Electronics Guide

Processor and Memory Emissions

The central processing unit and associated memory subsystem are among the most significant emission sources in any server. Modern processors contain billions of transistors switching at multi-gigahertz frequencies, with each transition generating current pulses that create electromagnetic fields. The memory interface, with dozens of lanes operating in parallel at gigabit-per-second rates, adds substantial additional emission sources.

Processor emissions occur across a broad frequency spectrum:

• Clock frequencies: The base clock and its harmonics create narrowband emissions at predictable frequencies. Spread-spectrum clocking techniques reduce peak emissions by distributing energy across a wider bandwidth.
• Switching noise: The aggregate effect of billions of transistors switching creates broadband noise extending from audio frequencies through gigahertz ranges. The spectral content depends on switching patterns influenced by software workload.
• Power delivery noise: Rapid changes in processor current demand create voltage fluctuations on power delivery networks that can radiate from PCB structures and cables.

Memory emissions follow similar patterns but add complexity from the parallel data bus structure. DDR interfaces create significant common-mode currents due to simultaneous switching of multiple data lines. The memory module sockets, cables in registered DIMMs, and the motherboard routing all contribute to emission characteristics.

High-Speed I/O Emissions

Server I/O interfaces, including PCIe, SAS, SATA, and networking ports, generate emissions from their high-speed signaling. These interfaces typically use differential signaling that inherently reduces emissions compared to single-ended alternatives, but careful design is still required to meet regulatory limits.

PCIe interfaces, particularly at Gen 4 and Gen 5 speeds, operate at 16-32 GT/s data rates requiring signal bandwidths extending beyond 30 GHz. At these frequencies, even minor PCB routing imperfections create impedance discontinuities that convert differential signals to common-mode currents, which radiate more efficiently. Proper PCB layer stack design, controlled impedance routing, and attention to via transitions minimize this conversion.

Network interface emissions depend on both the interface speed and physical layer technology. Copper Ethernet connections using twisted-pair cabling create conducted emissions on the cable while potentially radiating from improperly terminated cables. Fiber optic interfaces eliminate cable-conducted emissions but still generate emissions from the transceiver electronics.

Chassis Design for Emission Control

The server chassis provides the primary containment for electromagnetic emissions. Effective chassis EMC design addresses both the enclosure itself and the numerous penetrations required for cooling, cabling, and user access.

Key chassis EMC design elements include:

• Enclosure construction: Continuous metal construction provides the fundamental shield. Gaps between panels must be bridged by conductive gaskets, finger stock, or overlapping flanges to maintain shielding at high frequencies.
• Ventilation openings: Cooling airflow requirements create large apertures that can compromise shielding. Honeycomb panels or perforated metal screens with appropriately sized holes maintain shielding while allowing airflow.
• Cable penetrations: Every cable entering or leaving the chassis creates a potential emission path. Filtered connectors, proper cable shielding termination, and cable routing away from high-emission sources minimize this coupling.
• Front panel design: User interfaces, power buttons, and diagnostic LEDs require openings that must be designed for EMC. Conductive overlays on displays and proper grounding of control circuits maintain front panel shielding effectiveness.

Blade-to-Blade Coupling

Adjacent blade servers share the same chassis environment, creating opportunities for electromagnetic coupling between blades. This inter-blade coupling can occur through several mechanisms:

Radiated coupling: Fields from high-speed circuits on one blade can couple to adjacent blades through the minimal spacing between modules. While blade enclosures may include shielding, the blade insertion slots necessarily create apertures that allow some field coupling.

Backplane coupling: All blades connect to a common backplane that provides power, network connectivity, and management communications. Noise generated by one blade can conduct through the backplane to affect others. Proper backplane filtering and power plane design mitigate this coupling.

Power system coupling: Blades share power supplies and power distribution within the enclosure. Load variations on one blade can create power supply disturbances that affect other blades. The power distribution design must provide adequate isolation between blade circuits.

Enclosure-Level Considerations

Blade enclosures must meet EMC regulatory requirements as complete systems, not merely as collections of individual blades. The enclosure design integrates emissions from multiple blades, power supplies, network switches, and management modules into a combined emission profile that must meet applicable limits.

The hot-swap capability of blade systems creates EMC challenges during blade insertion and removal. The electrical connection sequence must prevent transients that could affect operating blades or create emissions exceeding limits. Contact sequencing in the backplane connectors typically grounds the blade before connecting power, with signal connections made last.

