Electronics Guide

Radio Frequency Spectrum

Wireless communication occurs through electromagnetic waves at specific frequencies allocated by regulatory bodies. Different frequency bands offer distinct characteristics that influence their suitability for various applications:

Sub-1 GHz bands: Frequencies below 1 GHz, including 433 MHz, 868 MHz (Europe), and 915 MHz (Americas), provide excellent propagation characteristics. Lower frequencies penetrate obstacles better and travel farther than higher frequencies at equivalent power levels. These bands are popular for long-range, low-power applications.

2.4 GHz ISM band: The industrial, scientific, and medical band at 2.4 GHz is globally available and hosts WiFi, Bluetooth, Zigbee, and many proprietary protocols. Its widespread use creates congestion in dense environments but ensures component availability and regulatory simplicity.

5 GHz and 6 GHz bands: Higher frequency bands offer more bandwidth and less congestion but shorter range. WiFi increasingly uses these frequencies for high-throughput applications where range requirements are modest.

Link Budget and Range

The link budget determines whether a wireless link can operate reliably at a given distance. It accounts for transmitter power, antenna gains, path loss, and receiver sensitivity:

Transmitter power: The RF power delivered to the antenna, typically measured in dBm. Higher power increases range but consumes more energy and may violate regulatory limits.

Antenna gain: Antennas can focus RF energy directionally, increasing effective radiated power in preferred directions while reducing it elsewhere. Gain is measured in dBi relative to an ideal isotropic radiator.

Path loss: Signal strength decreases with distance following the inverse-square law in free space. Real environments add additional losses from obstacles, reflections, and atmospheric absorption.

Receiver sensitivity: The minimum signal level a receiver can detect reliably, typically measured in dBm. More sensitive receivers extend range but cost more and may be more susceptible to interference.

A successful link requires the received signal strength to exceed the receiver sensitivity by a margin that accounts for fading and interference.

Modulation and Data Rates

Modulation encodes digital data onto RF carriers. Different modulation schemes trade off data rate, range, and immunity to interference:

Simple modulation: Techniques like on-off keying (OOK) and frequency shift keying (FSK) provide robust communication at modest data rates. Their simplicity makes them suitable for low-cost, low-power applications.

Spread spectrum: Direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) spread signals across wide frequency bands, improving interference immunity and enabling multiple devices to share spectrum.

Orthogonal frequency division multiplexing (OFDM): Divides the channel into many narrow subcarriers, achieving high data rates while handling multipath propagation. WiFi and cellular systems rely on OFDM variants.

Chirp spread spectrum: Used by LoRa, chirp modulation sweeps frequency continuously, providing exceptional range and interference immunity at low data rates.

WiFi Standards Overview

The IEEE 802.11 family defines WiFi standards, with each generation offering improved capabilities:

802.11b/g/n (WiFi 4): Operating at 2.4 GHz, these mature standards provide data rates from 11 Mbps to 150 Mbps. The 2.4 GHz band offers good range and obstacle penetration but faces congestion in populated areas.

802.11n/ac (WiFi 5): Adding 5 GHz operation and wider channels increases throughput to hundreds of megabits per second. The 5 GHz band provides cleaner spectrum but shorter range.

802.11ax (WiFi 6): Improves efficiency in dense environments through orthogonal frequency division multiple access (OFDMA), allowing multiple devices to share channels more effectively.

802.11ah (WiFi HaLow): Operating in sub-1 GHz bands, HaLow targets IoT applications requiring longer range and lower power than traditional WiFi, though with reduced data rates.

Module Selection

WiFi modules range from simple serial-to-WiFi converters to sophisticated system-on-chips with integrated processors:

AT command modules: Simple modules accept AT commands over UART, handling WiFi complexity internally. The popular ESP8266 began as an AT command module before its microcontroller capabilities were recognized. These modules suit applications where the host processor handles application logic while delegating networking.

