Table of Contents

Table of Figures

What is RTCA DO-160?

RTCA DO-160 has evolved into the primary commercial aviation standard for Environmental Conditions and Test Procedures for Airborne Equipment. Included in DO-160 various test methods and requirements are minimum performance standards for Environmental evaluations (climatic and dynamic conditions), power input variations and Electromagnetic Interference (EMI) / Electromagnetic Compatibility (EMC) conditions.

RTCA (Radio Technical Commission for Aeronautics) was founded in 1935 as a private association and reincorporated in 1991 as a private not-for-profit corporation. The mission of RTCA is creation and implementation of standards for the global aviation environment. As a standards development organization, RTCA works with the Federal Aviation Administration (FAA) and industry experts from around the world to develop standards.

DO-160 is one of the many standards managed by RTCA with the SC-135 committee responsible for the content. DO-357 is a user guide supplement to DO-160 that provides background information and guidance in applying the requirements.

DO-138, issued in 1958 (the precursor to DO-160), was used as the standard for environmental qualification testing. DO-160 was first issued in February 1975 with seven revisions spanning the 35-years until revision “G” was published in December 2010. Revision G, change one, came out in 2014 incorporating reference to DO-357 for user guide information. Revision “H” is with committee as of this writing with a target to publish by 2024.

Aircraft manufacturers frequently customize DO-160 for their specific standard where supplemental requirements may be added for their particular applications. This does not alter the DO-160 compliance for regulatory body approvals.

This DO-160 discussion is limited to the sections of DO-160 that deal with EMI/EMC and the section dealing with power input because of the close relationship with EMC. Sections included are:

  • Section 15 – Magnetic Effect
  • Section 16 – Power Input
  • Section 17 – Voltage Spike
  • Section 18 – Audio Frequency Conducted Susceptibility – Power Inputs
  • Section 19 – Induced Signal Susceptibility
  • Section 20 – Radio Frequency Susceptibility (Radiated and Conducted)
  • Section 21 – Emission of Radio Frequency Energy
  • Section 22 – Lightning Induced Transient Susceptibility
  • Section 23 – Lightning Direct Effects
  • Section 25 – Electrostatic Discharge

DO-160 General Information

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.

Test Report Layout and Content

The test configuration and test article operation are documented in the test report. This information should include the test article settings to confirm the evaluation used worst-case operation for the various tests.

A test report outline is not specified in DO-160 but many of the test methods require that certain information be included for that test. Test reports are normally left to the test laboratory format as long as the content requirements are met. The report should provide the reader with an understanding of the data and adequate detail to support repeating the test.

The report should include administrative data regarding what was tested and how it was operated during test. The “what was tested” needs to fully document the test article. A listing of critical features and identification of the sub-assembly revision levels, software/firmware revision, and other items are necessary to truly identify the test article. Deviations to an approved procedure or the test standard should be clearly stated with the technical rationale for the deviation.

The detailed results are necessary for the report providing the measurements along with the various data reduction parameters, such as transducer factors. If susceptibility is observed, the details and threshold measurements are recorded in the report. Unless expressly required, useful information pertinent to understanding the product and test configuration risks omission. For example, DO-160 does not call for listing the fundamental power frequency return current of the EUT in the test configuration. However, including that information in the test report can be essential to understanding if the power distribution path conforms to the installed current path.

An essential part of the documentation is completion of the Environmental Qualification Form. The form is provided in appendix A of DO-160. This form summarizes all the DO-160 testing for device qualification. Each test and section reference is provided for authors to incorporate test results. The test category should be included in the results block along with any pertinent information that may restrict installation locations or performance limitations.

Section 15 – Magnetic Effect

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.

  • Category Y – device placed immediately adjacent to the compass (0-meter)
  • Category Z – device placed between 0 and 0.3 meter from compass
  • Category A – device placed between 0.3 and 1 meter from compass
  • Category B – device placed between 1 and 3 meters from compass
  • Category C – device placed >3 meters from compass (report minimum distance in the equipment qualification form)

Often a device being considered for aircraft installation has a pre-determined location requiring compliance with an assigned category.

