In the previous article, we systematically organized the implementation of EMI countermeasures. In this article, which is FC28, we will cover EMC design. Moving a step beyond the EMI countermeasures discussed so far (managing noise generation, propagation, and interference), we will re-evaluate FC design from the perspective of regulatory compliance that a product must meet.
Difference between EMI and EMC
EMI (Electromagnetic Interference) and EMC (Electromagnetic Compatibility) are closely related but are different concepts.
EMI refers to noise itself or the interference phenomenon that noise causes in other equipment. All discussions up to the previous article focused on the generation and suppression of EMI.
EMC is a broader concept that refers to 'the ability of equipment to function normally in an electromagnetic environment without causing unacceptable electromagnetic interference to surrounding equipment.' EMC has two aspects: Emission, which regulates the noise generated by the equipment, and Immunity, which regulates how much resistance the equipment has against external noise.
While the series so far has focused on the emission side (not emitting noise), this article will cover systematic EMC design, including the immunity side (withstanding external noise).
Regulatory Frameworks Related to Drone FCs
We will organize the standards that may be referenced in FC development.
FCC Part 15 (USA) is a standard regarding intentional and unintentional radiation from radio frequency equipment. Drone radio transmitters (2.4GHz/5.8GHz) are subject to Subpart C of Part 15, and the FC itself may be subject to Subpart B as an unintentional radiator.
The CE/EMC Directive (Europe) typically references EN 55032 (Emission) and EN 55035 (Immunity). When obtaining CE marking for a drone as a whole, the FC, as a component, is also included in these test targets.
CISPR 25 (Automotive) is an EMC standard for automotive-mounted equipment, but it is increasingly being referenced for industrial drones (especially for logistics and infrastructure inspection). It includes stricter immunity requirements and has comprehensive test items assuming a vehicle environment.
RTCA DO-160 (Aviation) is an EMC test standard for manned aviation equipment, and it is sometimes referenced in the development of some commercial UAVs (especially large-scale and industrial ones). While test items like lightning surge immunity and high-intensity RF environment immunity may be excessive for drone applications, it is rich in knowledge that can be used as a reference for design.
Technical Conformity (Japan, Radio Act) is a certification system directly related to radio modules (telemetry, Wi-Fi, etc.) mounted on an FC. If the FC itself has radio functions, it is subject to Technical Conformity.
In practice, many small-scale drone manufacturers and custom FC developers do not obtain formal regulatory certification, but utilizing the test items of these standards as criteria for design reviews directly leads to improved product quality.
Emission Test Items and Reflection in Design
We will organize representative emission test items and their impact on FC design.
Radiated Emission testing measures electromagnetic waves radiated from equipment in an anechoic chamber in the range of 30MHz to 1GHz (up to 6GHz depending on the standard). Limits are specified in dBμV/m and are corrected according to the distance (usually 3m or 10m).
DC-DC switching noise, high-speed digital signals, and cable radiation, all of which were explained previously, affect this test. Near-field probe evaluation during the FC design stage (previous series, FC27) functions as an alternative early screening for this formal test.
Conducted Emission testing measures noise on power lines in the range of 150kHz to 30MHz via an LISN (Line Impedance Stabilization Network).
The design quality of the FC's power input filter (the pi-filter from the previous series) directly affects the results of this test. The LISN provides standardized impedance (usually 50Ω), enabling reproducible measurements.
Both tests involve two detection methods, Quasi-Peak and Average, and the applicable limit values differ depending on the standard. As a design margin, it is common industry practice to set a target 6 to 10 dB lower than the limit value during simulation and preliminary evaluation.
Immunity Test Items and Their Impact on Design
While attention tends to focus on emission countermeasures, resistance to external noise (immunity) is also a critical element of FC design.
The Electrostatic Discharge (ESD) test (IEC 61000-4-2) evaluates resistance to electrostatic discharge from humans or objects. There are test levels for contact discharge (±4kV, ±8kV) and air discharge (±8kV, ±15kV).
FC external connectors (USB, telemetry, etc.) are prone to becoming entry points for ESD surges, and the placement of TVS diodes, as explained in the series before last, is a direct countermeasure for this test. Connecting the connector shell to GND (explained in FC24) also functions as a path to properly divert ESD current.
The Radiated Immunity test (IEC 61000-4-3) evaluates the equipment's resistance to external electromagnetic waves (typically 80MHz to 6GHz, with a test field strength of 3 to 10V/m).
Since the drone's radio transmitter (2.4GHz) itself can be a source of radiated noise for the FC, the placement and shielding design of the radio module and FC within the airframe affect the results of this test. Ensuring physical distance between antennas, as explained in the previous series (GNSS design), is also relevant here.
