Currently Available Drafts of this Important Standard Reveal Significant Changes
For nearly 50 years, MIL‑STD‑461 has provided electromagnetic interference (EMI) control requirements for sub-systems and equipment deployed by all departments and agencies of the Department of Defense (DoD). These requirements are considered necessary to ensure that a subsystem or equipment will operate as intended when exposed to the anticipated electromagnetic environment (EME) of a particular platform or system. These requirements are also used to ensure that electromagnetic emissions are properly controlled such that equipment will not interfere with the operational performance of other subsystems or equipment. This article reviews some of the most significant changes outlined in the draft revision of MIL‑STD‑461G, dated March 2, 2015.
FFT Receivers Are Now Permitted For Use
The MIL‑STD‑461G draft proposes a number of significant changes to the standard including the introduction of Fast Fourier transfer (FFT) receivers. FFT measurement techniques may now be used provided that FFT operation is in accordance with ANSI C63.2. Additionally, the receiver must allow for the direct input of the parameters for both FFT time domain and frequency stepped modes of measurement in the same manner, without the necessity or opportunity to control FFT functions directly.
FFT-based measurement receivers perform time samples of signals in a relatively large bandwidth (typically in the MHz range) and mathematically convert the results to the frequency domain representation of the emission profile using the bandwidths specified by the standard. MIL‑STD‑461G points out that FFT techniques provide a significant reduction in measurement time over conventional analyzers and receivers, and can be used to capture frequency agile signals that jump around in a designated frequency band.
However, the standard also points out a concern that an FFT receiver has a potential to saturate sooner than a conventional receiver at lower signal levels when detecting broadband signals (with respect to the receiver band-pass), primarily due to the larger measurement FFT bandwidths used. An additional concern exists where low repetition rate signals can be totally missed if the dwell time is shorter than the pulse repetition interval of the emission. This can be remedied simply by increasing the FFT measurement dwell time.
The bandwidth and measurement time table has been updated to reflect the specific measurement parameters in stepped-tuned receivers and FFT receivers, as well as analog tuned receivers (Table 1).
Interconnecting cable routing now specified for floor standing equipment
MIL‑STD‑461 has provided specific cabling requirements for equipment setups in order to help standardize RF coupling paths. The total interconnecting cable lengths in the setup shall be the same as in the actual platform installation. If a cable is longer than 10 meters, at least 10 meters shall be included. When cable lengths are not specified for the installation, cables shall be sufficiently long to satisfy the conditions specified below.
At least the first two meters (except for cables which are shorter in the actual installation)
of each interconnecting cable associated with each enclosure of the equipment under test (EUT) shall be run parallel to the front boundary of the setup. Remaining cable lengths shall be routed to the back of the setup, positioned five cm above the ground plane, and shall be placed in a zigzagged arrangement, minimizing cable overlap or crossing.
When the setup includes more than one cable, individual cables shall be separated by two cm measured from their outer circumference. For bench top setups using ground planes, the cable closest to the front boundary shall be placed 10 cm from the front edge of the ground plane. All cables shall be supported five cm above the ground plane with non-conductive material such as wood or foam.
These requirements were largely based on tabletop setups or large free-standing equipment with cabling that would normally be routed along the floor. However, a long standing issue existed regarding what to do with the excessive cable routing for top or bottom fed equipment cabinets. Six foot equipment cabinets with top fed cable entry point required extra cabling to be vertically routed down the side of the cabinet to the required two meter horizontal coupling path. Test labs were required to carefully document the extra length and routing arrangement in the test report for duplication purposes.
The draft revision of MIL‑STD‑461G now provides guidance specifically for testing equipment cabinets (Figure 1) and free-standing equipment (Figure 2), eliminating the option to route cables on the floor of the test chamber. All setups will now require a bench top ground plane for routing cables regardless of the device under test. If cables are routed from top or near the top, then the cables shall be routed down to the ground plane bench and then two meters shall be run parallel to the front edge of the boundary. If the EUT is a floor standing unit and the cables are routed from the top, then the cables shall be routed down to the ground plane bench and then two meters shall be run parallel to the front edge of the boundary. If the cables are routed from the bottom, then the cables shall be routed up to the ground plane bench and then two meters shall be run parallel to the front edge of the boundary.
Additionally, you will notice that the minimum ground plane spacing around free-standing equipment setups has been extended to 2.5 meters from 1.5 meters. This is an important factor to be aware of when testing large equipment outside of a shielded enclosure/chamber.
Test Method CS106 Has Been Removed
Among the many additions found in the draft revision of MIL‑STD‑461G, test method CS106 has been removed from the applicable test requirements for Navy ships and submarines. CS106 was intended to evaluate equipment performance when exposed to voltage transients experienced on shipboard power systems coupling to interface wiring inside enclosures. Electrical transients occur on all electrical distribution systems and can cause problems in circuitry which tend to be sensitive to voltage transients, such as latching circuits expecting a single trigger signal.