Assessing the compliance involves using a compass or magnetic sensor with resolution adequate to measure the required deflection. The compass needle needs to be level with the earth’s surface to ensure the horizontal component of the earth’s field is used for measurements. A compass with a floating needle meets that need.

The equipment under test (EUT) is placed on a non-magnetic stand and the compass on another stand (see Figure 1) with the direction between the compass and EUT along the East-West axis. Using a wheeled cart for the EUT allows movement of the EUT without movement of the compass that could result in a misalignment or field uniformity variations. Route the EUT cables directly away from the compass face. The EUT is placed at a long distance (far enough to not observe compass movement if the EUT is moved) from the compass to allow the compass to be aligned North-South (0°). Support equipment needs to be far enough away to not influence the testing with sufficient cable length to not require support equipment movement.

The earth’s magnetic field North-South poles vary from the geographic North-South poles depending on any location and date. The reference basis uses a horizontal magnetic field of 14.4 A/m (18129 nT). If the earth’s magnetic field is not equal to the reference basis, we need to determine the deflection constant (dc) for the test laboratory. For example, the earth’s horizontal magnetic field at 38 °N / 120 °W with an elevation of 150 meters on June 25th, 2021, was 22840.6 nT (dc-0.79). That same location on June 25th, 2024, will be 22715.9 nT (dc-0.80).

Conversion of nT to A/m is equal to B (in Tesla) / ?o (4? x e-7 Henries/m). This conversion can be used to keep the A/m reference if desired.

Once aligned, move the EUT toward the compass until the compass deflects by the dc (dc replaces 1° for the compass movement) and measure the distance between the EUT face and the compass center to obtain the test result in meters. Assign the compliance category based on the distance.

Testing is accomplished for each operational mode to determine the worst-case mode for the category classification. Note that power off is to be included as an operational mode (it is possible that current could cancel part of the deflection depending on placement).

Figure 1: Section 15 Test Layout
Figure 1: Section 15 Test Layout

Issues with compliance are resolved by positioning in the installation. If critical, the device can be fitted with a cancellation circuit where a field is generated in the opposite direction to negate the magnetic effects. For example, a current-carrying wire routed on the left side of a compass would cause a deflection. Routing the return current wire on the right side would cause a deflection in the opposite direction resulting in cancellation of the deflection with equal current in the opposite direction. This approach requires that the distance of the wires to the compass be equal for full cancellation.

Section 16 – Power Input

Section 16 evaluates the ability of the device to tolerate variations of the aircraft power. Several categories are available for various standard power buses of 115/230 Vac, 14/28 Vdc or 270 Vdc. If the utilization voltage is not one of the standard voltages, the compliance requirements must be defined in the equipment performance standards.

We will first discuss the ac power compliance criteria which is designated as 115 Vac/400 Hz and may be single or three phase distribution. If the power source is 230 Vac instead of 115 Vac, the test amplitudes are multiplied by 2.

Figure 2 lists various ac test designations where the common tests are grouped by the ac power frequency ranges. Note that in Figure 2 the designation column can be associated with any row. Testing to the (WF) range supports the use with more restrictive ranges (NF or CF) but testing to (CF) does not support use in less restrictive ranges.

Three test levels are assigned based on the status of the aircraft’s power generation capacity: 1) Normal, 2) Abnormal, and 3) Emergency. This supports operation based on the criticality of the device for aircraft functionality.

Figure 2: AC Power Input Test Designations
Figure 2: AC Power Input Test Designations

Tests for ac equipment include:

  1. Normal operating conditions
    • Steady state voltage and frequency
    • Voltage modulation
    • Frequency modulation
    • Momentary power interruption
    • Surge voltage
    • Frequency transients
    • Frequency variations
    • Voltage dc content on ac power
    • Total harmonic distortion (voltage)
    • Individual harmonic distortion (voltage)
  1. Abnormal operating conditions
    • Steady state voltage and frequency
    • Momentary undervoltage operation
    • Surge voltage
    • Frequency transients
    • Frequency variations
    • Loss of phase input (3-phase equipment only)
  1. Emergency operating conditions
    • Steady state voltage
    • Phase unbalance (3-phase equipment only)
  1. Designated categories
    • Current distortion (designation H)
    • Inrush current (designation I)
    • Current modulation (designation L)
    • Power factor (designation P)

Figure 3 lists various dc test designations where the common tests are grouped by the dc power voltage ranges. Note that in Figure 3 the designation column can be associated with any row. Testing to category Z is acceptable in lieu of Category A or Category B.

