MIL-STD-1377 is a military standard that provides guidelines and procedures for testing the electrical power systems of military and aerospace vehicles. The standard covers the requirements for measuring and evaluating the performance, reliability, and safety of these systems in a variety of operating environments and conditions.
One of the important aspects of MIL-STD-1377 testing is evaluating the shielding effectiveness of the power system. Shielding effectiveness refers to the ability of the power system to protect against electromagnetic interference (EMI) and radio frequency interference (RFI). This is a critical concern for military and aerospace applications, where the presence of electronic systems and communication equipment can generate significant levels of EMI and RFI that can interfere with the performance and reliability of other systems.
The testing process for MIL-STD-1377 involves a series of tests and evaluations designed to assess the performance and reliability of the power system. These tests typically include:
- Power supply analysis: The power supply is analyzed to ensure that it meets the electrical requirements and specifications specified in the standard. This includes measurements of voltage, current, and power output.
- Load testing: The power system is subjected to various load conditions to determine its ability to deliver power under different conditions. This includes tests of load regulation, line regulation, and transient response.
- Reliability testing: The reliability of the power system is assessed through a series of tests designed to evaluate the system’s ability to operate under a variety of conditions. This includes tests of durability, shock, vibration, and temperature.
- Safety testing: The safety of the power system is evaluated through a series of tests designed to assess the system’s ability to operate safely and without risk of damage to the vehicle or its occupants. This includes tests of over-voltage, over-current, and short circuit protection.
- Environmental testing: The performance of the power system in different environmental conditions is evaluated through a series of tests designed to assess the system’s ability to operate in extreme temperatures, humid conditions, and other environmental factors.
The test is performed over a frequency range of 100 kilohertz (kHz) up to 30 megahertz (MHz), then from 1 gigahertz (GHz) to 10 gigahertz (GHz).
As depicted in the figure below, the test setup is the same as other shielding effectiveness, with the determination of the dynamic range and the actual measurement of the signal for the sample in question. If the system involves a weapon enclosure, all internal components are removed so only the enclosure is tested.
Scope of the Military Shielding Effectiveness Compliance Standard
To gain a better understanding of the EMC test requirements of this standard, it is important to know the definitions of some of the key words and phrases. A weapon enclosure is the metal shell surrounding a weapon circuit, such as the weapon skin.
The weapon cable is any wiring outside of the weapon enclosure designed to be connected to circuits inside the enclosure. Surface transfer impedance is the ratio of the magnitudes of the longitudinal voltage drop on the outer surface of the shield to the current on the inside of the shield.
Testing to MIL-1377 shielding valuation requires a number of various pieces of equipment. The list below is not all inclusive, but it does provide an overview of the most common equipment needed.
- The signal source shall be any RF signal generator or power oscillator with a current output capability compatible with the ammeter and voltmeter. Any shielded RF current measuring device which can measure the center conductor current of a coaxial cable over the desired frequency range shall be used. Such a device can be constructed from a small metal box and an RF panel ammeter.
- The ammeter, mounted inside the box, is connected to the center conductors of two bulkhead connectors mounted on opposite sides of the box. A small screen-shielded window in front of the box is used for viewing the meter. The sensitivity of the ammeter shall be compatible with the signal generator output current capability.
- It is generally necessary to construct an adapter to link the signal source and ammeter to the test cable. This can be constructed by placing the appropriate connectors on opposite sides of a small metal box and joining the proper center conductors inside. If a multiconductor cable is to be tested, the selection of the conductor(s) to be connected to the signal source is arbitrary.
- One end of the test cable shall be terminated with a short that provides shield integrity. An RF voltage measuring device or balanced or near balanced input design shall be wed. The voltmeter probe should be fitted with balanced probe leads, each approximately 3 inches in length. The voltmeter shall be coordinated with the signal generator such that it is capable of measuring RF voltages as small as 1 millivolt per ampere of signal generator output current capability.
- To avoid direct coupling to the voltmeter probe, the circuit from the signal source to the cable under test should be shielded. The effectiveness of this shield should be greater than that of the cable under test. This can be determined by shorting the cable adapter and measuring the voltage and the center conducting current with the signal generator on. The surface transfer impedance (STI) calculated from this arrangement shall be at least 20 decibels (db) below the STI of the test cable when calculated.
Expert MIL-STD 1377 Laboratory Testing
Keystone Compliance tailors each test plan to individual customer needs minimizing unnecessary costs and over-testing. Thus, our proven process helps avoid product launch delays. In addition to MIL-STD shielding effectiveness, Keystone has a full scope of expertise including environmental and package testing such as vibration, cold chain, and accelerated aging.
Are you ready to partner with Keystone Compliance to determine the shielding effectiveness of your product when tested to the requirements of MIL-STD 1377? Let’s get started!
Diagram of MIL-STD 1377 Testing
For more information on Keystone’s shielding effectiveness testing capabilities, please visit: www.keystonecompliance.com