SPI/SDIO modules: Higher-performance modules connect via SPI or SDIO interfaces, offering better throughput and more control. They require the host to implement more of the network stack but provide greater flexibility.

System-on-chip solutions: Devices like the ESP32 integrate WiFi radio, microcontroller, and memory in single packages. Running application code directly on these chips simplifies designs and reduces component count.

Certified modules: Pre-certified modules have completed regulatory testing, simplifying product certification. Using modules with integrated antennas and shields often enables certification by reference rather than full testing.

Integration Considerations

Successfully integrating WiFi requires attention to several factors:

Power supply: WiFi transmission creates significant current spikes, often exceeding 300 mA during transmission. Power supplies must handle these transients without voltage droop that could cause module resets or erratic behavior.

Antenna placement: WiFi performance depends heavily on antenna positioning. Keep antennas away from metal enclosures, ground planes, and high-frequency switching circuits. RF simulation or empirical testing validates antenna placement.

Network stack complexity: WiFi involves complex protocols including association, authentication, DHCP, TCP/IP, and application-layer protocols. Using modules with integrated stacks or mature software frameworks reduces development time.

Security: WiFi networks face security threats including eavesdropping, unauthorized access, and denial of service. Implement WPA2 or WPA3 encryption, use TLS for sensitive communications, and keep firmware updated to address vulnerabilities.

Power Management

WiFi consumes significant power, challenging battery-operated applications. Several techniques reduce power consumption:

Station power save mode: WiFi clients can enter power save mode between access point beacons, waking periodically to check for pending data. This reduces average current but adds latency.

Deep sleep: When communication is unnecessary, completely powering down the WiFi radio dramatically reduces consumption. Reconnection takes time, making this suitable for infrequent communication patterns.

Target wake time: WiFi 6 introduces target wake time, allowing devices to negotiate specific wake schedules with access points, further reducing unnecessary radio activity.

Classic Bluetooth versus BLE

Understanding the distinction between Classic Bluetooth and BLE helps select the appropriate technology:

Classic Bluetooth: Designed for continuous streaming applications like audio and serial port replacement. It maintains persistent connections and supports data rates up to 3 Mbps but consumes substantial power.

Bluetooth Low Energy: Optimized for intermittent data transfer with minimal power consumption. BLE devices can operate for years on coin cell batteries, making it ideal for sensors, beacons, and wearables.

Dual-mode devices: Many modern devices support both Classic Bluetooth and BLE, enabling compatibility with legacy equipment while benefiting from BLE efficiency for appropriate applications.

BLE Architecture

BLE employs a client-server model with devices acting as peripherals (servers) or centrals (clients):

Peripheral devices: Peripherals advertise their presence and provide data through services and characteristics. A heart rate sensor is a typical peripheral, broadcasting measurements to connected centrals.

Central devices: Centrals scan for peripherals, initiate connections, and read or write peripheral data. Smartphones typically act as centrals, connecting to BLE peripherals to collect data or control them.

GATT protocol: The Generic Attribute Profile defines how data is organized and exchanged. Services group related characteristics; characteristics contain actual data values with defined read, write, and notification properties.

Advertising: Peripherals broadcast advertising packets containing device information and service UUIDs. Advertising intervals balance discoverability against power consumption.

BLE Integration

Integrating BLE into embedded systems involves several components:

BLE SoCs: System-on-chip solutions like the Nordic nRF52 series and Silicon Labs EFR32 integrate BLE radio, processor, and memory. These devices run both the BLE stack and application code, minimizing external components.

BLE modules: Pre-certified modules simplify integration by encapsulating radio, antenna, and regulatory compliance. Modules often expose AT command interfaces or HCI (Host Controller Interface) for more advanced control.

Software stacks: BLE requires complex software including the link layer, security manager, attribute protocol, and GATT. Vendor-provided stacks or open-source implementations like Zephyr RTOS provide these layers.