As with ac, three test levels are assigned based on the status of the aircraft’s power generation capacity: 1) Normal, 2) Abnormal, and 3) Emergency. This supports operation based on critically of the device for aircraft functionality. The test levels for the 14 Vdc applications are normally ½ of the 28 Vdc test levels.

Figure 3: DC Power Input Test Designations
Figure 3: DC Power Input Test Designations

Tests for dc equipment include:

  1. Normal operating conditions
    • Steady state voltage and frequency
    • Voltage modulation
    • Frequency modulation
    • Momentary power interruption
    • Surge voltage
    • Frequency transients
    • Frequency variations
    • Voltage dc content on ac power
    • Total harmonic distortion (voltage)
    • Individual harmonic distortion (voltage)
  1. Abnormal operating conditions
    • Steady state voltage
    • Low voltage (category B only)
    • Momentary undervoltage operation
    • Surge voltage
  1. Emergency operating conditions
    • Steady state voltage
  1. Designated categories
    • Regenerated energy (category D only)
    • Inrush current (designation I)
    • Current ripple (designation R)

As you can see by the lists, many tests are associated with power input qualification. A programmable power source accomplishes many of the tests, and suppliers of these power sources have programs for most of the tests. If not available from the vendors, programs can be developed to execute the various test parameters and used multiple times.

In many cases, testing at the abnormal or emergency levels satisfies the normal test requirements if the device performance is maintained without degradation. Look for these options to reduce the test time by eliminating the normal test levels if applicable.

Note that DO-357 (user guide for DO-160) includes some information that supplements the test guidance contained in DO-160.

Issues with power input compliance normally involve circuit design changes to incorporate or modify transient suppressors, power filters, hold-up capacitors, or other control measures depending on the specific issue.

Section 17 – Voltage Spike

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.

Figure 4: Section 17 Waveform
Figure 4: Section 17 Waveform

Figure 5: Section 17 Test Configuration
Figure 5: Section 17 Test Configuration

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 18 – Audio Frequency Conducted Susceptibility – Power Inputs

The Section 18 test goal is to determine if power line conducted interference over the frequency range of 10 Hz to 150 kHz causes the device performance degradation. Often the frequency components are related to power source harmonics, or switched mode power supplies that couple energy onto the power distribution bus.

Compliance categories are assigned based on the aircraft power performance with ac power categories of R(CF), R(NF) and R(WF) associated with the power fundamental frequency variance. These classifications follow the Section 16 category A power ranges. In addition, a designation of “K” that replaces the “R” if the power source distortion levels are higher than the standard aircraft distortion control.

The dc categories are R, B or Z based on the dc bus power source; “R” is associated with dc power from an ac power system. “B” is associated with power from an engine driven alternator with a battery floating on the bus. “Z” is associated with other types of aircraft power systems and “Z” is allowed to be used in place of “R” or “B” categories. Category “Z” applies to the 270 Vdc system. The “K” designator is also available for dc power if appropriate. Note that test levels for 14 Vdc systems are 0.5 times the 28 Vdc test levels.

The susceptibility testing pass/fail is related to the EUT performance with the test voltage being applied to the power terminals. Because the input circuit impedance could be very low at certain frequencies allowing the current to be excessive, a maximum current to be applied is established. This limitation establishes that – 1) a test voltage applied to the EUT or 2) a maximum applied current is reached without producing susceptibility of the EUT and indicates compliance.

Figure 6 provides a general test configuration for an ac powered device. A coupling transformer secondary is placed in series to support modulation of the applied power. The signal source (signal generator and amplifier) is adjusted to produce the test voltage or current limit at the input terminals. The signal source is adjusted over the test frequency range maintaining the test voltage or current limit while monitoring the EUT performance for indications of susceptibility.

The testing calls for applying the interfering signal at a minimum of 30 logarithmically spaced frequency points per decade over the test frequency range. The dwell time at each frequency is at least 1-minute.

