Electronics Guide

Cable Coupling

Cables connecting equipment act as antennas, converting electromagnetic field energy into conducted interference on the cable conductors. The coupling efficiency depends on cable length relative to wavelength, cable geometry and shielding, and the impedance of the circuits at each end. At frequencies where the cable length approaches a quarter wavelength (approximately 0.75 meters at 100 MHz), cable coupling becomes particularly efficient.

Unshielded cables are especially effective antennas, with field-induced currents flowing on all conductors. Shielded cables provide protection when the shield is properly terminated to the enclosure at both ends, but shield effectiveness degrades at frequencies where the termination impedance becomes significant. Even well-shielded cables allow some field penetration through shield imperfections and at connector interfaces.

Direct Field Penetration

Electromagnetic fields penetrate enclosures through apertures including ventilation slots, display windows, seams between mating surfaces, and any gap in the conductive enclosure. The penetration efficiency depends on the aperture dimensions relative to wavelength, with apertures approaching a half wavelength providing efficient coupling.

Once inside the enclosure, fields induce currents on PCB traces, component leads, and internal wiring. The induced currents depend on the loop areas presented by the circuits and the field orientation. Circuits with large loop areas or high-impedance sensitive nodes are most vulnerable to direct field coupling.

Antenna Effects

Some equipment configurations create unintended antenna structures that efficiently capture electromagnetic field energy. Long internal cables, heatsink structures, and extended ground planes can resonate at frequencies within the test range, producing enhanced susceptibility at specific frequencies. These resonances may not be apparent from circuit analysis but become evident during immunity testing.

Test Environment

Testing is performed in a controlled environment that ensures uniform field illumination of the equipment under test. Anechoic chambers lined with RF absorbing material are most commonly used, providing a reflection-free environment for accurate field generation. Semi-anechoic chambers with absorbing walls and ceiling but a reflective floor provide a defined ground plane reference. GTEM cells (gigahertz transverse electromagnetic cells) offer a compact alternative for smaller equipment.

Field uniformity must meet defined criteria within the test volume. The standard requires that field strength be within +6/-0 dB of the target level over 75% of the test area. Achieving uniform field distribution requires careful test setup including proper antenna selection, appropriate distances, and correct absorber placement.

Test Levels

Four test levels are defined based on the intended operating environment. Level 1 (1 V/m) represents protected environments with minimal RF exposure. Level 2 (3 V/m) covers typical commercial environments. Level 3 (10 V/m) addresses industrial environments with significant RF sources. Level 4 (30 V/m) covers heavy industrial and outdoor environments with high RF exposure.

Some applications require higher test levels. Military equipment often must withstand 200 V/m or more. Automotive electronics may be tested to 30-200 V/m depending on location within the vehicle. Medical equipment standards specify levels appropriate to the device classification and intended use environment.

Frequency Range and Modulation

The standard test range of 80 MHz to 1 GHz covers the frequency range where cable coupling is most efficient and common RF sources operate. Extended testing to 6 GHz addresses the increasing use of higher frequencies for wireless communications and the susceptibility of high-speed digital circuits.

The test signal is amplitude-modulated at 1 kHz with 80% modulation depth. This modulation simulates the effect of pulsed and modulated communication signals and reveals detection effects where semiconductor junctions demodulate the RF envelope, producing audio-frequency interference. Continuous-wave (unmodulated) testing may miss these susceptibility mechanisms.

Equipment Orientation

Testing is performed with the equipment in multiple orientations to identify the most susceptible exposure direction. The field is typically applied from four sides (front, back, left, right) with both horizontal and vertical polarization. Some standards require additional orientations including top exposure for floor-standing equipment.

Each orientation is tested across the entire frequency range, making radiated immunity testing a time-consuming process. Sweep rates must be slow enough to allow equipment response time and detection of transient malfunctions. Frequency dwell at susceptibility peaks helps characterize the failure mode and margin.

Detection Effects

Semiconductor junctions can function as RF detectors, demodulating amplitude-modulated signals to produce baseband interference. This detection occurs when RF voltage exceeds the small-signal range where junction behavior is linear. The resulting detected signal can affect analog circuits, corrupt digital communications, and cause erratic microprocessor behavior.

Operational amplifier inputs are particularly susceptible to detection because of their high gain and the nonlinear voltage-current characteristic at the input transistors. Even signals well below normal input levels can produce detection when the RF amplitude is large enough to cause significant nonlinear operation.

Direct RF Interference

In addition to detection effects, the RF carrier itself can interfere with circuit operation. High-frequency noise on power supply rails affects voltage regulation and introduces modulation on sensitive analog circuits. RF currents in ground paths produce ground bounce that corrupts logic levels and affects signal integrity.

Radio receivers and other RF-sensitive circuits experience direct interference when the illuminating field falls within their operating frequency range. Even shielded receivers can be affected through antenna connections and I/O ports that provide coupling paths into the RF front end.

Resonance Effects

Mechanical and electrical resonances within equipment can produce enhanced susceptibility at specific frequencies. Cable resonances, PCB resonances, and enclosure cavity resonances all create frequency-dependent variations in immunity performance. These resonances may cause dramatic susceptibility peaks that are not evident from broadband immunity considerations.

Shielding

Conductive enclosures provide the first line of defense against radiated fields. The shielding effectiveness depends on enclosure material, construction, and the treatment of necessary openings. Continuous welded or brazed seams provide better shielding than mechanical joints, which require conductive gaskets or spring fingers to maintain shield integrity.

Apertures in shields must be managed to maintain shielding effectiveness at the frequencies of concern. The critical dimension of an aperture determines its shielding effectiveness, with smaller apertures providing better shielding at higher frequencies. Multiple small apertures provide better shielding than a single large aperture of the same total area.

Ventilation openings can be covered with honeycomb panels or perforated screens that maintain shielding while allowing airflow. The cell size or hole spacing should be small compared to the wavelength at the highest frequency of concern. Conductive mesh on display windows provides shielding while maintaining visibility.

Cable Shielding and Filtering

Shielded cables with proper termination prevent cables from acting as efficient antennas. The shield should make a 360-degree, low-impedance connection to the enclosure at the entry point. Pigtail connections that gather the shield braid and connect with a wire create inductance that severely degrades high-frequency shielding effectiveness.

Filtering at cable entry points attenuates RF currents before they reach internal circuits. Feed-through capacitors, ferrite filters, and pi-filters provide increasing attenuation for progressively more demanding applications. Filter placement at the enclosure boundary maintains the shield integrity by preventing RF currents from entering the shielded volume.

Circuit Design

Circuit-level measures complement enclosure shielding and provide defense-in-depth against fields that penetrate external barriers. Series resistance in sensitive inputs limits peak RF current and reduces detection. Local bypass capacitors provide low-impedance AC ground near sensitive nodes. Differential circuits inherently reject common-mode RF interference when properly balanced.

High-frequency supply decoupling reduces power rail susceptibility. Multiple capacitor values in parallel provide low impedance over a wide frequency range. Ferrite beads in series with supply traces increase supply impedance to RF while passing DC and low-frequency supply current.