The field of avionics and aerospace electronics has unique aspects not present in other EMI/EMC testing environments. Although many tests are similar to those performed in military applications, the environment of an aircraft brings in other electromagnetic aspects which have to be considered in the design of the product. For this article, RTCA DO-160G will be considered.
Setting Up EMI and EMC Tests
The similarities of Section 21’s conducted and radiated emissions to military standards is mainly in the test setup. Both military and aerospace products perform testing on a conductive bench, using LISNs on the power lines. But DO-160 does not perform a low frequency conducted emissions measurement, and those performed starting at 150 kHz measure currents on the power lines, not voltages from an LISN. As a result, filter designs may need to be modified to address this difference. Also, interconnecting cables are also measured for conducted emissions, unlike other standards. Conducted emissions is performed up to 152 MHz, not 10 MHz or 30 MHz as in many standards.
Radiated emissions, which start at 100 MHz, much higher start frequency than other standards, may require using one of the Section 21 limit lines with notches. These notches are frequency bands where the limit line is reduced by 20 dB or more. This provides a guard band around the receivers on the aircraft for both communication and navigation purposes, especially for electronics which may be near or exposed to the aircraft antennas.
Many conducted susceptibility tests, such as in Section 20, have strong similarities to military testing, but have added impulses typically not imposed on other electronics. Induced lightning testing performed in Section 22 can be tested on cable bundles or the full chassis of the equipment (called ground injection) but can also be imposed on each individual pin of the device. Test levels are based on the aircraft structure (composite which is called resistance, or metallic), proximity of the devices cabling to apertures (doors, windows, or access panels), or if the cable bundle has a shield or simply a ground wire in the bundle.
Additional Testing Considerations
Other conducted tests are performed on the interconnecting bundles (non-power line related). These induced signal tests in Section 19 replicate the type of fields which may be electrically or magnetically coupled onto the signal lines from adjacent power lines or other sources. If relays are opened and closed on the power lines, this can create a high frequency interference on nearby signal lines. This environment is replicated by a “Chattering Relay” test, using a normally closed, unsuppressed relay, connected as a buzzer, to create these fields. It should be noted that most relays used on aircraft are unsuppressed, since they are lighter and more reliable over time.
Power line testing is included in Section 16. These tests require a variety of under voltage, over voltage, voltage transients, dropouts, and ripple tests to be performed. The ripple test is typically covered under Section 18 testing. It is recommended no to perform these tests using an LISN on the power line, since this will interfere with the quality of the test, such as the transition time of voltage steps.
For radiated susceptibility in Section 22, test levels performed may be extremely high. Amplitudes up to 7,200 V/m are required for some electronics on rotary wing aircraft. These aircraft tend to have open flight decks with little or no shielding and are allowed to fly closer to some ground transmitters than fixed wing aircraft. As a result, the electronics must withstand very intense fields without interference or loss of function.
In addition to the induced lightning test of Section 22, direct lightning of Section 23 may also be imposed. This is typically performed on external structures and devices such as antennas, and other nose, wing or tail mounted items. Peak currents of 200 kA can be obtained, which can damage or fuse the item being tested.
The magnetic effects test of Section 15 may also be required. The measurement will establish the distance the avionics will be to create a 1-degree deflection on a compass or magnetic sensor used on the aircraft. The test is best performed on a non-conductive table, and away from any metallic objects, such as shielded enclosures, which can interfere with the uniformity of the magnetic field in the area. This test should not be performed inside a shielded enclosure, nor on a metallic bench where currents can flow and induce a magnetic field.
The final two tests to address voltage spikes on the power lines in Section 17, and electrostatic discharge in Section 25. For the voltage spike test, if performing induced lightning in Section 22, this should be easy to pass. For ESD, the commonly used conductive chasses used in avionics highly protect the electronics from these discharges. The ESD tests are performed like the commercial ESD tests, using air discharge methods at 15 kV test levels.
In all cases, except for Section 15 Magnetic Effects, the term DETERMINE COMPLIANCE WITH APPLICABLE EQUIPMENT PERFORMANCE STANDARDS is used. These performance standards refer to the minimum operational performance standard (MPS or MOPS) established by the RTCA or EUROCAE, or by the manufacturers equipment specification when that is applicable. Therefore, when performing these tests, it must be well understood what these standards or specifications are, and what will constitute a failure of the test.