Electromagnetic Compatibility (EMC) in Aircraft

Electromagnetic Compatibility (EMC) in Aircraft

An aircraft packs hundreds of electronic units close together, under lightning, radar and radio transmitters. EMC is their ability to keep working correctly in that same electromagnetic environment. This module goes from the definition and sources of interference, through signals, spectra and the dB unit, to the test standards (CISPR, IEC 61000, DO-160) and the EMI reduction measures: filtering, ferrites, decoupling capacitors, wiring, grounding and bonding, with three live demos on the page.

Tooley ch.14 (p.218-237) · Lecture "EMC and Aircraft" (46 slides) · Floyd ch.13 section 13-1, ch.6 section 6-11, ch.15 section 15-3

Opening case: Wi-Fi and the Boeing 757 cockpit displays

The aircraft Wi-Fi system transmits at 2.4 or 5 GHz. Some of the emissions (intended or not) can couple into the wiring or electronics of the Honeywell Phase 3 displays (PFD, ND) in the Boeing 757 cockpit and cause display anomalies. Hence the cockpit placard banning Wi-Fi devices while the engines run for the purpose of flight.

Definition of EMC

Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment by limiting the unintentional generation, propagation and reception of electromagnetic energy that may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage. The goal: different equipment working correctly in a common electromagnetic environment.

The electromagnetic environment of an aircraft

External sources: lightning and weather, ground radar, VHF/UHF and navigation radio stations (VOR, DME, ILS), other aircraft (TCAS), solar and cosmic radiation. Internal sources: generators, motors, converters, avionics and switching devices. Four steps of the energy: unintentional generation, propagation, reflection/scattering, reception (through the aircraft's antennas and wiring).

Strength of electromagnetic sources

Bands: VLF 3 to 30 kHz, LF 30 to 300 kHz, MF 300 kHz to 3 MHz, HF 3 to 30 MHz, VHF 30 to 300 MHz, UHF 300 MHz to 3 GHz, SHF 3 to 30 GHz.

Sources and effects of EMI (Tooley 14.4)

Sources: fluorescent lights, radio and radar transmitters, power lines, window heat controllers, induction motors, switching and light-dimming circuits, microprocessors and associated circuitry, pulsed high-frequency circuits, bus cables (but not fibre-optic cables), static discharge and lightning.

Effects: errors in instrument indications (above and below true), heterodyne whistles on audio, herringbone patterns on video displays, buzz on intercom and cabin phones, desensitised radio and radar receivers, false indications on radar and DME, unwarranted alarms.

Signal types in EMC

All waveforms must be considered: low and high frequency, periodic and non-periodic, sinusoidal and modulated (AM, FM), narrowband (one frequency: VHF, VOR/ILS) and broadband (pulses, noise: switching supplies, fast-edge digital circuits, lightning pulses).

Time domain (oscilloscope): amplitude, period, rise and fall time. Frequency domain (spectrum analyser): frequency content, narrowband or broadband, level versus frequency.

Spectrum of a rectangular pulse: a sum of odd harmonics

Every complex wave comprises a fundamental plus many harmonics, each with its own amplitude and phase (Fourier analysis). Square wave: v = V[sin ωt + (1/3) sin 3ωt + (1/5) sin 5ωt + (1/7) sin 7ωt + …], only odd harmonics (f, 3f, 5f, 7f, …) with amplitude V/n; the spectral envelope is proportional to 1/n. Try Demo 2 at the bottom of the page.

Signals in the aircraft electromagnetic environment

The spectrum analyser

A spectrum analyser measures the frequency spectrum of an electrical signal and is one of the most important devices for EMC measurements. Block diagram (lecture): attenuator → mixer → intermediate-frequency (IF) amplifier → resolution bandwidth filter RBW → detector → video bandwidth filter VBW → display; the local oscillator LO sweeps from f_start to f_stop in sync with the display.

Noise floor and Tooley's spectrum examples

The display has an amplitude scale in dBm (−100 to +10 dBm) and a frequency scale (0.1 to 1000 MHz); signals appear as lines (a line spectrum) and two cursors measure amplitude and frequency (Figure 14.7: −26 dBm at 121 MHz). The measurement range is 110 dB, and the ultimate sensitivity is set by the instrument's own noise floor, typically −95 to −110 dB.

