Electronics Guide

WiFi Spectral Characteristics

Understanding WiFi's spectral properties is essential for EMC analysis:

2.4 GHz band (2.400-2.4835 GHz): Contains only three non-overlapping 20 MHz channels in most regulatory domains. High congestion from WiFi, Bluetooth, Zigbee, and numerous other ISM band users creates challenging coexistence scenarios.

5 GHz bands (5.150-5.850 GHz): More available spectrum with wider channels (up to 160 MHz). Dynamic Frequency Selection (DFS) requirements in portions of the band add complexity but provide interference avoidance with radar systems.

6 GHz band (5.925-7.125 GHz): WiFi 6E and WiFi 7 operate here with reduced regulatory burden compared to 5 GHz DFS bands. Lower congestion initially, but potential for interference with existing users in some regions.

WiFi modulation (OFDM) spreads energy across the channel bandwidth, producing a relatively flat spectral signature. However, out-of-band emissions, harmonics, and spurious responses can cause interference outside the intended channel.

WiFi Interference Sources and Victims

Common WiFi interference scenarios include:

In-device coexistence: WiFi radios in smartphones, laptops, and IoT devices share space with Bluetooth, cellular, and other radios. Direct coupling through the antenna, antenna feed networks, and shared platform resources creates interference paths.

USB 3.x interference: USB 3.0 and later standards spread spectral energy into the 2.4 GHz band, directly impacting WiFi performance. Shielded cables, proper connector grounding, and frequency planning help mitigate this issue.

Harmonic interference: Digital circuits with clocks near 2.4 GHz or its subharmonics can generate interference falling in WiFi channels. Spread spectrum clocking and careful PCB layout reduce this risk.

Adjacent channel interference: Other WiFi networks on overlapping or adjacent channels degrade performance. While not strictly an EMC issue, it demonstrates the importance of channel planning and transmit power control.

WiFi EMC Design Practices

Effective WiFi EMC design involves:

• Antenna placement: Locate WiFi antennas away from high-speed digital circuits and other radio antennas. Use ground planes and shielding to reduce coupling.
• Front-end filtering: Band-pass or low-pass filters in the antenna path reduce out-of-band emissions and improve immunity to out-of-band interference.
• Power amplifier linearity: Nonlinear PA operation generates harmonics and intermodulation products. Back-off from maximum power and predistortion techniques improve spectral purity.
• Reference oscillator isolation: The reference oscillator driving the WiFi radio should be isolated from noise sources to maintain phase noise performance.
• Conducted emissions filtering: Power supply connections to the WiFi module require filtering to prevent conducted emissions through the DC lines.

Bluetooth Spectral Behavior

Classic Bluetooth and BLE use different approaches to spectrum access:

Classic Bluetooth: Uses frequency hopping spread spectrum (FHSS) across 79 channels of 1 MHz width, hopping up to 1600 times per second. This spreading provides inherent interference resistance but means Bluetooth transmissions periodically land on any frequency in the band.

Bluetooth Low Energy: Uses 40 channels of 2 MHz width with adaptive frequency hopping. BLE advertising occurs on three fixed channels (37, 38, 39) located at band edges and center to minimize overlap with common WiFi channels.

Power levels: Bluetooth Class 1 devices can transmit up to 100 mW (20 dBm), while Class 2 and Class 3 devices use lower power. BLE typically operates at lower power levels than classic Bluetooth.

WiFi-Bluetooth Coexistence Mechanisms

Several mechanisms enable WiFi and Bluetooth to share the 2.4 GHz band:

Collaborative coexistence: When WiFi and Bluetooth radios are in the same device, they can coordinate through dedicated coexistence interfaces:

• Packet Traffic Arbitration (PTA): A three-wire interface allowing radios to signal activity, priority, and frequency to avoid simultaneous transmission
• Enhanced PTA: Additional signaling for better coordination in complex scenarios
• Integrated solutions: Combo chips with single antenna and sophisticated internal coexistence management

Adaptive Frequency Hopping (AFH): Bluetooth can characterize the interference environment and avoid hopping to frequencies occupied by WiFi or other strong signals. This standard feature significantly improves coexistence in congested environments.

Time-division coordination: The radios can time-division multiplex access to the shared medium, with one radio pausing while the other transmits critical packets.

Bluetooth Isolation Requirements

Achieving adequate isolation between Bluetooth and other radios requires attention to:

Antenna isolation: Physical separation between antennas provides frequency-independent isolation. The minimum required isolation depends on transmit power and receiver sensitivity but typically ranges from 10-20 dB.

