RTCA DO-160 Requirements and Tests

DO-160 allots the first three sections to establish some common elements applicable to the specific test sections.

Section 1 discusses the purpose of this standard is to standardize conditions and procedures to instill confidence that the tested device will perform properly when exposed to various environmental (including the electromagnetic environment) conditions. It is noted that proper performance is established by the equipment performance standard. The performance standard defines the allowed operation during less-than-optimal conditions. For example, a particular device may reset if a power voltage transient exceeding 1,000 volts occurs. The performance standard may allow a reset if the device does not require operator actions to accomplish the reset and the reset is complete within 3-seconds. If this performance is acceptable for the aircraft operation, then a reset that meets the performance criteria is acceptable for that application. The performance variation should not be a surprise to the user – full disclosure is documented on the Environmental Qualification Form.

Other conditions not included in the standard may need to be included in the overall acceptance for a particular device. These conditions may need to be evaluated if that condition’s compliance is necessary. It is the obligation of the device performance author to include the special conditions and the means to evaluate.

Not all conditions apply to each device. When applying the standard, the minimum operational performance standards (MOPS) for devices are examined to determine applicability. The party responsible for device acceptance must approve applicability for each of the test methods. As part of the applicability process, various categories are assigned based on device vulnerability to reduce applying excessive requirements unnecessarily.

Section 1 also refers to user guides for selected test methods. The user guides are meant to aid in understanding the goals of the test and assist in preparing procedures for the evaluation.

Section 2 is listed as definitions of terms. This is not just a list of acronyms but a guide in understanding the meaning behind the terms. Things like determining thermal stabilization, assigning categories and the like are discussed in this section.

Section 3 discusses conditions of tests providing general test setup guidance. Some test methods include details that may alter the general conditions. Unless otherwise stated, section 3 conditions apply.

Equipment connections and orientations should follow the manufacturer’s recommended installation practices, including required ancillary items such as cooling fans or remote controls. The interconnecting cables are at least 1.5 meters and configured to allow one common cable bundle of 1.2 meters. For most of the EMI/EMC test configurations, the cables have a different arrangement defined by the individual test methods.

Using multiple test articles is permitted except for when cumulative testing is necessary. The manufacturer is responsible for assessing the need for cumulative testing. For example, it may be appropriate to specify that conducted emission testing be accomplished on a unit that had been subjected to induced lightning. This sequence could identify defective filters that were damaged by high current induced by the lightning transient. Section 3 identifies certain tests that have a required sequence. Combining tests are also permitted, as long as the required conditions are met (combining EMI/EMC tests is limited to verifying multiple categories by accomplishing only the worst-case test).

Section 3 specifies the ambient conditions (temperature, humidity atmospheric pressure) for the test facilities.

Susceptibility tests are to be accomplished with the test article operating in the most vulnerable mode for that particular test. This could be the weakest transmit signal amplitude for a radio transceiver operating in the receive mode during radiated RF susceptibility testing. Although not stated, emission tests should use the mode that would produce the highest emission levels. Sufficient operational modes should be used to evaluate all circuits.

Section 15 testing determines the effect the equipment has on compasses or compass sensors (flux gates). Current flow in electronic circuits produces magnetic fields, and permeable materials may distort the earth’s magnetic field. Either field variance has the potential to vary compass indications. Since the compass provides aircraft heading information it is critical that the readings are accurate.

Modern technology provides more accurate positioning instruments but the compass maintains its place as an emergency indicator in case of signal loss or power failures.

Several factors determine how the magnetic fields affect the compass deflection, such as current amplitude, proximity of compass to the circuit, permeability of material, direction of conductors, etc. An electronic device brings these factors into a complex arrangement for creating a magnetic field. Including the device mass and material without power may distort the earth’s field and influence the compass reading. During a simple experiment placing a wire in alignment with the compass’s North-South axis, the compass changed the reading by over 20° when only 325ma current was allowed to flow. Were the wire placed above the compass, the direction of change would be opposite to the direction of deflection with the wire placed beneath the compass. If the wire was aligned on the East-West axis, the deflection may be unchanged.

Distance between the device and the compass significantly affects the deflection. DO-160 establishes categories to classify the potential influence based on the distance. The assigned category provides data to installing the device relative to the compass position. Five categories are assigned based on a 1° change of the compass reading.