Why use dB

The dB is a compact way to express ratios of quantities spanning many orders of magnitude, used for the amplitude of voltage, current, power, electric and magnetic field. Rules: a positive dB value means an increase, a negative one a decrease; 20 dB = 10 × amplitude, 40 dB = 100 ×, 60 dB = 1000 × (or 100, 10,000, 1,000,000 × in power).

Examples: 1 mV to 1 V = +60 dB; 1 mW to 1 W = +30 dB; 10 µV/m to 1 mV/m = +40 dB; H from 1 to 0.1 A/m = −20 dB.

Power ratio and voltage ratio

L_P = 10 log(P₂/P₁); L = 20 log(V₂/V₁) = 20 log(I₂/I₁) because P ∝ V² (same impedance).

Absolute amplitudes: dBm, dBµV, dBµV/m

Exercises on slides 17 and 19: dB ratios and the dBm table

Slide 19 table: 0.001 W = 0 dBm; 0.1 W = 20; 0.5 W = 27; 1 W = 30; 2 W = 33; 10 W = 40; 1000 W = 60 dBm.

Adding and subtracting dB: gain and loss chains

With the same impedance, multiplying ratios becomes adding dB and dividing becomes subtracting dB. Two levels with the same reference can be subtracted: Ratio(dB) = P₂(dB) − P₁(dB); e.g. receiving 10 W (10 dB) against a 5 W specification (7 dB) gives 3 dB, i.e. double. Chains: 0 dBm through +10 dB and +6 dB amplifiers gives +16 dBm; 0 dBm through a −3 dB cable and a −10 dB filter gives −13 dBm.

Exercises on slide 21: four step-by-step solutions

What EMC standards contain

An EMC standard = terms and definitions + rules and requirements + test methods + limits and minimum test levels. Two groups: emissions (limit the unwanted electromagnetic energy the product emits, conducted and radiated) and immunity (the product works correctly under external disturbance: ESD, radiated RF field, fast transient EFT/burst, surge, conducted RF).

Example limit: CISPR 11 (Group 1, Class A)

Radiated emission limit for the electric field, measured in situ at 30 m with a quasi-peak detector: 30 dBµV/m from 30 MHz to 230 MHz and 37 dBµV/m above 230 MHz. The vertical axis is dBµV/m (relative to 1 µV/m), the horizontal axis is frequency on a logarithmic scale from 10 to 1000 MHz.

DO-160 in aerospace

DO-160 is published by the industry group Radio Technical Commission for Aeronautics (RTCA). RTCA is not a government body, but DO-160 is widely accepted by aviation authorities as a means of compliance. DO-160 is a test procedure, not a requirement: it gives guidance on which tests apply and describes test categories, methods and procedures.

The three factors of an EMI problem

Address at least one of the three factors. EMC must be considered from the design phase.

1. Suppress interference at the source

1. Enclose the interference source in a screened metal enclosure and ground it adequately.
2. Use transient suppression (TVS diode, MOV, RC snubber) on relays, switches and contactors.
3. Twist and/or shield bus wires and data bus connections.
4. Use screened (coaxial) cables for audio and radio-frequency signals, and connect the shield to ground properly.
5. Keep pulse rise times as slow and long as possible (reduce high-frequency content).
6. Check that enclosures, racks and supporting structures are grounded effectively (low impedance).

2. Reduce noise coupling and wiring practice

• Separate power wiring from signal wiring; route strategically (signals away from power).
• Twist/shield noisy wires and data buses; use optical fibre where possible (immune to EMI, Module 08).
• Keep ground leads short; break ground loops with isolation transformers, differential amplifiers or balanced circuits.
• Filter noisy output leads (ferrite cores, LC filters); relocate sensitive equipment away from the noise source.
• Keep wires short; keep shield pigtails to their designed length; do not extend equipment ground wires.