BLE 5 and Beyond

Recent Bluetooth specifications expand BLE capabilities:

Extended range: BLE 5 introduces coded PHY options that trade data rate for range, achieving four times the range of BLE 4.2 in optimal conditions.

Higher throughput: The 2 Mbps PHY doubles data rate compared to the original 1 Mbps, reducing transmission time and energy consumption for bulk transfers.

Advertising extensions: Extended advertising supports larger advertising payloads and more sophisticated advertising patterns, beneficial for beacon and asset tracking applications.

Direction finding: Bluetooth 5.1 adds angle of arrival and angle of departure capabilities, enabling centimeter-level location accuracy for indoor positioning systems.

LE Audio: Bluetooth LE Audio brings high-quality audio to BLE with the LC3 codec, broadcast audio capabilities, and hearing aid support.

Mesh Networking

Zigbee's mesh architecture distinguishes it from point-to-point wireless technologies:

Network topology: Zigbee networks form mesh topologies where messages can route through intermediate nodes to reach destinations beyond direct radio range. This extends effective network coverage without requiring all devices to reach a central coordinator.

Device roles: Zigbee defines three device types. Coordinators establish and manage networks. Routers extend network range by forwarding messages and typically remain powered continuously. End devices conserve power by sleeping between communications, relying on routers to buffer messages.

Self-healing: When nodes fail or move, the mesh network automatically discovers new routes. This resilience makes Zigbee suitable for applications where individual device failures should not disrupt overall system operation.

Application Profiles

Zigbee defines application profiles ensuring interoperability between devices from different manufacturers:

Zigbee Home Automation: Standardizes smart home devices including lights, switches, thermostats, and sensors. Devices certified to this profile interoperate regardless of manufacturer.

Zigbee Light Link: Simplified profile for lighting products, enabling easy setup without complex network management. Now incorporated into Zigbee 3.0.

Zigbee 3.0: Unifies previous profiles into a single standard with consistent security and commissioning procedures. Zigbee 3.0 devices interoperate across application domains.

Green Power: Enables energy harvesting devices like kinetic switches to participate in Zigbee networks without batteries.

Integration Approach

Zigbee integration typically follows one of several approaches:

Certified modules: Complete modules with integrated Zigbee stack and certification simplify development. The Digi XBee series popularized this approach, offering AT command interfaces that abstract Zigbee complexity.

SoC development: System-on-chip solutions from vendors like Silicon Labs and Texas Instruments provide complete Zigbee capability with custom firmware development. This approach offers maximum flexibility but requires deeper Zigbee expertise.

Gateway integration: For applications requiring cloud connectivity, Zigbee devices often connect through gateways that bridge Zigbee networks to WiFi or Ethernet. This hybrid approach combines Zigbee's mesh benefits with internet connectivity.

LoRa Physical Layer

LoRa uses chirp spread spectrum modulation to achieve exceptional range and interference immunity:

Chirp modulation: Rather than transmitting at fixed frequencies, LoRa signals continuously sweep across a frequency band. This spreading provides processing gain that allows receivers to extract signals below the noise floor.

Spreading factors: LoRa offers configurable spreading factors from SF7 to SF12. Higher spreading factors increase range and interference immunity but reduce data rate and increase transmission time. SF7 might achieve 250 meters with high data rate; SF12 might reach 15 kilometers with data rates of only hundreds of bits per second.

Bandwidth options: LoRa supports various channel bandwidths. Narrower bandwidths improve sensitivity but reduce data rate. Common configurations use 125 kHz or 250 kHz bandwidth in the sub-1 GHz bands.

LoRaWAN Network Architecture

LoRaWAN defines a network architecture for deploying LoRa at scale:

End devices: Battery-powered sensors and actuators communicate with the network. LoRaWAN defines device classes with different power and latency tradeoffs.

Gateways: Gateways receive transmissions from all end devices within range and forward them to network servers via IP backhaul. A single gateway might serve thousands of end devices over several kilometers.