Section 17 determines if the device can withstand voltage spikes arriving at the power input terminals. Testing applies to ac and dc powered equipment.

Two categories are associated with section 17.

• Category A – equipment with a high degree of protection required
• Category B – equipment with a lower degree of protection required

Figure 4 shows the test waveform with an open circuit termination and 50? source impedance. Some spike generators that produce the prescribed waveform have a low impedance source requiring that a series resistor be added to create the 50? source. The generator impedance can be checked by setting a voltage with an open circuit and then terminating the generator output with 50? and verifying that the voltage is 0.5 times the open-circuit voltage.

After verification of the waveform, record the spike generator settings for both positive and negative spikes. Connect the spike generator to the EUT power terminals and a 10?F capacitor across the power source to limit the voltage drop across the power source, as indicated in Figure 5. The isolation transformer is needed if the power return is ungrounded.

The settings for the open circuit waveform verification are used when applying the spikes to the EUT power input. The waveform will normally experience a lot of distortion from the EUT reactance characteristics, and the amplitude will be limited if the characteristic impedance loads the generator. The settings are not adjusted to compensate for loading during test – the applied voltage is recorded with the established generator settings.

Issues with voltage spike normally involve incorporating transient suppressors, filter or filters inserts to reduce the spike amplitude entering the power input circuit. Wire routing that may allow field to couple energy around the filter or cable-to-cable coupling of the spike into a sensitive circuit may provide mitigation of the problem.

Section 19 evaluated the susceptibility of induced voltage and current from fields associated with the installation environment. Magnetic and electric fields from power, signal and transients induce voltage via field-to-cable, cable-to-cable, and field-to-equipment coupling paths when equipment and wiring are close to each other.

As with other test methods, categories are assigned based on the power type for the device, the required performance, and potential exposure. Figure 7 provides information on category assignments.

The various applicable tests are listed with the required test levels for each category. The five tests for Section 19 are:

1. Magnetic fields induced into the equipment
2. Electric fields induced into the equipment
3. Magnetic fields induced into the interconnecting cables
4. Electric fields induced into interconnecting cables
5. Spikes induced into interconnecting cables

As an example of testing, we will examine one of the potential tests – Magnetic fields induced into interconnecting cables. Figure 8 provides a sketch of the basic test configuration. The test level for category CC equipment is 120 A-m (40-amps x 3-meters of cable) between 380 and 420Hz and 60 A-m at 400 Hz decreasing to 1.6 A-m at 15 kHz. The high level between 380 and 420 Hz is established because the fundamental power frequency would exhibit more current in the power distribution wiring providing a greater coupling level. As the frequency increases the harmonic currents diminish, reducing the threat level.

The interfering signal is stepped over the test frequency range in 30 logarithmically spaced steps per frequency decade with a minimum 10-second dwell at each step. During test, the EUT performance is monitored for indications of susceptibility.

Testing is applicable to cables external to the EUT. The induced current is normally associated with exposure to radiated fields that couple energy into the cable. Except at lower frequencies, interference signals are attenuated by the distributed wire inductance of the cables if the cable is long.

Categories are assigned based on the aircraft type and locations within the aircraft for various threat levels. Conducted susceptibility categories are based on:

• Category Y/O – equipment located in a very severe electromagnetic environment where failure is catastrophic (level A) impact to flight operations
• Category W – level A equipment located in a severe electromagnetic environment
• Category M – level A equipment located in a moderate electromagnetic environment
• Category R – level A equipment located in a protected electromagnetic environment or where failure is hazardous (level B) impact to flight operations
• Category T – equipment located in a protected electromagnetic environment where failure is major (level C) impact to flight operations
• Category S – equipment where failure is minor (level D) or no (level E) impact to flight operations

Positioning of the cables and the injection/monitoring probes is critical to maintaining a standardized test. Current is induced into the circuit from fields or cable-to-cable where the closed loop necessary for current flow is from the point of coupling through the EUT and returns to the cable via the ground plane. Figure 10 shows the general cable layout with the probes positioned as required. The figure indicates the presence of distributed inductance and capacitance for the cable based on the wire characteristics and position relative to the ground plane. At low test frequencies, the distributed inductive reactance is low, and the distributed capacitive reactance is high, supporting the interfering signal current flow from the injection probe through the EUT to ground the ground plane to the support equipment to return to the injection probe via the cable. As the test frequency increases the distributed inductive reactance increases and distributed capacitive reactance decreases allowing more current to return to the injection probe through the distributed capacitance and less current returns via the support equipment. As the frequency reaches the upper test frequency range the current loop is very short, as indicated by the loops in Figure 10.