Cooling systems for blade enclosures involve high-power fans operating in the high-velocity airflows required to cool dense blade configurations. These fans, often variable-speed with electronic commutation, generate both acoustic and electromagnetic noise. Fan placement, filtering of fan power connections, and attention to motor-drive EMC are essential for blade enclosure compliance.

Midplane and Backplane Design

The midplane or backplane connecting blade servers carries high-speed signals between blades, network traffic, and management communications. This critical interconnect substrate requires careful EMC design to prevent it from acting as a coupling path between blades or an emission source in itself.

High-speed differential signals crossing the backplane must maintain controlled impedance through the connector interfaces. Impedance discontinuities at connectors convert differential signals to common-mode, increasing emissions. Proper connector selection, pin assignment, and PCB design around connector footprints minimize this conversion.

Power distribution on the backplane requires extensive decoupling to prevent high-frequency noise from propagating between blades. Multiple decoupling capacitor types, from bulk electrolytics through small ceramics, provide low impedance across the frequency range of concern. Power plane impedance analysis ensures adequate decoupling throughout the operating frequency range.

Hard Disk Drive EMC

Hard disk drives contain spinning platters, voice coil actuators, and high-speed read/write electronics within each drive enclosure. While individual drives are designed to meet EMC requirements, dense arrays of drives create aggregate effects requiring system-level attention.

HDD EMC considerations include:

• Spindle motor emissions: The brushless DC motors spinning disk platters generate both conducted and radiated emissions from their drive electronics. In arrays with dozens or hundreds of drives, these emissions accumulate.
• Actuator noise: The voice coil motors positioning read/write heads create current transients during seek operations. Random seek patterns generate broadband noise, while sequential access patterns can create stronger narrowband emissions at the seek rate.
• Data interface emissions: SATA and SAS interfaces carry high-speed data between drives and controllers. Proper cable shielding and connector termination contain these emissions.
• Vibration effects: While not electromagnetic, vibration from spinning drives can affect other electronic components and must be considered in storage system design.

Solid-State Storage EMC

Solid-state drives eliminate the mechanical components of hard drives but introduce different EMC characteristics. The high-speed flash memory interfaces and controller electronics generate emissions that, while different in character from HDD emissions, still require careful management.

SSD controllers typically employ multiple flash memory channels operating in parallel, each running at hundreds of megabytes per second. The aggregate data throughput of high-performance SSDs exceeds that of HDDs, creating correspondingly higher-frequency emissions from the data paths.

NVMe SSDs connecting directly to PCIe interfaces generate emissions from the high-speed serial links. At PCIe Gen 4 and Gen 5 speeds, these emissions extend well into the gigahertz range and require proper trace routing, impedance control, and connector shielding.

The power consumption patterns of SSDs differ from HDDs, with rapid current variations as flash programming and reading operations occur. These current transients can create power supply noise affecting other system components if not properly decoupled.

Storage Controller Emissions

Storage controllers managing arrays of drives contain high-performance processors, large memory caches, and multiple high-speed interfaces. These controllers can be significant emission sources requiring attention to chassis design, cable management, and power supply filtering.

RAID controllers performing data striping and parity calculations process data at rates limited only by drive and interface bandwidth. The memory and processor activity supporting these calculations generates emissions similar to server processors, requiring similar mitigation strategies.

The interface between controllers and external hosts, whether Fibre Channel, iSCSI, or direct-attached SAS/SATA, carries high-speed data requiring proper cable and connector design for EMC compliance. External cable emissions can dominate the overall storage system emission profile if not properly controlled.

PCIe EMC Considerations

The PCI Express interface dominates high-speed peripheral connectivity in servers and storage systems. Each generation of PCIe doubles the data rate, with Gen 5 operating at 32 GT/s and Gen 6 planning for 64 GT/s. These extreme speeds require increasingly sophisticated EMC design.

PCIe EMC design considerations include:

• Connector design: PCIe connectors must maintain signal integrity and provide adequate shielding at multi-gigahertz frequencies. Connector selection, mounting, and PCB footprint design all affect emissions.
• Cable assemblies: External PCIe cabling for connections beyond the system enclosure requires high-performance shielded assemblies with controlled impedance and low skew.
• Retimer/redriver placement: Signal conditioning devices for long PCIe links can affect EMC by re-amplifying and re-shaping signals. Proper placement minimizes additional emissions.
• Reference clock distribution: The low-skew reference clocks required for PCIe can radiate if not properly distributed on inner PCB layers with adequate ground shielding.