The Surge test (IEC 61000-4-5) evaluates resistance to transient high-energy surges (such as lightning induction) on power lines. While surges originating from commercial power are unlikely for battery-powered drones, transient phenomena during rapid charger connection or discharges from the airframe's static charge can pose similar threats.
The Electrical Fast Transient/Burst (EFT/B) test (IEC 61000-4-4) evaluates resistance to high-frequency repetitive burst noise, which has aspects similar to evaluating resistance to noise from motor drive circuits (ESC switching noise).
Practical Countermeasures for Immunity Design
We will organize the design methods corresponding to these immunity tests.
Strengthening the input protection stage involves a combination of TVS diodes, ferrite beads, and proper grounding to block paths through which external transient noise can enter internal circuits. The 'connector peripheral techniques' from the series before last serve as the foundation here as well.
Considering the bidirectionality of filters is an important design perspective. While EMI filters have been designed primarily to 'keep internal noise from getting out,' the same filter structures (LC filters, ferrite beads) also have the effect of 'keeping external noise from getting in.' Since pi-filters and common-mode chokes function bidirectionally, it is possible to design for both emission and immunity countermeasures simultaneously.
Lowering GND impedance: The ground design principles from previous sessions (continuous GND plane, shortest return current path) also contribute to improved immunity. Noise current that has entered from the outside is quickly diffused and attenuated through the low-impedance GND path, reducing its impact on internal circuits.
As a software-side countermeasure, if instantaneous noise bursts like EFT/B appear as abnormal values in sensor data, outlier removal via EKF innovation monitoring (see FC13–FC14) acts as a final line of defense to mitigate the effects of noise that cannot be completely prevented by hardware. Multi-layered defense on both the hardware and software sides is the practical approach to immunity design.
Planning the Test Schedule
We will organize the practical workflow for planning EMC tests.
Preliminary evaluation (pre-compliance) is an evaluation conducted in-house or with simple equipment before the official test at an EMC laboratory. The combination of near-field probes and a spectrum analyzer, as explained in the series before last, functions as a preliminary evaluation for emissions.
Preliminary immunity evaluation requires more specialized equipment (ESD guns, antennas for radiated immunity, and signal generators). If it is difficult to conduct this with in-house equipment, utilize the preliminary evaluation services of a specialized laboratory.
Ensuring design margins by maintaining sufficient leeway (6dB or more) against limit values during pre-compliance testing increases the first-pass success rate in official tests. Incorporating this pre-compliance evaluation into the early stages of FC prototyping directly leads to risk reduction in the overall development schedule.
As a response flow for non-compliance, typical actions taken when non-compliance is identified during formal testing tend to be 'retroactive measures' such as adding ferrite beads, adding shielding, or adjusting software filters. This means being forced to rely on the second and third priorities (low-cost passive components, high-cost measures) in the 'priority of countermeasures' discussed previously, and it reaffirms the importance of fundamental measures (return current, loop area minimization) at the initial design stage.
EMC Requirements Specific to Industrial Applications
We will organize the additional EMC requirements that should be considered for industrial drone FCs.
Operation near high-voltage power lines (for power infrastructure inspection) requires immunity to strong 50/60Hz electromagnetic fields and their harmonics. This is a special electromagnetic environment not covered by standard EMC regulations, making on-site field evaluation important.
Operation within factories and warehouses (for indoor inspection) requires guaranteed operation in environments with diverse noise sources such as large motors, welding machines, and wireless LAN access points.
Dense aircraft operation (simultaneous flight of multiple drones) requires consideration of mutual electromagnetic interference between aircraft (especially wireless communication in the same frequency band).
Documentation of EMC Design
In development aimed at regulatory compliance, recording design decisions (documentation) becomes an important practical element.
The EMC design review checklist will be developed as a document that clearly states the correspondence with the test items of each standard, based on the implementation checklist organized last time (FC27).
By recording the rationale for component selection, such as the clamping voltage of TVS diodes, impedance characteristics of ferrite beads, and cutoff frequencies of filters, responses to future design changes or standard revisions will be streamlined.
As for test result traceability, we will establish a system to record the results of pre-compliance tests and formal tests for each design version, allowing us to track the impact of design changes on EMC performance.
Summary
EMC design is a broader concept that encompasses previous EMI countermeasures, dealing with both emission (not emitting noise) and immunity (withstanding noise). Standards such as FCC Part 15, the CE/EMC Directive, and CISPR 25 are referenced in drone FC development, and radiated/conducted emission tests and immunity tests such as ESD, radiated immunity, and surge tests are representative evaluation items. Many filter and ground designs are bidirectional measures that are effective for both emission and immunity, and fundamental responses at the initial design stage (return current management, loop area minimization) most effectively reduce the risk of test non-compliance. In the next issue, FC29, we will explain thermal design—heat dissipation design and thermal simulation for heat-generating components on the FC board.