Network server: The network server manages device authentication, message routing, and duplicate filtering. Multiple gateways might receive the same transmission; the network server deduplicates these.

Application server: Application servers receive decoded data from the network server and implement application-specific logic.

Device Classes

LoRaWAN defines three device classes addressing different application requirements:

Class A: Most power-efficient, supporting bidirectional communication with receive windows only after uplink transmission. Ideal for battery-powered sensors transmitting periodically.

Class B: Adds scheduled receive windows synchronized via beacons, enabling predictable downlink latency without continuous reception.

Class C: Devices receive continuously except when transmitting, minimizing downlink latency but requiring constant power. Suitable for mains-powered actuators.

Deployment Considerations

Successfully deploying LoRa requires understanding its characteristics:

Duty cycle limits: Regulatory requirements limit transmission duty cycles in sub-1 GHz bands. Applications must budget airtime carefully, especially with high spreading factors that extend transmission duration.

Gateway density: While single gateways cover large areas, redundant coverage improves reliability and reduces collisions. Dense deployments might use multiple overlapping gateways.

Network options: Organizations can deploy private LoRaWAN networks, use public network operators like The Things Network, or participate in commercial LoRaWAN networks offering professional service levels.

Cellular Technologies

Multiple cellular generations serve embedded applications:

2G (GSM/GPRS): While largely deprecated in many regions, 2G provided initial embedded cellular connectivity. Its low bandwidth suits simple telemetry but network shutdowns limit new deployments.

3G: Higher bandwidth than 2G but also facing network sunsets. 3G remains available in some regions for transitional applications.

4G LTE: Current mainstream technology offering megabit-per-second throughput with reasonable power consumption. LTE Category M1 (Cat-M1) and Cat-1 address IoT applications with reduced capability and cost.

LTE-M (Cat-M1): Optimized for IoT with lower bandwidth (1 Mbps), deeper coverage, and power-saving features. LTE-M supports voice and mobility handover.

NB-IoT: Narrowband IoT targets stationary devices with small data volumes. Extremely low power consumption and deep building penetration suit smart metering and environmental sensing.

5G: Fifth-generation cellular offers massive bandwidth and low latency but currently targets high-performance applications rather than power-constrained IoT devices.

Module Integration

Cellular modules encapsulate radio, baseband processor, and cellular stack:

Interface options: Modules typically expose UART interfaces for AT commands, USB for higher-bandwidth applications, or native interfaces for tight processor integration.

SIM handling: Traditional modules accept physical SIM cards. Modern modules may support embedded SIM (eSIM) or integrated SIM (iSIM), enabling remote provisioning without physical card access.

GNSS integration: Many cellular modules include GNSS receivers, providing location capability alongside connectivity. This combination suits asset tracking and fleet management.

Certification: Cellular modules require extensive certification including RF type approval, carrier certification, and regulatory compliance. Using pre-certified modules dramatically simplifies product certification.

Power Management

Cellular power consumption presents challenges for battery applications:

Power states: Modules support various power states from full operation through idle to power-down. Managing transitions between states optimizes battery life while maintaining connectivity.

PSM and eDRX: Power Saving Mode and extended Discontinuous Reception reduce power consumption during idle periods. PSM allows devices to sleep for extended periods while maintaining network registration. eDRX extends the interval between paging occasions, reducing wake-ups.

Current profiles: Cellular transmission creates substantial current spikes, sometimes exceeding 2 amperes during high-power transmission. Power supply design must accommodate these transients.

Carrier and Network Considerations

Cellular deployment involves network operator relationships:

Carrier selection: Choose carriers based on coverage in target deployment areas, support for required cellular technologies, and IoT-specific offerings.

Roaming: Devices deployed across regions may require roaming agreements. Global IoT SIM providers offer simplified multi-carrier coverage.