3. Increase susceptibility thresholds

1. Limit bandwidth to only what is strictly necessary (a band-pass filter removes wideband interference).
2. Limit gain and sensitivity to only what is strictly necessary (excess gain also amplifies interference).
3. Ensure enclosures are grounded and internal screens are fitted.
4. Fit less susceptible components against stray radiated fields: short leads (SMD), shielded modules in metal cans, shielded inductors, rather than long leads, unshielded devices and open inductors.

Filtering: the LC low-pass and power filters

There are many ways to filter EMI: on cables (ferrite core), at equipment inputs (LC-type EMI filter), on boards (small RC or LC). The idea is always the same: attenuate unwanted high-frequency noise while letting the useful signal pass.

A low-pass L + C filter: Z_L = jωL rises with frequency (blocks high-frequency noise), Z_C = 1/(jωC) falls with frequency (shunts high-frequency noise to ground). At 50/60 Hz Z_L is small and Z_C is large, so the filter barely affects the supply. Configurations: single-phase L + C, or the Pi filter (C, L, C) for better attenuation.

Ferrite cores: against common-mode noise

High-frequency noise current on a cable creates a magnetic field around it. A cylindrical ferrite core has high magnetic permeability μ, so it concentrates the flux; magnetic losses (hysteresis and eddy-current loss) convert energy into heat, so the cable impedance rises at high frequency and the noise current is attenuated.

If the current is too large the core saturates and loses effectiveness.

Decoupling capacitors at board level (Floyd 6-11)

A decoupling capacitor sits between VCC and ground of every component, placed as close as possible to the IC power pins with short traces, to give high-frequency noise a low-impedance path instead of running along the supply line and radiating. Several values in parallel cover a wide band: 10 µF (low), 1 µF (medium), 0.1 µF (high frequency).

Floyd 6-11 (p.364): current and voltage spikes on the VCC and ground lines from the fast switching of digital circuits; absorb them with about a 1 µF capacitor from VCC to ground for the whole board, plus small 0.022 to 0.1 µF capacitors distributed around, especially near devices switching at high rates or driving many loads (oscillators, counters, buffers, bus drivers).

A real capacitor at high frequency: ESL, ESR and resonance

A real capacitor = R (ESR) + L (ESL) + C in series: Z = R + jωL + 1/(jωC), |Z| = √(R² + (ωL − 1/(ωC))²). Resonant frequency f_r = 1/(2π√(LC)): below f_r it behaves as a capacitor (Z falls with frequency), at f_r the impedance is minimum ≈ ESR, above f_r it becomes an inductor (Z rises) and no longer decouples well. That is why aircraft equipment uses many capacitor values (more than 10): small capacitors (100 nF, 1 nF) for high frequency, large ones (1, 10 µF) for low frequency.

Grounding and bonding

The electrical integrity of the aircraft structure is very important to reduce EMI and protect the aircraft, passengers, crew and systems from lightning and static discharge; it also reduces the effect of high-intensity radio-frequency fields (HIRF) from high-power transmitters and radar. Use low-resistance bonding straps. Grounding and bonding resistances of less than 0.001 to 0.003 Ω are usually required.

Grounding is connecting conductive objects to a conductive structure or other return path to safely complete a normal or fault circuit; bonding is the electrical connecting of two or more conducting objects not otherwise adequately connected.

The six purposes of grounding and bonding in an aircraft

1. Protect aircraft, crew and passengers against lightning discharge: lightning current flows through the airframe to ground.
2. Provide return paths for normal and fault currents.
3. Prevent unwanted RF voltages and currents: all metal parts at the same potential.
4. Protect personnel from shock hazards: all accessible metal parts are bonded and grounded.
5. Maintain effective radio transmission and reception: a well-grounded antenna has low noise.
6. Prevent accumulation of static charge (e.g. fuel pipes, airflow).

Bonding procedures and checks

1. Bond or ground to the primary aircraft structure where possible; do not weaken the structure; bond parts individually if feasible.
2. Install against smooth, clean surfaces (remove paint and corrosion).
3. Install so that vibration, expansion or contraction, or relative movement will not break or loosen it; use flexible straps, proper clamps and strain relief.
4. Check integrity and effectiveness with an approved bonding tester: measure the resistance between part and structure and compare it with the maintenance data.