Prior to test, a 0.6? resistor is connected to the coupling transformer secondary to verify that the signal source amplifier can produce the required voltage and current without distortion. The voltage and current values are based on dissipating 100W by the 0.6? resistor. The maximum current limit is 12.91 amps rms or amps peak-to-peak.

During tests, current flows in a loop from the coupling transformer secondary through the EUT and is returned via the power supply mains. As the test frequency increases the mains power impedance increases creating a voltage drop reducing the voltage drop at the EUT. The capacitor across the mains terminals reduces the voltage drop by providing a low impedance path for the current loop.

Notice that the measurement oscilloscope uses an isolation transformer to prevent accidental grounding of the power line under test. This allows the oscilloscope to become a potential shock hazard if connected to an ungrounded neutral or phase lead. Use caution! Differential probe measurements can be used as an alternative to isolating the oscilloscope.

Figure 6: Section 18 Test Configuration
Figure 6: Section 18 Test Configuration

If issues are identified, solutions tend to be somewhat difficult at lower frequencies for AC powered devices reacting to power frequency harmonics. At higher frequencies, filtered connectors or filter inserts can be very effective.

Section 19 – Induced Signal Susceptibility

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

Figure 7: Section 19 Category Definitions
Figure 7: Section 19 Category Definitions

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.

Figure 8: Section 19 Magnetic Fields Induced Into Cables
Figure 8: Section 19 Magnetic Fields Induced Into Cables

Note that the coupling wire is near the cable under test and the remaining coupling is kept away from the cable under test to prevent cancellation of the interfering signal field.

The electric field test is similar, but the coupling wire is open circuit to support an electric field without the associated magnetic field and the associated signal source energy demands. Refer to DO-160 for details of other tests and their configurations and test levels.

Susceptibility issues associated with the induced fields are generally resolved by installation practices segregating sensitive cables and threat sources. Avoid long parallel cable runs to minimize cable-to-cable coupling. Place sensitive circuits away from the equipment chassis where threats could be present. Filters or filter inserts are not normally used because of the low frequencies involved, but do not rule them out when analysis indicates their feasibility, especially when spikes are the issue.

Quell_Aeronautics CTA image

Section 20 – Radio Frequency Susceptibility

Section 20 provides a lot of configuration information that applies to both the conducted and radiated testing. Testing is accomplished in a shielded enclosure to allow testing throughout the frequency range without interference affecting other items in the test laboratory vicinity.

Figure 9 shows a general test layout. Excess items are removed when accomplishing a specific test, such as the removal of the current probes during radiated testing.

The figure shows a layout with cables routed in different directions. If the installation calls for cable routing in the same direction, the cable layout will follow that approach. The goal is to use a standardized layout so all laboratories will attain the same measurements within the uncertainty. Cable layout greatly affects the measurements. It is reasonable to duplicate an actual installation if all installations are identical, but the standardized configuration is used when multiple installation options would apply. The layout is critical to obtaining repeatable measurements and to support coupling of energy into the device.

Testing is accomplished with the test article on a ground plane. Attaching the ground plane to the enclosure calls for using multiple ground straps spaced no greater than 1-meter apart. Straps are attached to the rear and sides of the ground plane with a dc bonding resistance of less than 2.5 milliohms. The ground straps should have a length to width ratio no greater than 5:1 to minimize the inductive reactance.

Grounding of the test article affects the testing so only the provisions included in the design and installation instructions are to be used in the test configuration. The resistance is not specified but resistance measurements should be consistent with the installation specification. If a ground terminal is provided and the installation fails to define the connection method, a 30 cm wire of the same gauge as the power wire would be used for EUT grounding. It is inappropriate to create an artificial grounding system that would not be duplicated in an installation.

The cable layout places a length of the cable 10 cm behind the front edge of the ground plane elevated 5 cm above the ground plane. Excess cable length is routed toward the rear of the ground plane in a zigzag arrangement to avoid cable loops. The cable length is normally 3.3-meters unless otherwise stated. Cables up to 15-meters may be used if the installation calls for long cables.

