Basic Elements of MIL-STD-461
MIL-STD-461 establishes interface and verification requirements for the control of the electromagnetic interference (EMI) emission and susceptibility characteristics of electronic equipment and subsystems designed for or procured by the U.S. Department of Defense (DoD). This white paper supplies a summary of the standard and presents some ideas regarding ways to achieve compliant designs.
Background on MIL-STD-461
EMI (electromagnetic interference) and EMC (electromagnetic compatibility) relate to the unwanted effects in an electrical system due to electromagnetic radiation and electromagnetic conduction. The worst part of a high-frequency electromagnetic signal is that it makes every conductor an antenna, in the sense that they can generate and absorb electromagnetic fields. The control of EMI is an art and science that seeks to prevent the various interdependent and independent electronic systems on military platforms from interfering with each other's normal operation.
MIL-STD-461 is concerned with specifying technical requirements for controlling EMI emissions and susceptibility at the subsystem-level and equipment level. MIL-STD-461 is not intended to be directly applied to subassemblies of equipment such as modules or circuit cards, although these must be designed with EMI containment in mind in order for subsystem-level assemblies to conform to MIL-STD-461 requirements.
At the time of this writing, MIL-STD-461 has been issued in revisions A through F. Unlike many specifications, the release of a newer standard does not make earlier revisions unnecessary. There are still military platforms in service which were designed around MIL-STD-461A. Technology refreshes on older platforms will continue to express the EMI requirements in terms of the version of MIL-STD-461 that was in effect when the platform was first designed. In some cases, the earlier revisions are more stringent than later versions.
Comparison with Commercial Electronics
The use of commercial and industrial items presents a dilemma between the need for EMI control on the one hand and the desire to take advantage of existing low-cost commercial designs which may exhibit undesirable EMI characteristics on the other. For some applications of commercially developed products, such as commercial transport aircraft, EMI requirements similar to those in MIL-STD-461 are usually imposed on equipment. Most commercial aircraft equipment is required to meet the EMI requirements in RTCA DO-160 or an equivalent contractor in-house document. Later revisions to RTCA DO-160 are making the document more compatible with MIL-STD-461. Equipment qualified to revisions C, D, or E of RTCA DO-160 is often suitable for military aircraft applications.
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