Digital noise limits: noise margin (Floyd 15-3)

The digital design side has its own "susceptibility threshold": the noise margin (Floyd p.859) measures how much noise a logic gate tolerates. V_NH = V_OH(min) − V_IH(min) and V_NL = V_IL(max) − V_OL(max). For 5 V CMOS: V_NH = 4.4 − 3.5 = 0.9 V, V_NL = 1.5 − 0.33 = 1.17 V. Noise smaller than the noise margin does not flip the logic level.

dB, dBm, square-wave harmonics and the resonance of a real capacitor

• L_P, L_Apower ratio and amplitude ratio in dB (the factor 20 because P ∝ V² at equal impedance)
• dBmpower level relative to 1 mW
• V/namplitude of the n-th odd harmonic of a square wave (n = 1, 3, 5, …)
• L, CESL inductance and capacitance of a real capacitor
• f_rresonant frequency; above f_r the capacitor behaves as an inductor

📜 Theory and historical background

According to Tooley (section 14.2), in recent years EMC and EMI have become a very important consideration for avionic equipment designers: both the unwanted signals the equipment generates and its susceptibility to outside interference must be considered, to meet increasingly demanding standards. The lecture opens with aircraft being tested in anechoic chambers (KC-46 Pegasus, Global Hawk) and the example of Wi-Fi interfering with Honeywell Phase 3 displays on the Boeing 757; the lecture's 'Historical facts' item has only a heading and no content. The RTCA's DO-160 is not a government regulation but is widely accepted by aviation authorities as a means of showing compliance.

🔎 Case study: a 400 Hz DC supply and the parts added to meet EMC

Tooley takes a simple low-voltage DC supply (Figure 14.1) and modifies it to meet stringent EMC requirements (Figure 14.5). The added parts have four jobs: C5 to C8 give high-frequency decoupling so the regulator ICs do not oscillate; C9 and C10 remove noise on the output rails; C11, L1, L2, C12, C13 form a low-pass supply filter that removes harmonics from the diode rectifiers' switching and also blocks noise entering the equipment from the supply; finally a low-resistance ground connection links the equipment chassis to aircraft ground. All three groups of measures of the lecture are present: reduce the source (filtering harmonics), reduce coupling (grounding, screening) and raise the susceptibility threshold (decoupling). A numerical example in the same chapter: a defective switching supply raised the noise floor in the 500 kHz to 1.75 MHz band from below −110 dB to about −84 dB, i.e. 26 dB, enough to badly degrade MF reception.

⚠️ Common traps (and mismatches found in the material)

General trap: confusing 0 dBm with 0 W (0 dBm is 1 mW) and mixing the factor 10 with 20 in the dB formulas (10 for power, 20 for voltage and current). Mismatches: slide 21 (exercise 2) prints "P_out = 10000 W (10 kW)" although the calculation right next to it gives 100,000 W; the correct answer is 100 kW because 10 W × 10⁴ = 100,000 W (see Demo 1 and the notebook). Slide 43 requires a bonding resistance of 0.001 to 0.003 Ω, whereas slide 45 says "typically below 2.5 mΩ" and the tester picture there reads 0.003 Ω, i.e. 3 mΩ, above 2.5 mΩ and therefore failing by that slide's own rule; Tooley says below 0.001 to 0.003 Ω and quiz question 6 says below 0.005 Ω, so the sources use different thresholds and the manufacturer's maintenance data must be followed. Tooley (Figure 14.14) says the noise floor rose by 26 dB "or 50 times"; 26 dB is about 20 times in amplitude (about 400 times in power), not 50. Slide 10 and Tooley 14.1.2 write the square wave as V sin ωt + (V/3) sin 3ωt + …; that is a simplified scaled form, since the fundamental of a square wave of amplitude V is really 4V/π, so read V there as the fundamental amplitude, not the square-wave amplitude. Slide 1 misspells "Dndergoes" in the Global Hawk caption, slide 16 repeats slide 15, and the slide 2 heading "Historical facts" has no content. On the Floyd side: Floyd has no EMC chapter; it mentions EMI only at cable level (twisted pairs reduce crosstalk, shielded STP resists EMI), board-level decoupling capacitors, and the noise margin of logic families.