On submarines and surface ships, these transients can be caused by switching of inductive loads, circuit breaker (or relay) bounce, and load feedback onto the power distribution system. Measurements of transients on Navy platforms have shown the transient durations (widths) are predominantly in the 1 – 10 microsecond range. The large majority (> 90%) of the transients measured on both the 115 volt and 440 volt ac power distribution systems were between 50 and 500 volts peak. MIL‑STD‑461F defined the five microsecond 400 Vpk pulse as a suitable representation of the typical transient observed on Navy platforms.
The Navy submarine community has found the obsolete CS06 of MIL‑STD‑461 (through revision C) requirement to be an effective method to minimize risk of transient-related equipment and subsystem susceptibility. This test was removed in revisions D and E, but was reintroduced as CS106 in MIL‑STD‑461F. This type transient test has been successful in early identification of transient-related EMI problems in naval equipment and subsystems, and the Navy has traditionally found good correlation between transient related shipboard EMI problems (including longevity, degraded performance and premature failures) and CS106 susceptibilities. However, it was removed in the latest draft revision of this standard. No technical rationale has been provided at this time.
Test Method CS114 System Check Updates
The CS114 field calibration procedure now requires that the current monitoring clamp traditionally used to measure injected current during line testing is included in the calibration circuit path and terminated into 50 ohms (see Figure 3). This is intended to compensate for the monitor probe loading effects encountered during EUT testing.
Additionally, CS114 requires a verification check of the test system with the current monitoring clamp left in the circuit meant to emulate an actual EUT line test (see Figure 4). This verification check is performed over the entire frequency range with minimal dwell times and step sizes up to twice those specified by the standard while maintaining the calibrated forward power level. The intent is to ensure that the forward power follows the calibration and that the developed current is within a three dB tolerance of the current test limit.
In bulk cable injection (BCI) techniques, the test signal is inductively coupled. Faraday’s law predicts that an induced voltage in a circuit loop with the resultant current and voltage distribution is dependent on the various impedances presented by the cable under test. For this reason, the test method under MIL‑STD‑461G reverts to an older method than was used in MIL‑STD‑461D in which, instead of leveling primarily on the cable-induced current, the pre-calibrated forward power is the primary target. With this older method, which is found in SAE ARP1972, and RTCA DO-160C/D, the relationship between the BCI-induced cable currents and those induced by radiated fields will agree more closely with respect to shielded cables. In these cases, testing is conducted where either the calibrated forward power or injected current limit is achieved (whichever is less severe).
The Addition of Test Method CS117, Conducted Susceptibility, Lightning Induced Transients, Cables and
The draft revision of MIL‑STD‑461G has introduced lightning induced transients with limited applicability to all aircraft safety-critical equipment interconnecting cables, including complete power cables, and individual high side power leads. This includes non-safety critical equipment with interconnecting cables/electrical interfaces that are part of or connected to equipment performing safety critical functions. CS117 may also be applicable to aircraft equipment performing non-safety critical functions when specified by the procuring entity. Lastly, CS117 has limited applicability to surface ship equipment located above deck or which includes interconnecting cables which are routed above deck.
This requirement is intended to address the equipment-level indirect effects of lightning as outlined in the platform electromagnetic environments defined within MIL‑STD‑464. Test method CS117 does not provide test coverage for either the direct effects or nearby lightning requirement from MIL‑STD‑464. However, for most equipment, testing with some combination of CS116 and CS115 may provide sufficient coverage to address the environment for nearby lightning called out in MIL‑STD‑464. This is because the changing electromagnetic fields primarily result in currents being induced on equipment cables.
Test methodology is similar to RTCA DO160 cable bundle including Multiple Stroke (MS) waveforms 1 through 5, as well as Multiple Burst (MB) waveforms 3 at 1 MHz and 10 MHz and waveform 6. CS117 applies the Single Stroke (SS) and MS waveforms as a combined test. For this reason, the first stroke of the MS application has been adjusted to the applicable SS test level from RTCA DO-160. The MS and MB tests are not synchronized to any particular EUT critical frequency so an irregular pattern is applied and is not intended to be synchronized with the EUT timing processes. This excites the system so that the probability of upsetting periodic signals in the EUT is increased. This irregular timing approach results in excitation of the cable resonance points.
The CS117 test levels are segregated into two categories, internal equipment and external equipment environments. As shown in Table 4, internal equipment limits are comparable to the RTCA DO160G test levels three and four, whereas external equipment limits align with test levels four and five. The reason for the category segregation is that lightning induced coupling to wiring that is external to the aircraft is expected to be significantly higher than coupling to wiring fully contained inside the protection of the aircraft main fuselage/shielded interior.
As indicated by the standard, in cases where line–replaceable units (LRUs) are installed within the aircraft fuselage but have wire bundles routed through exterior portions of the aircraft, the LRU to be tested to external levels even though the LRU itself was inside the fuselage. In addition, aircraft with portions of structure that are constructed of non-conductive materials would be considered external and any LRUs or wiring installed inside such structures would therefore need to meet the external lightning threat. Examples of this could be an empennage or fairing made of non-conductive composite materials. Aircraft often have wiring routed in wheel wells, bomb bays and leading edge areas of the wings that are exposed to external threats during certain flight phases. Therefore, wiring or LRUs in these areas would need to meet the external threat levels.
