnetic Compatibility (EMC), Electromagnetic Pulse (EMP) and Electrostatic Discharge (ESD).” In 1999, 461 and 462 were combined, requirements updated and published in the currently enforced standard: MIL-STD 461E.
MIL-STD 461E is a set of EMC requirements intended to serve a wide range of applications, from trucks to ships to aircraft to fixed installations (Figure 1). It specifically provides the opportunity to tailor the requirements for each application without having to issue exceptions to the standard. Although the more modest sections are not much different from the common IEC and FCC commercial requirements, most sections are decidedly harsh. Table 1 and Table 2 are adapted from the MIL-STD 461E standard and identify the emissions and susceptibility requirements. It is important to remember that MIL-STD 461E doesn’t directly cover a number of commercial EMI situations, including lightning and ESD. Thus, a number of related requirements are added to the list—some from commercial standards and some from DEFSTAN (UK) or STANAG (NATO) requirements.
Table 1 is adapted from the MIL-STD 461E standard and identifies the emissions and susceptibility requirements. Each test is defined using the following syntax:
|C = Conducted||CExxx = Conducted emissions test|
|R = Radiated||RExxx = Radiated emissions test|
|E = Emission||CSxxx = Conducted susceptibility test|
|S = Susceptibility||RSxxx = Radiated susceptibility test|
The table also summarizes how the tests apply to computers with the text lifted and edited directly from the applicability sections of the standard. Major changes from revision D are also shown and summarized.
|Requirement||Application||Frequency Range||Description||Changes From MIL-STD 461E|
|CE101||Power & Interconnecting Leads||30Hz – 10kHz||No longer applicable to shipboard equip.|
|CE102||Power & Interconnecting Leads||10kHz – 10MHz|
|CE106||Antenna Terminals||10kHz – 40GHz|
|CS101||30Hz – 150kHz||The requirement is applicable to power input leads that obtain power from other sources not part of the computer including those that are rechargeable. There is no requirement on power output leads.This test ensures that performance is not degraded from the ripple voltages associated with generators, airborne, ship-borne, and vehicle-borne power sources.||Applicability and limits extended to 150kHz|
|CS103||Antenna Port , Intermodulation||15kHz – 10GHz||Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like.)|
|CS104||Antenna Port , Signal Rejection||30Hz – 20GHz||Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like.)|
|CS105||Antenna Port , Cross modulation||30Hz – 20GHz||Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like, which extract information from the amplitude modulation of a carrier.)|
|CS109||Structure current||60Hz – 100kHz||Not applicable to computers unless it is directly attached to very sensitive equipment (sensitive to 1µV or better) such as tuned receivers operating over the frequency range of the test.||Measurement procedures revised|
|CS114||Bulk Cable Injection||10kHz – 200MHz||The requirements are applicable to all electrical cables interfacing with the computer. It simulates currents that may be developed on the platform cabling from electromagnetic fields generated by antenna transmissions both on and off the platform.||Limits reduced to 200MHz|
|CS115||Bulk Cable Injection, Impulse Excitation||Transients – 2nS x 30nS||The requirements are applicable to all electrical cables interfacing with the computer. It simulates the fast rise and fall time transients that may be present due to platform switching operations and external transients such as lightning and electromagnetic pulses.||Applicability revised|
|CS116||Damped Sinusoid Transients – I/O & Power Cables||10kHz – 100MHz||The requirements are applicable to all electrical cables interfacing with the computer. It simulates electrical current and voltage waveforms occurring in platforms from excitation of natural resonance.In contrast to fast transients of CS115, this test uses dampened sine waveforms to simulate the occurrence on platforms resulting from lightning, electromagnetic pulses, and the electrical switching phenomena of the platform.||Measurement procedures & applicability revised|
|RE101||Magnetic Field||30Hz – 100kHz||This is applicable to computers and is intended primarily to control magnetic fields for applications where other equipment is sensitive to magnetic induction at lower frequencies. The most common example is a tuned receiver. (RS101 is a complementary requirement imposed on equipment to ensure compatibility with the anticipated magnetic fields.)||50cm requirement deleted; limits more stringent|
|RE102||Electric Field||10kHz – 18GHz||The requirements are applicable to electric field emissions from the computer and associated cables. The intent is to protect sensitive receivers from interference radiated from the computer and coupled through the antennas associated with the receiver (many receivers have sensitivities on the order of one microvolt).||Limits revised for submarine equipment|
|RE103||Antenna Spurious & Harmonic Outputs||10kHz – 40GHz||Not applicable to computers, laptops, and notebooks.|
|RS101||Magnetic Field||30Hz – 100kHz||This requirement is applicable to computers and ensures that performance is not degraded when subjected to low frequency magnetic fields.||Limits revised for Navy applications; Added Helmholtz coil test|
|RS103||Electric Field||2MHz – 40GHz||Added use of mode-tuned reverberation chambers above 200MHz|
|RS105||Electromagnetic Pulse Field Transient||This requirement is primarily intended for computers that could be subject to the fast rise time, free-field, transient environment of an electromagnetic pulse (EMP). It applies only to those computer equipments and enclosures that are directly exposed to the incident field outside of the platform structure, or for equipment inside poorly shielded or unshielded platforms.This requirement is applicable only for EUT enclosures (electrical interface cabling should be protected in shielded conduit). Potential equipment responses due to cable coupling are controlled under CS116.|