Power lines that are part of an overall bundle remain with the bundle until the cable exits the test area where the power leads are separated and attached to LISNs. When the power return is locally grounded, only the positive lead is routed through a LISN – the return is connected to the ground plane.

Antenna cables are terminated in a load impedance characteristic of the cable. This dummy antenna is shielded, and the antenna cable is tested as any other cable. Test levels of the antenna cable may be adjusted for sensitive receiver ports within the receive operating band. If susceptibility is observed from other cable tests or radiated tests, the results must be evaluated to acceptability.

The interfering signal is stepped over the test frequency range in 100 logarithmically spaced steps per frequency decade (10 steps per decade at test frequencies below 100 kHz) with a minimum 1-second dwell at each step. During test, the EUT performance is monitored for indications of susceptibility.

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
  • Pass/fail criteria
  • Detailed test results

Figure 9: Section 20 General Test Configuration
Figure 9: Section 20 General Test Configuration

Section 20 – Radio Frequency Susceptibility – Conducted

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.

Figure 10: Section 20 Conducted Current Flow
Figure 10: Section 20 Conducted Current Flow

The testing phase applies the test current to each of the interface cables. A monitor current probe is placed 5-cm from the interface connector and the injection current probe is 5-cm from the monitor probe. The positioning prevents the induced current from being returned by the parasitic elements without flowing in the equipment circuitry.

Prior to test, a calibration sweep is accomplished to record the forward power required to produce a calibration current defined by the applicable limit. This establishes the maximum signal amplitude to be applied.

After the calibration process, the probes are placed on the cable selected for test. The signal source is adjusted to the start frequency and the amplitude to the lesser of the specified current or calibration setting for that frequency. CW is applied for the dwell period and then the square wave modulation is enabled for the dwell period.

Figure 11: Section 20 Conducted Test Configuration
Figure 11: Section 20 Conducted Test Configuration

It is instinctive to attack problem resolution through filter connectors, filter inserts or cable shielding to reduce current in the wiring. However, before applying cable control measures, verify that the issue is not from radiation. Since the test is simulating radiated interference inducing current it is logical that inducing a current would create radiation from the cables. To eliminate radiation from the cables as the culprit, place a temporary shield over the EUT. If the threshold of susceptibility sees a significant change, consider that a chassis aperture may be the path for the susceptibility signal.

Section 20 – Radio Frequency Susceptibility – Radiated

Testing is applicable to equipment and cables exposed to radiated fields. Categories are assigned based on the threat level for the installed location. The electromagnetic environment poses both external and internal threats to the aircraft. Ten category designations are listed along with the associated test levels. Test levels may vary for different frequency bands within a category and if pulse modulation applies. The test levels range from 1 V/m up to 490 V/m for selected CW or square wave modulation. Some categories specify pulse modulation with peak field strengths up to 7200 V/m.

Two fundamental test methods are used. The anechoic chamber method uses a shielded enclosure lined with RF absorber to reduce the effects of reflected signals illuminating the EUT. The reverberation chamber method takes advantage of reflected signals to illuminate all faces of the EUT.

Figure 9 show the general layout for radiated susceptibility testing (current probes are not present for radiated testing). The antenna is located at least 1-meter from the EUT boundary. Prior to test, the field is calibrated by placing a RF field probe where the EUT will be positioned and the radiating antenna at the test location. The RF source illuminates the RF field probe, and the source is adjusted to produce the specified field strength. The source drive levels are recorded to be used during test. If the EUT removal is prohibitive, field calibration can be accomplished at another location in the anechoic chamber, as long as the probe to antenna maintains the same relative position–and the reflection area of the chamber is avoided.

After the field calibration, the EUT is installed in the test configuration and operation established. The recorded RF source settings are then used to illuminate the EUT. As the source is stepped through the test frequency range, the EUT is monitored for susceptibility indications.

The EUT and cables up to 0.5 times the test frequency wavelength are to be exposed within the 3 dB beamwidth of the radiating antenna. This may require multiple antenna positions depending on the EUT size and cable arrangement. Antenna beamwidth information is available from the antenna manufacturer, and the coverage area can be calculated by 2 * d * tan (BW/2) where d = distance between EUT and radiating antenna and BW is the antenna beamwidth. Horizontal and vertical antenna polarizations provide different beamwidths.