Data plans: IoT data plans differ from consumer plans, often based on data volume or device count rather than unlimited usage. Plan costs significantly impact deployed system economics.

Antenna Types

Various antenna types suit different embedded applications:

Chip antennas: Surface-mount ceramic antennas provide compact solutions for space-constrained designs. Performance depends heavily on ground plane size and placement, requiring careful PCB layout.

PCB antennas: Printed antennas integrate directly into circuit boards, eliminating separate components. Common types include inverted-F antennas (IFA), meander lines, and planar designs. Performance varies with PCB material, ground plane, and clearance areas.

Wire antennas: Simple wire antennas, often quarter-wave monopoles, provide good performance when space permits. The required wire length depends on frequency.

External antennas: Connector-mounted external antennas offer flexibility and often superior performance. They enable antenna placement independent of electronics location and simplify enclosure design.

Flex PCB antennas: Flexible printed antennas conform to enclosure shapes, maximizing available space. Common in wearables and compact consumer devices.

Ground Plane Requirements

Most embedded antennas require proper ground planes to function correctly:

Size: Ground plane dimensions affect antenna efficiency and pattern. Chip and PCB antennas typically specify minimum ground plane sizes; insufficient ground plane degrades performance significantly.

Clearance: Areas adjacent to antennas must remain free of components, traces, and ground fill. Manufacturer recommendations specify required clearance distances.

Orientation: Ground plane orientation affects radiation patterns. Understanding the relationship between ground plane and antenna helps optimize placement for intended coverage.

Matching Networks

Impedance matching ensures maximum power transfer between RF circuits and antennas:

Impedance mismatch: When antenna impedance differs from the standard 50 ohms, power reflects back toward the transmitter rather than radiating. Return loss or VSWR quantifies this mismatch.

Matching components: LC networks using inductors and capacitors transform impedances for optimal matching. Component values depend on frequency and mismatch characteristics.

Tuning: Even well-designed antennas require tuning when integrated into products. Enclosures, batteries, and nearby components affect antenna characteristics. Vector network analyzer measurements guide matching network adjustments.

Environmental Effects

Real-world conditions affect antenna performance:

Enclosures: Plastic enclosures typically have minimal effect, though material selection matters. Metal enclosures require apertures or external antennas. Conductive coatings for EMI shielding can attenuate RF signals.

Human body: Body proximity significantly affects antenna performance, especially at lower frequencies. Wearable devices require designs accounting for body loading.

Temperature: Antenna dimensions change with temperature, potentially shifting resonant frequency. Outdoor and industrial applications may need wider bandwidth designs accommodating thermal variation.

Stack Architecture

Wireless protocol stacks follow layered architectures:

Physical layer: Handles modulation, frequency synthesis, and RF transmission/reception. Usually implemented in dedicated radio hardware.

Link layer: Manages channel access, framing, error detection, and retransmission. Often implemented in radio firmware or dedicated hardware.

Network layer: Provides addressing and routing. For IP-based protocols, this includes TCP/IP or UDP/IP. Mesh networks require routing protocols at this layer.

Application layer: Implements application-specific protocols and data formats. May include security, compression, and session management.

Stack Implementation Options

Several approaches exist for implementing wireless stacks:

Module-resident stacks: AT command modules run complete stacks internally, exposing simple command interfaces to host processors. This simplifies host software but limits control and visibility.

Host-resident stacks: Running stacks on the host processor provides maximum control and flexibility. This approach requires more host resources and development effort but enables customization and optimization.

Split stacks: Some architectures split stacks between host and radio processors. HCI interfaces for Bluetooth exemplify this approach, with link layer running on the radio and higher layers on the host.

Operating System Integration

Wireless stacks interact extensively with operating systems:

Bare-metal: Simple applications may implement wireless communication without an operating system, using polling or interrupt-driven state machines. This minimizes overhead but complicates complex protocol implementation.