Figure 11 shows the probe positioning requirements to ensure that the higher frequency actual current flow includes the EUT. If the probes were placed at the cable center, the higher frequency interference would simply flow around the cable at that location without reaching the EUT circuits.

DO-160 supports testing of multiple cables simultaneously with separate probes. This approach is needed when redundant circuits are present that could be exposed at the same time in a common mode fashion. Cables that are routed in different directions should be tested separately because the exposure is not common to the different cables.

Pigtail exclusion from the cable test is recommended if the wire is providing cable shield termination and it is not bonded to the ancillary equipment chassis. If the pigtail is less than one meter, it is not tested. If it is greater than one meter, it is tested separately.

Conducted susceptibility testing specifies that a continuous wave (CW) and square wave modulation be used during test. The square wave modulation depth is at least 90% indicating that amplitude modulation (AM) is used. The interfering signal amplitude is set using the CW signal. When AM is applied, the signal peak amplitude is almost double the CW amplitude. No provisions are made to adjust the amplitude, so the square wave peak is equal to the CW level.

Emission testing quantifies the unintended energy emitted from a device. Measurements for conducted emissions assess the potential for undesired signals to enter connected devices or to radiate from the cables. Radiated emission measurements provide field strength measurements.

Categories are assigned based on the aircraft type and locations within the aircraft for where sensitive items are located. Categories are:

• Category B – locations where interference should be controlled to a tolerable level
• Category L – equipment and cables are located far from apertures of the aircraft
• Category M – equipment and cables are in areas where apertures are significant but not in view of radio receiver antennas
• Category H – equipment and cables are in direct view of radio receiver antennas
• Category P – equipment or cables are located close to HF, VHF or GPS radio receiver antennas of where aircraft structure provided little shielding
• Category Q – equipment or cables are located close to VHF or GPS radio receiver antennas of the aircraft structure provides little shielding

The test configuration generally follows the layout of Section 20 (see Figure 9) above as far as the cable and equipment layout.

Measurements are made with the receiver in the peak detector mode and the 6 dB bandwidth set based on the test frequency range listed in DO-160, Section 21, Table I. A minimum dwell time is listed in the table, but the actual dwell time is based on the time it takes the EUT to perform the functions being measured. If it takes 300 mS for a device to complete an operation, then the dwell time at each frequency would be expanded to at least 300 mS. Stepped receivers need to step in increments of 0.5 * BW to prevent measurements that are potentially 6 dB below the peak amplitude. Steps of ½ BW increments reduce the measurement accuracy to about 2 dB.

At a minimum, the test report shall include the following test setup and data items:

• Cable configuration (length, type, termination, shielding, etc.)
• Block diagrams or photographs of each test setup
• EUT operating modes
• Loads and stimulation equipment
• Detailed test results with comparison to the applicable limit

Radiated emission testing covers the frequency range of 100 MHz to 6 GHz with the measurement system antenna 1-meter from the EUT boundary (0.9-meter from the ground plane). Table-top equipment layouts use a table height of 90 cm.

Once the measurement system is established and EUT operation stabilized, scan the test frequency range using the proper bandwidth and dwell time measuring the radiated emissions. Apply any correction and conversion factors to the measurements and plot the measurements with the applicable limit on the plot.

Multiple antenna positions may be necessary to ensure that the peaks emission arrives within the 3 dB beamwidth of the antenna. Antenna positioning guidance is presented in the discussion of Section 20 radiated susceptibility testing.

Section 21 provides for using a reverberation chamber test method instead of the anechoic chamber method. Some advantages include testing of all EUT faces simultaneously and the improved measurement system sensitivity from reflected signal amplification without amplifier noise gain. Disadvantages include the test time is longer and over-limit emissions do not lead to the source direction. The major challenge is measurement of the chamber loading associated with the EUT installation which varies based on the EUT size.

If over-limit emissions are detected, resolution is usually required. Several methods are used once the source and point of radiation are determined. If cables are the escape point, adding filtering is often the initial approach with filter inserts providing a quick answer. If the chassis is the escape point, either suppressing the signal source or improving the shielding are typical resolution approaches.