The use of a reverberation chamber is listed in the standard as an alternate means of radiated susceptibility testing with a mode-stirred approach. This method uses an unlined chamber with highly conductive walls and mode tuning paddles to distribute the field within the test volume. The reverberation chamber provides an advantage in exposing all EUT faces simultaneously and creating very high-level fields with relatively low power amplifiers. The test time may be extended because of the physical movement of the tuning paddles and if susceptibility is noted, determining the entry location is a little more difficult. Loading effects of the EUT are needed to manage control of the test levels. Or a maximum loading must be applied for the calibration, and the resulting over-test allowed.

Radiated field susceptibility issues take on a variety of solutions from filter connectors or filter inserts for cable coupled interference to shielding and aperture closure if the chassis is the path for signals to enter.

The test uses a 3-port network to input two signals simultaneously into the receiver. One of the signals is set to the receiver tuned frequency with the EUT modulation applied. The amplitude is set to be just above the threshold of the receiver sensitivity to establish receiver operation. The second frequency signal is set to the test frequency with the amplitude set to the rejection requirement level above the receiver sensitivity. The second frequency signal is tuned through the test frequency range while the receiver is monitored for indications of susceptibility. If susceptibility is observed, lower the second signal amplitude until the receiver recovers to determine the rejection capability.

The requirements and test frequency range are determined for the specific technology associated with the receiver. Therefore, the procurement specification provides the details on testing and the required rejection. It is common for the rejection level requirement to be somewhat less restrictive for frequencies within the tuning range of the receiver but outside the tuned frequency bandwidth.

Band-pass filtering is typically used to reject out-of-band frequencies in the receiver design, and additional antenna port filtering tends to be avoided because of excess capacitance affecting the RF signals intentionally received.

Section 21 – Emission of Radio Frequency Energy

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

Section 21 – Emission of Radio Frequency Energy – Conducted

Conducted emissions measurements are made over the frequency range of 50 kHz to 152 MHz. The RF current monitor probe is positioned on the cable 5 cm from the EUT. If the EUT connector plus backshell length exceeds 5 cm, position the probe as close to the backshell as possible.

Once the probe is positioned and EUT operation stabilized, scan the test frequency range using the proper bandwidth and dwell time measuring the conducted emissions. Apply any correction and conversion factors to the measurements and plot the measurements with the applicable limit on the plot. Use caution about including a grounding pigtail within the probe as discussed in Section 20 conducted susceptibility.

Conducted emission over limit conditions are typically resolved by filters or filter inserts located at the EUT chassis boundary. Filter selection is a function of the frequency that needs attenuation and may be limited to the amount of capacitance or inductance that maintains the intended signal integrity. Cable shielding is considered but often encounters problems with acceptability because the installation may not be changed.

Section 21 – Emission of Radio Frequency Energy – Radiated

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.

Section 22 – Lightning Induced Transient Susceptibility

The purpose of Section 22 testing is to evaluate the EUT’s ability to withstand lightning induced transients coupled onto the cables. Two groups of tests support the induced lightning to provide damage tolerance or functional upset of the device.

Test categories are associated with test levels and waveforms consistent with the use and aircraft installation. A six-character series designates the applicable tests and test levels as indicated in Figure 12. A letter “Z” in any position indicates the detection of a different waveform or configuration from the designated identifier. The letter “X” indicated that the test is not applicable.

Figure 12: Section 22 Category Designations
Figure 12: Section 22 Category Designations

Several test configurations are provided in DO-160 for the various tests but basic principles apply to each configuration. The test waveforms are based on current or voltage with one being the test target value and the other providing a limiting function. The various waveforms and applicability of each are discussed in DO-160. In the example (see Figure 13), waveform WF1 provides the test current (IT) and WF2 provides the limit voltage (VL) parameters.

Figure 13: Section 22 Waveform Example
Figure 13: Section 22 Waveform Example

In the calibration configuration (see Figure 14) adjust the WF1 to the designated test current (IT) with the shorted loop setting and verify the waveform parameters. Using the open loop setting adjust the WF2 to the designated test voltage (VT) and verify the waveform parameters. It is not required that the current or voltage limit (IL or VL) be verified but if the generator can attain those levels, record, and verify the waveforms. Guidance on acceptability is provided in the user guide if the limit level is reached without attaining the test level.