Antenna pattern optimization: Orienting antenna patterns to minimize coupling between radios can supplement physical separation.

Receiver blocking: Strong signals from nearby transmitters can desensitize or block receivers. Front-end filtering and sufficient antenna isolation prevent this degradation.

Intermodulation: Two strong signals mixing in a nonlinear element can produce products that fall on a third radio's frequency. System-level frequency planning identifies and avoids dangerous combinations.

Cellular Band Complexity

Modern cellular devices support dozens of frequency bands with complex configurations:

FDD bands: Frequency Division Duplex bands use separate frequencies for uplink and downlink, with specified guard bands. The duplex spacing varies by band, affecting filter requirements.

TDD bands: Time Division Duplex bands use the same frequency for both directions, eliminating the need for duplexers but requiring precise timing.

Carrier aggregation: LTE and 5G can simultaneously use multiple bands (intra-band or inter-band carrier aggregation), creating complex simultaneous transmission and reception scenarios.

Band proximity: Some cellular bands are adjacent to WiFi (LAA/LTE-U in 5 GHz), GPS (multiple bands), and other services, requiring careful filtering to prevent interference.

Desense and Self-Interference

Cellular receiver desensitization is a critical EMC issue in mobile devices:

Harmonic desense: Digital clocks, display interfaces, camera modules, and other circuits generate harmonics that can fall in cellular receive bands. Identifying and mitigating these harmonics is a major design effort.

Platform noise: The aggregate noise floor from the mobile device platform can raise the effective noise figure of cellular receivers. This noise comes from digital circuits, power management, and other radios.

Self-interference: In FDD operation, the transmitter operates simultaneously with the receiver. Despite duplexer filtering, transmitter noise in the receive band can degrade sensitivity.

IMD from other radios: Intermodulation between cellular and WiFi or between cellular bands can produce products that fall in sensitive receive bands.

Cellular EMC Design Approaches

Managing cellular EMC requires a comprehensive approach:

• Harmonic analysis: Systematically analyze all clock frequencies and their harmonics against all cellular receive bands to identify conflicts early in design
• Clock frequency planning: Choose system clocks to place harmonics in gaps between cellular bands where possible
• Shield planning: Identify circuits requiring shielding based on their emission potential and proximity to cellular antennas
• Antenna isolation optimization: Use 3D electromagnetic simulation to optimize antenna placement and achieve required isolation
• Filter specification: Specify front-end filters based on system-level interference analysis rather than generic requirements

GNSS Signal Characteristics

GNSS signals arrive at receivers well below the thermal noise floor:

Signal levels: GPS L1 signals arrive at approximately -130 dBm, requiring about 14 dB of processing gain to achieve usable signal-to-noise ratios. This extreme weakness makes GPS susceptible to even low-level interference.

Frequency bands: GPS L1 (1575.42 MHz) is most common in consumer devices. L2 (1227.60 MHz) and L5 (1176.45 MHz) provide additional capabilities in professional receivers. Other GNSS systems use similar or adjacent frequencies.

Bandwidth: The narrow GNSS signal bandwidth (approximately 2 MHz for C/A code) allows tight filtering, but this also means that narrowband interference is particularly damaging.

GNSS Interference Sources

Numerous interference sources threaten GNSS reception:

Harmonics: Digital circuits operating at frequencies whose harmonics fall near 1575 MHz are primary concerns. For example, a 157.5 MHz clock has a 10th harmonic directly on GPS L1.

LTE Band 13 uplink: The LTE Band 13 uplink (777-787 MHz) has a second harmonic near GPS L1. Devices with Band 13 cellular require careful filtering to prevent GPS degradation during transmission.

Display interfaces: HDMI, DisplayPort, and embedded display interfaces can produce harmonics or spurious emissions in GNSS bands, particularly when driving certain resolutions.

Camera and image sensor noise: High-speed camera interfaces and image sensor clocks can generate interference in GNSS bands.

Intentional jammers: While illegal in most jurisdictions, GPS jammers represent a real threat. GNSS receiver design should consider graceful degradation under jamming conditions.

GNSS Protection Techniques

Protecting GNSS receivers requires multiple layers of defense:

Antenna placement: Position GNSS antennas away from digital circuits, preferably with clear sky view. Ground plane design under the antenna affects both pattern and shielding.

SAW filters: Surface acoustic wave (SAW) filters provide excellent selectivity with small size. A SAW filter at the antenna input rejects out-of-band interference before it can overload the LNA.

LNA design: The low-noise amplifier must have sufficient dynamic range to handle in-band interference without compression while maintaining low noise figure.

