Impulse testing requirements in the military

Testing equipment to ensure it meets military standards has a long history and is usually more demanding than tests carried out on other equipment. While this story looks at American-specific methods, the requirements in Australia would be similar and does give a good indication to prospective exporters of just what is needed to meet military specifications.

American military experience gained around the world is now embodied in a single document, MIL-STD-461E superseding MIL 461D and MIL 462.

The earlier standards are still among the most widely used for military electronics suppliers, due largely to the fact that they cover inter-service operability issues.

In line with other international standards, the transient event in MIL 461E is now specified at the point of injection to equipment under test (EUT) which includes all cabling and coupling elements in a test system verification.

Differences between the current and previous editions of MIL standards extend to changes to the test requirements both in terms of equipment and verification procedures.

MIL-STD-461 and 462 are themselves the combination of about 20 service-dependent documents previously in use for military procurement.

Each edition has been systematically replaced by revised versions, however older MIL-STD editions can still be called out in procurement specifications to qualify replacement hardware added into systems older than 25 years.

Pre-test verification of the test equipment became mandatory as part of the overall test procedure. This introduced the need to include all cables and clamps in the procedure, as a result shifting the emphasis from individual equipment specifications to a system specification defined at the actual point of injection.

461E is intended to serve applications from trucks to ships to aircraft. It can tailor requirements for a specific platform without having to make exceptions to the standard. Most sections are now broadly similar to IEC and FCC, but the requirements are, by necessity, harsher.

The MIL-STD-461E test types required to be performed on a specific platform are clearly defined (see figure 1).

Impulse tests within the MIL-STD document are relatively few compared to the conducted and radiated continuous wave (CW) tests that have to be performed this has one unfortunate side effect in that some of the techniques and equipment have been adapted, where possible, from the RF requirements and are not necessarily optimum for meeting the impulse requirements.

Power supply cables are to be tested in common mode with the return looped outside the injection clamp. This situation simulates the platform structure being used as the power return path (see figure 2).

The sharp (2 nS) rise and fall times are important as they should simulate the incidence of platform switching transients. The pulse duration approximates the energy content of such transients.

A 30 Hz repetition has been chosen to ensure equipment resistibility.

Frequency characteristics of the clamp are critical to maintaining the CS115 transient characteristics. A clamp with reduced bandwidth will result in drooping over the pulse duration and distortion of the rise and fall times.

The specified pulse can only be achieved in a verification jig with defined impedance avoiding reflections which will influence waveshape and amplitude. The waveshape is a current, verified by measuring across one of the 50 ohm shunt resistors (see figure 3 and figure 4).

A selection of damped sinusoidal transients is used to simulate oscillations arising from excitation of the platform wiring due to incidence of lightning, EMP or platform switching (see figure 5).

The test signal amplitude has its maximum between 1 and 30 MHz reducing at lower and higher frequencies.

The damping rate or Q factor is measured from the first peak to the peak closest to the 50% point.

The open circuit voltage is not defined in this requirement, only the peak current driven into a 100 ohm circuit.

The maximum specified current into 100 ohm load is 10 A plus a 100% margin giving 20 A. Therefore the maximum voltage at the output of the generator is about 2000 V (20 A x 100 ). The test and verification equipment must be selected to withstand such high voltage transients.

The specified pulse can only be achieved in the verification jig as the actual waveshape and amplitude are dependent on cable impedances. The waveshape is a current, verified by measuring across one of the 50 ohm shunt resistors.

Example of measured current in a 1 m calibration jig (see figure 6, figure 7 and figure 8).

A fast rise time free space phenomena to simulate the effects of NEMP transients. The EM field is generated by transient voltage impulses applied to a parallel plate antenna.

The EUT is placed in the centre of a test volume which provides a uniform field between the antenna plates.

Applied to units only, potential equipment responses caused by cable induction are simulated using the CS116 test.

The EUT must be rotated to test in all three axes.

Injection of all impulse types, with the exception of RS105, are on cable bundles only, including signal and power lines. There are no longer any PIN injection requirements as in the equivalent civilian standard RTCA DO160 or direct injection as used in IEC and ANSI/IEEE standards.

The frequencies for damped sinusoidal transients range from tens of kilohertz to hundreds of megahertz, placing particular demands on the injection coupler. Most coupling devices are designed for RF operation in defined frequency bands and in general have poor low frequency characteristics.

The implications of using such couplers translate directly into the cost of a test system as more energy or several couplers are required to achieve the same test level at higher frequencies (see figure 9).

Verification of the test system is an integral part of the test procedure with requirements for waveform plots to be included in the final test report.

The complete test set-up, including all cabling and injection clamps, is to be arranged as closely as possible to the final test layout.

The idea being to perform the system verification using the verification jig, then remove the jig and replace it with the EUT, disturbing the test equipment layout as little as possible.

MIL-STD-461E makes no reference to open circuit voltage requirements, emphasis is on achieving the prescribed current limit in a 100 ohm circuit.

All test amplitude values are defined as LIMITS. Because of the well-defined test procedure, it is not necessary to monitor transients during testing. System verification provides a value for each frequency required to meet the standard limits in a test jig.

The test jig is removed and a test cable fitted into the coupling clamp. Transients are injected according to the equipment set-up values obtained during verification.

MIL-STD-461E requires three distinct impulse test types, with very diverse waveform characteristics and energy content (see figure 10).

CS115 and CS116 impulses, however, are similar in terms of energy requirement and can be incorporated into a single test system.

RS105 transients are a completely different dimension and must be a standalone system. Because of the high EM field intensities, RS105 testing should be in a shielded enclosure or at least sufficiently far away from other equipment/installations that may be subject to interference.

The test equipment requires a high-voltage generator of several hundred kilovolts, definitely not tabletop equipment.

By its very nature, defence tends to have a nationalistic orientation; this applies not least to the test and procurement requirements for military hardware. MIL-STD-461E is an American standard widely used to procure military equipment for all branches of the services.

Defence Standard 59-41, which has recently undergone changes, GAM-EG-13B and many other standards have parallel test requirements. Unfortunately for manufacturers wishing to export their products to several markets, the standards are not always compatible and each calls out local requirements to be applied during equipment testing at a manufacturer's plant.

One advantage of the clear system definition in MIL-STD-461E is a faster set-up and test time. A primary objective of this standard is to increase repeatability of testing results through an easy-to-understand tests application; EMC tests are well defined and do not require highly qualified personnel.

MIL-STD-461E is to be treated as a generic standard, offering a basic level of immunity across a broad spectrum of equipment. As a generic standard, it cannot provide an absolute test catalogue for every product, so that application-specific standards could be more appropriate.

An example is the DO-160D avionics standard which contains many more detailed requirements specifically for airborne equipment.