Figure 14: Section 22 Calibration Configuration
Figure 14: Section 22 Calibration Configuration

After the waveform levels and parameters are verified, configure the EUT cable under test through the current monitor probe and injection transformer. Adjust the generator output to the test level or until the limit level is attained. Figure 15 shows an example test configuration.

Figure 15: Section 22 Test Configuration
Figure 15: Section 22 Test Configuration

If designated, multiple stroke and burst testing are applied to the testing for each of the defined waveforms.

  • Multiple stroke testing applies an initial transient at the designated level followed by thirteen subsequent transients at a decreased level over a period of up to 1.5-seconds. Ten multiple stroke applications are applied with up to five minutes between each application.
  • Multiple burst testing applies a group of twenty transients with 50 to 1000 µS between transients with three sets of bursts spaced between 30 and 300 mS. Burst groups are applied every three seconds for a minimum of five minutes

Transient suppressors are the primary means to protect sensitive circuitry from the effects of the induced lightning. Cable shields and installation wire routing to prevent the coupling are potential solutions, but many applications do not support these approaches.

Section 23 – Lightning Direct Effects

Section 23 testing applies to externally mounted equipment or is covered by a dielectric skin that is an integral part of the device. High voltage strike attachment tests and high current physical damage tests are evaluated by Section 23 tests. High voltage tests identify the locations of attachment followed by high current damage test applied to the identified location. In many cases, attachment points are not identified for a device. In those cases, current distribution analysis identifies the primary point for current test application.

Categories are assigned based on locating the equipment in designated lightning attachment zones. Test peak amplitudes up to 2400 kV and 200 kA are associated with the direct strike evaluation depending on the designated category.

Section 23 testing is very specialized with demands for customized configurations and test equipment and facilities. Calibration of waveforms and test levels are accomplished on a dummy test item that has the approximate dimensions and conductivity as the test article. After the waveform calibration is complete, the EUT is exposed to the lightning transient. A post-test evaluation is accomplished to determine acceptability of the device.

Refer to DO-160 and DO-357 for the details of direct strike lighting test requirements.

Quell_MIL STD 461 CTA image

Section 25 – Electrostatic Discharge (ESD)

Section 25 testing is to verify the ability of the EUT to withstand ESD from personnel-borne discharges.

The ESD simulator follows the human-body model established in the IEC (International Electrotechnical Commission) 61000-4-2 with 150 pF capacitor and 330 ? resistance.

Although not required by DO-160, signal integrity checks (refer to Figure 16) are accomplished to verity the ESD simulator performance.

  • The simulator tip voltage is checked with an electrostatic voltmeter to verify that the voltage is at the correct level for the setting. Checks are made at each voltage setting to be used during test.
  • The discharge current waveform characteristics are verified by applying a contact discharge of 8 kV to a test target and measuring the timing and current levels associated with the waveform.

Figure 16: ESD Signal Integrity Check
Figure 16: ESD Signal Integrity Check

After the signal integrity checks are complete, EUT testing can proceed while maintaining the standard test configuration. Test points are selected based on accessibility to human contact. Points to consider for testing include operator controls and indicators, ventilation openings, connector shells and seams/slots where a gap could be present.

Testing is accomplished via an air discharge method with testing at the 15 kV level. Air discharges are applied with a charged tip moved perpendicular toward the test point no faster than 0.3 meters/second until the discharge occurs or contact with the test point is made. Between discharge applications, if a discharge did not occur, remove the residual charge by grounding the point through a resistor or waiting until the charge dissipates. Testing is to be accomplished at each of the defined voltages with ten positive and ten negative discharges.

ESD issues typically are managed through positioning of sensitive circuits in a manner that separates the circuit from the discharge path including the radiated field associated with the discharge. Filtered connectors or filter inserts reduce the potential for cable coupled discharges. Often, a TVS or MOV can be placed into a filter insert to protect the equipment from ESD. Filter inserts offer a quick solution with a shorter lead time than alternative solutions.