The Addition of Test Method CS118, Personnel Borne Electrostatic Discharge
The CS118 test presents a controlled method to evaluate the susceptibility of electrical and electronic subsystems exposed to human body electrostatic discharges.
Electrostatic discharge (ESD) testing is applicable to all electrical, electronic, and electromechanical subsystems and equipment except for those which interface with or control ordnance/munitions. In accordance with MIL‑STD‑464C, system level ESD requirements for ordnance and munitions will still be covered under standards such as MIL‑STD‑331, Fuze and Fuze Components, Environmental and Performance Tests.
The CS118 test equipment and procedures are based on those provided by internationally-recognized standards such as IEC 61000-4-2. This standard represents the human body RC model consisting of a 150 pF / 330 ohm network (see Figure 5 for its corresponding current waveform). However, one significant difference is with respect to the EUT test setup. In the CS118 test, MIL‑STD‑461G requires that the EUT shall be electrically bonded in accordance with the product installation requirements. The purpose for this requirement is to maximize the charge transfer through the equipment chassis to ground. This configuration varies from the IEC 61000-4-2 conventional setup where the current return path is limited via a resistor network in an effort to avoid component burnout. Direct electrical bonding of the equipment chassis allows maximum current flow of the ESD pulse and is considered to be a better representation of the product installation environment.
Verification of the ESD simulator’s performance is required prior to testing. This is normally performed using the contact discharge waveform into a two ohm coaxial resistor network (commonly referred to as a “target”) to validate the ESD current waveform. This approach is preferred over verification using the air discharge waveform due to variances in the approach speed and angle of approach to the target introduce variability into the waveform which is observable as a change in the rise time, peak current, and duration of the waveform. Annex E of ISO 10605:2008 provides further information regarding the use of air discharge to verify the ESD simulator’s performance. In addition to a contact discharge verification, MIL‑STD‑461G will require the charge voltage to be verified using an electrostatic voltmeter, or charge meter.
Contact discharges will be applied to all external and accessible conductive parts at test levels ranging from 2,000 to 8,000 volts. Air discharges at 15,000 volts will be applied to locations where contact discharges cannot be made.
Test Methods RE102 and RS103 Technical Updates
The RE102 test method is used to quantify and control the electric field emissions propagated from the equipment enclosure and associated cabling. In previous versions of the standard, RE102 testing was required from a specified test frequency (either 10 kHz or 2 MHz, depending on the installation platform and/or procurement) to 1 GHz or 10 times (x10) the highest intentionally generated frequency up to 18 GHz. The draft revision of MIL‑STD‑461G now requires testing to 18 GHz regardless of the EUT’s highest generated frequency or installation platforms (see Figure 6).
In addition to the extended test frequency range, RE102 now includes new antenna positioning requirements above 200 MHz for free-standing and tall equipment:
For testing from 200 MHz to one GHz, place the antenna in a sufficient number of positions such that the entire area of each EUT enclosure and the first 35 cm of cables and leads interfacing with the EUT enclosure are within the three dB beam width of the antenna.
For testing at one GHz and above, place the antenna in a sufficient number of positions such that the entire area of each EUT enclosure and the first seven cm of cables and leads interfacing with the EUT enclosure are within the three dB beam width of the antenna.
This change has been made to specifically address issues concerning insufficient antenna coverage of tall equipment, such as cabinets. In previous versions of the standard, RE102 testing above 200 MHz required that the entire “width” of the EUT fell within the half power beam width of the measurement antenna. This was based a standard height of 120 cm above the ground plane, or 30 cm above the typical bench top EUT setup where vertical height was not a significant concern. However, this is not the case for floor standing cabinetry which often times reaches six feet in height. A cabinet’s internal shelving provides the ability to mount equipment at various heights (at the top or bottom), oftentimes allowing such equipment to reside outside of the half power beam width of the large or small double-ridged horn antenna.
The draft revision of MIL‑STD‑461G also requires the entire area of the EUT to be tested in the RS103 test method above 200 MHz for similar reasons. Unless sound technical justifications can be made to exclude specific measurement locations, manufacturers can expect a significant increase in qualification test time for tall cabinets and equipment. No guidance is provided as to performing full surface area coverage for RE102 or RS103 above 200 MHz, leaving open the prospect of different interpretations of the new requirement. One should select a method which best suits the equipment under test. A logical choice is to consider bore sighting and/or adjusting the horizontal centerline height (see Figures 8 and 9). However, there are drawbacks to both proposed options. Test labs and manufacturers should anticipate that EMI test procedure concurrence will require a lengthy and iterative process.
Jeffrey Viel, is the Corporate Director of EMI/EMC services for National Technical Systems with over 20 years of electromagnetic interference design and compliance testing experience. He can be reached at email@example.com.