Memory Interconnects

Processor-to-memory interconnects carry the highest bandwidth of any system connection, with DDR5 systems approaching terabytes per second of aggregate memory bandwidth. This extreme bandwidth requires dense, high-speed signaling that presents substantial EMC challenges.

Memory channel EMC involves:

• Simultaneous switching noise: Memory data buses with 64 or more data bits switching simultaneously create large aggregate current transients that couple to power planes and radiate from PCB structures.
• Socket and module design: Memory module sockets must maintain signal integrity while providing adequate grounding. Improperly designed sockets can create resonances that increase emissions at specific frequencies.
• Termination: DDR memory interfaces require precise termination to maintain signal integrity and minimize reflections that increase emissions. On-die termination in modern memories helps, but board design remains critical.

Multi-Processor Interconnects

Multi-processor servers use high-speed interconnects between processor sockets, such as Intel UPI, AMD Infinity Fabric, or various proprietary links. These interconnects carry cache coherence traffic, memory access requests, and I/O transactions at data rates matching or exceeding PCIe.

The routing of processor interconnects on server motherboards requires careful attention to maintain signal integrity while minimizing radiation. These links typically route on inner layers with ground plane shielding, with proper via design for layer transitions.

Switching Converter Emissions

Modern server power supplies use resonant switching topologies operating at frequencies from hundreds of kilohertz to several megahertz. These converters generate conducted and radiated emissions from their switching waveforms, with spectral content extending well above the fundamental switching frequency.

Primary converter emissions include:

Differential mode: Current ripple at the switching frequency and its harmonics flows in the input and output circuits. Input EMC filters attenuate differential mode emissions propagating back to the facility power system, while output filtering reduces noise on the DC outputs.

Common mode: Capacitive coupling between primary and secondary circuits, particularly through the isolation transformer, creates common-mode currents that are difficult to filter and radiate efficiently from cables. Y-capacitors and common-mode chokes address this emission type.

Power supply internal design affects emissions through component selection, layout, and shielding. Higher-quality power supplies intended for EMC-sensitive applications include additional filtering and may use shielded magnetics to reduce radiated emissions.

Voltage Regulator Module EMC

Voltage regulator modules (VRMs) on server motherboards convert the 12V or 48V power supply output to the sub-1V voltages required by processors and memory. These multi-phase buck converters operate at frequencies from 300 kHz to over 1 MHz, switching high currents at each phase.

VRM EMC considerations include:

• Switching frequency selection: Higher switching frequencies allow smaller inductors but create emissions at frequencies that may be more problematic. The switching frequency should avoid harmonics that fall at sensitive frequencies.
• Phase interleaving: Multi-phase VRMs can interleave switching to reduce input ripple current and associated emissions. Proper interleaving spreads switching events in time, reducing peak current demand.
• Inductor shielding: Power inductors with exposed windings radiate magnetic fields that can couple to nearby circuits. Shielded or semi-shielded inductors contain these fields at modest cost and space penalties.
• Layout practices: The high-current switching loops in VRM circuits should have minimal area to reduce both inductance and radiation. Proper component placement and PCB layer assignment minimize loop areas.

Power Distribution Network EMC

The power distribution network (PDN) connecting power supplies to loads affects EMC through its impedance characteristics and potential for resonances. A properly designed PDN maintains low impedance across the frequency range of load current variations, preventing voltage disturbances that could radiate or couple to sensitive circuits.

PDN design for EMC requires distributed decoupling capacitors sized and placed to provide low impedance at frequencies from DC through gigahertz. Multiple capacitor values address different frequency ranges, with careful attention to avoiding parallel resonances that create impedance peaks.

Power plane design affects both PDN impedance and radiation. Solid power and ground planes provide low-inductance current paths, but plane edges can radiate. Proper plane layer pairing in PCB stack-ups minimizes edge radiation while maintaining low PDN impedance.

Fan Motor EMC

Server and storage fans typically use brushless DC (BLDC) motors with electronic commutation. The motor drive electronics switching current to the motor windings generate emissions similar to other PWM motor drives, scaled to the fan power level.

Fan motor EMC concerns include:

• Commutation noise: The switching of current between motor phases creates current transients that propagate on fan power wiring and radiate from motor leads.
• PWM speed control: Variable-speed fans use PWM duty cycle control to adjust fan speed. The PWM frequency and its harmonics appear in fan emissions.
• Tachometer signals: Fan speed monitoring typically uses pulse outputs that can be emission sources if not properly filtered at the monitoring input.

Fan Placement and Wiring

The placement of fans within server and storage enclosures affects their EMC impact. Fans positioned near high-speed circuits can couple noise into those circuits, while fan wiring routed with signal cables can create coupling paths.