Shielded LNA/filter modules: Integrated front-end modules combining LNA, filtering, and shielding provide optimized performance in a compact package.

Active interference cancellation: Some receivers implement active cancellation of known interference sources using reference signals from the interferer.

NFC/RFID Frequency Bands

Different NFC and RFID systems operate across a wide frequency range:

LF RFID (125-134 kHz): Used for access control, animal tracking, and automotive immobilizers. Long wavelengths enable operation through materials but limit data rates and range.

HF/NFC (13.56 MHz): NFC and HF RFID share this frequency. The 13.56 MHz ISM band allows reasonable data rates with near-field coupling range of several centimeters.

UHF RFID (860-960 MHz): Longer range (several meters) enables supply chain and inventory applications. The specific frequency varies by region due to regulatory allocation.

Microwave RFID (2.45 GHz): Limited use due to ISM band congestion from WiFi and Bluetooth.

NFC EMC Considerations

NFC in mobile devices presents specific EMC challenges:

Antenna design: NFC antennas are typically large loops (several centimeters) that must be integrated into device housings. Metal cases require special antenna designs or case integration.

13.56 MHz emissions: NFC transmitters generate strong magnetic fields that can induce currents in nearby conductors. These currents can couple to other circuits or reradiate as electric fields.

Harmonic emissions: The 13.56 MHz NFC carrier and its harmonics can interfere with FM broadcast, VHF communications, and other services if not adequately filtered.

Conducted emissions: NFC reader circuits can inject 13.56 MHz and harmonics onto power supply and signal lines.

Immunity: NFC cards and tags must operate reliably in the presence of environmental interference, including from the reader's own electronics.

RFID Reader EMC

RFID readers, particularly those with higher power and longer range, require careful EMC design:

• Power amplifier harmonic filtering: High-power PA stages generate significant harmonics requiring multiple filter stages
• Antenna matching: Impedance mismatch causes reflection and increases emissions at unintended frequencies
• Multi-reader interference: Dense reader deployments can cause reader-to-reader interference requiring coordination protocols
• Regulatory compliance: UHF RFID readers must meet stringent emissions limits in adjacent frequency bands to protect cellular and other services

802.15.4 Physical Layer EMC

The IEEE 802.15.4 standard defines the physical layer used by Zigbee, Thread, and other protocols:

2.4 GHz operation: Sixteen channels of 2 MHz bandwidth span the 2.4 GHz ISM band with 5 MHz channel spacing. This leaves gaps between channels that help reduce adjacent channel interference.

Low power: Maximum transmit power is typically 0 dBm (1 mW), much lower than WiFi or Bluetooth Class 1. This reduces interference potential but also limits range.

DSSS modulation: Direct Sequence Spread Spectrum provides processing gain that improves interference resistance and reduces spectral density.

Sub-GHz bands: 802.15.4 also defines operation at 868 MHz (Europe) and 915 MHz (Americas), which face less congestion than 2.4 GHz.

Mesh Network Coexistence

Mesh topology affects EMC behavior in several ways:

Aggregate emissions: Dense mesh deployments result in many simultaneous transmitters. While individual nodes have low power, the aggregate can create significant local field strengths.

Retransmissions: Mesh routing involves multiple hops and potential retransmissions, increasing average channel occupancy and interference potential.

Channel selection: Zigbee can operate on any of sixteen channels. Network-wide channel selection should avoid the most congested WiFi channels (1, 6, 11 in the US).

Coexistence mechanisms: Clear Channel Assessment (CCA) before transmission reduces collisions with other 2.4 GHz users. Some implementations support frequency agility to avoid persistent interference.

IoT Device EMC Design

Zigbee and similar protocols often appear in cost-sensitive IoT devices:

• Integrated solutions: System-on-chip (SoC) radios simplify design but limit EMC optimization options
• Antenna design: PCB antennas in small form factors require careful design to achieve acceptable efficiency and pattern
• Battery power: Battery-operated devices may have less filtering and shielding due to size and cost constraints
• Enclosure effects: Plastic enclosures provide no shielding; nearby metal can detune antennas

LoRa Physical Layer Characteristics

LoRa's chirp spread spectrum modulation differs from traditional wireless technologies:

Frequency bands: LoRa typically operates in 868 MHz (Europe), 915 MHz (Americas), or 433 MHz (various regions). These sub-GHz bands face less congestion than 2.4 GHz.

Chirp spread spectrum: LoRa uses frequency-chirped signals that sweep across the channel bandwidth. This spreading provides high sensitivity (-137 dBm or better) and interference resistance.