EMC-aware fan integration practices include:

• Routing fan power wiring separately from signal cables
• Using twisted-pair wiring for fan connections to reduce differential mode radiation
• Adding filtering at fan connectors or at the fan control circuits
• Grounding fan frames to chassis to reduce common-mode currents on fan wiring

Variable Speed Fan Control

Thermal management in modern servers adjusts fan speed dynamically based on temperature, workload, and inlet air conditions. This variable-speed operation creates time-varying EMC emissions that must be considered in system compliance.

EMC testing must consider fan operation across the speed range, as emission levels may vary with fan speed. Some frequencies may be problematic only at certain fan speeds where switching harmonics align with chassis or cable resonances.

The control signals for fan speed adjustment, typically PWM signals from the baseboard management controller, should be properly filtered to prevent the control interface from contributing to emissions.

PCB Density and EMC

Dense PCB layouts place high-speed signals in close proximity, increasing crosstalk and potentially creating coupling paths between circuits that should be isolated. Proper stack-up design with adequate ground plane coverage, controlled impedance routing, and attention to signal layer assignment mitigates these effects.

The number of PCB layers available for routing affects EMC. More layers allow better isolation of signal types on different layers with intervening ground planes, but increase cost and manufacturing complexity. The layer count must balance signal integrity, EMC, and economic considerations.

Component package selection affects density and EMC. Smaller packages allow higher density but may have less effective power/ground pin arrangements and less internal shielding. Ball grid array packages with power and ground balls distributed among signal balls provide better EMC performance than packages with peripheral power/ground pins.

Enclosure Design for Dense Systems

Dense server and storage designs leave limited space for EMC treatments within the enclosure. Shielding partitions, cable management structures, and filter components compete for space with computing components. Creative mechanical design can integrate EMC functions into structural elements.

Card guides, chassis walls, and heat sink assemblies can all contribute to EMC shielding if designed with EMC in mind. Conductive gaskets between components can maintain shielding continuity without dedicated shielding structures.

Thermal and EMC Tradeoffs

Thermal management and EMC requirements can conflict in dense designs. Cooling airflow requires openings that compromise shielding, while metal components desirable for shielding may impede airflow. Balancing these requirements is a key challenge in high-density system design.

Honeycomb air filters that maintain shielding while allowing airflow represent one solution. Directing airflow through shielded channels rather than directly through electronics areas maintains both cooling and EMC performance. Computational fluid dynamics and electromagnetic simulation tools help optimize designs that meet both thermal and EMC requirements.

Temperature Effects on Emissions

Electronic component characteristics vary with temperature, affecting EMC performance. Crystal oscillator frequencies shift with temperature, potentially moving emission frequencies. Semiconductor switching speeds may change, affecting emission spectral content. Capacitor values and ESR vary with temperature, changing filter and decoupling performance.

EMC testing at temperature extremes may reveal emission levels different from room-temperature testing. Comprehensive EMC verification includes testing at expected operating temperature limits to ensure compliance across the operating range.

Mechanical Effects on Shielding

Thermal expansion can affect shielding effectiveness by opening gaps at chassis joints, changing gasket compression, and altering the fit of covers and access panels. Designs should accommodate thermal expansion without compromising EMC.

Cable connections can loosen with thermal cycling, degrading shield connections. Proper connector selection and installation practices maintain reliable connections despite thermal cycling.

Hot-Swap Component EMC

Hot-swappable components, including power supplies, fans, storage drives, and sometimes computing modules, must insert and remove without creating EMC disturbances affecting other equipment. The connector sequencing, chassis design, and component EMC characteristics all influence hot-swap EMC performance.

Contact sequencing ensures ground connections make first and break last, maintaining chassis shielding during insertion and removal. Staged power connections prevent in-rush current transients from affecting operating equipment. Signal connections last in, first out prevents active signals from creating transients.

Service Panel Design

Service panels and access doors must maintain shielding effectiveness when closed while allowing access when needed. Panel edge treatments, gaskets, and latching mechanisms should maintain EMC performance despite repeated opening and closing.

Captive hardware that ensures proper panel installation reduces the risk of maintenance personnel leaving panels improperly secured. Visual indicators of proper panel closure help ensure EMC is maintained after maintenance.

Cable Service Loops

Service loops in cables allowing component movement during maintenance can affect EMC if they create antennas or coupling paths. Cable management that maintains organization during service while controlling loop size and routing minimizes EMC impact.