Spreading factors: Variable spreading factors trade data rate for range and interference immunity. Higher spreading factors create longer symbols with more processing gain.

Channel bandwidth: Bandwidths from 125 kHz to 500 kHz are common. Narrower bandwidths improve sensitivity but limit data rate.

LoRaWAN Network EMC

LoRaWAN defines the network protocol for LoRa devices:

Duty cycle limits: Regional regulations strictly limit transmission duty cycle (typically 1% or less in Europe). This inherently limits average interference potential.

Adaptive data rate: LoRaWAN adjusts transmission parameters based on link quality, optimizing power and spectrum use while reducing unnecessary interference.

Gateway receive sensitivity: Gateways must receive signals from distant, low-power end devices. High sensitivity makes gateways vulnerable to local interference from power supplies, computers, and other electronics.

Dense deployments: As LoRaWAN networks scale, same-spreading-factor collisions increase. Network capacity depends on managing aggregate interference levels.

5G Frequency Considerations

5G NR operates across an unprecedented range of frequencies:

FR1 (sub-6 GHz): Frequency Range 1 includes bands from 410 MHz to 7125 MHz. Many FR1 bands overlap with existing LTE allocations, while new bands like n77 (3.3-4.2 GHz) and n78 (3.3-3.8 GHz) open new spectrum.

FR2 (millimeter wave): Frequency Range 2 covers 24.25-52.6 GHz initially, with extensions to higher frequencies planned. These bands enable very high bandwidth but require line-of-sight propagation.

Bandwidth: 5G NR supports channel bandwidths up to 100 MHz in FR1 and 400 MHz in FR2, far exceeding LTE capabilities. Wider bandwidth means higher instantaneous power and more potential for interference.

5G Antenna Array EMC

Massive MIMO and beamforming in 5G create unique EMC scenarios:

Beamforming: 5G base stations focus energy in specific directions, creating localized high field strengths rather than omnidirectional coverage. EMC analysis must consider beam steering patterns.

Device antenna arrays: 5G smartphones may include multiple antenna arrays for different bands and MIMO operation. These arrays must be isolated from each other and from internal circuits.

Millimeter-wave integration: FR2 antennas are integrated directly with RF front-end modules due to high transmission line losses. This integration affects thermal design and shielding strategies.

Human exposure: Compliance with RF exposure limits (SAR and power density) becomes more complex with steerable beams and the move to millimeter-wave frequencies.

5G Coexistence Challenges

5G introduces new coexistence scenarios:

• C-band and WiFi 6E: 5G n77/n78/n79 bands are adjacent to the 6 GHz WiFi band, requiring careful filtering to prevent mutual interference
• Satellite interference: C-band 5G deployments near satellite earth stations require coordination to prevent interference with satellite services
• Legacy system coexistence: 5G dynamic spectrum sharing with LTE requires careful management to prevent cross-system interference
• Millimeter-wave blocking: At FR2 frequencies, human bodies, vehicles, and buildings significantly attenuate signals, creating coverage challenges and potential interference variability

In-Device Coexistence Testing

Testing wireless coexistence within a single device:

Antenna isolation measurement: Vector network analyzer measurements characterize coupling between antenna ports. Minimum isolation requirements depend on transmit power and receiver sensitivity.

Desense testing: With one radio transmitting, measure degradation in another radio's receiver sensitivity. This captures platform noise and direct coupling effects.

Throughput testing: Measure simultaneous throughput of multiple radios under various operating conditions to verify real-world performance.

Power consumption: Interference that causes retransmissions increases power consumption. Battery life testing with multiple radios active reveals coexistence problems.

System-Level Coexistence Testing

Testing coexistence between devices in a system:

RF chamber testing: Shielded chambers provide a controlled environment for testing without external interference. Anechoic and reverberation chambers offer different test scenarios.

CTIA and carrier testing: Mobile device certification includes extensive coexistence testing defined by CTIA and individual cellular carriers.

Field testing: Real-world testing captures effects that laboratory environments cannot replicate, including multipath, moving interferers, and aggregate interference from multiple sources.

Coexistence Test Standards

Industry standards guide coexistence testing:

• IEEE 1528: SAR testing methodology that affects antenna placement and power control
• 3GPP TS 36.101/38.101: LTE and 5G NR receiver performance requirements
• CTIA test plans: Comprehensive over-the-air and conducted test procedures for mobile devices
• WiFi Alliance test specifications: WiFi certification testing including coexistence aspects
• Bluetooth SIG test specifications: Bluetooth qualification testing with coexistence components