RTOS integration: Real-time operating systems provide scheduling, synchronization, and memory management that wireless stacks require. Many vendor stacks target specific RTOS platforms.

Linux: Embedded Linux systems leverage mature wireless subsystems including wpa_supplicant for WiFi, BlueZ for Bluetooth, and various network managers. This approach provides comprehensive functionality but requires more system resources.

2.4 GHz Congestion

The 2.4 GHz ISM band hosts multiple technologies that can interfere:

WiFi channels: WiFi channels 1, 6, and 11 avoid overlap in North America; other regions use different channel plans. Operating WiFi, Bluetooth, and Zigbee simultaneously requires frequency management.

Bluetooth frequency hopping: Bluetooth hops across 79 channels, occasionally colliding with other 2.4 GHz systems. Adaptive frequency hopping avoids channels with persistent interference.

Zigbee channel selection: Zigbee channels 15, 20, 25, and 26 avoid WiFi channels 1, 6, and 11. Choosing appropriate Zigbee channels minimizes WiFi interference.

Coexistence Mechanisms

Various techniques improve coexistence:

Time division: Sharing time between radios prevents simultaneous transmission. Coexistence interfaces like the Packet Traffic Arbitration (PTA) signal coordinate timing between WiFi and Bluetooth.

Frequency planning: Assigning non-overlapping frequencies to colocated systems eliminates direct interference. This requires coordination and may not be possible in all deployments.

Spatial separation: Physically separating antennas reduces coupling between radios. Orthogonal antenna polarization further improves isolation.

Power control: Reducing transmit power when possible minimizes interference with nearby receivers. Adaptive power control responds to actual operating conditions.

Encryption

Encrypting wireless communications prevents eavesdropping:

Link-layer encryption: WiFi WPA2/WPA3, Bluetooth pairing, and Zigbee network keys encrypt data at the link layer, protecting against casual interception.

Transport-layer encryption: TLS provides end-to-end encryption regardless of underlying wireless security. Essential when link-layer encryption cannot be trusted.

Application-layer encryption: Encrypting sensitive data before transmission provides defense in depth, protecting against compromised intermediate systems.

Authentication

Authentication ensures devices communicate only with intended parties:

Network authentication: WiFi enterprise authentication, cellular SIM authentication, and LoRaWAN device authentication verify device identity before granting network access.

Device authentication: Mutual authentication ensures both parties verify each other's identity. Certificate-based authentication provides strong assurance.

Secure provisioning: Initial credential provisioning requires careful design. Manufacturing processes must protect keys; field provisioning needs secure channels.

Firmware Security

Wireless interfaces create attack surfaces for firmware compromise:

Secure boot: Verifying firmware authenticity before execution prevents attackers from installing malicious code through wireless updates.

Update authentication: Over-the-air updates must authenticate to prevent malicious firmware installation. Signed firmware and secure update protocols are essential.

Vulnerability management: Wireless protocol implementations contain vulnerabilities discovered over time. Maintaining update capability and monitoring for security advisories enables timely patching.

Regulatory Bodies

Different regions have different regulatory authorities:

FCC (United States): The Federal Communications Commission regulates RF devices in the United States. Part 15 covers unlicensed devices including WiFi, Bluetooth, and IoT radios.

CE (Europe): The Radio Equipment Directive (RED) governs wireless devices in the European Union. Compliance enables CE marking and market access.

Other regions: Japan, Canada, Australia, and other markets have their own regulations. Global products require compliance with all target markets.

Testing Requirements

Certification requires testing various parameters:

Conducted power: Transmitter output power must remain within regulatory limits across all operating conditions.

Radiated emissions: Both intentional emissions (the wireless signal) and unintentional emissions (spurious outputs) must comply with limits.

Receiver performance: Some regulations specify minimum receiver performance to ensure efficient spectrum use.

Dynamic frequency selection: 5 GHz devices may require radar detection to avoid interfering with weather and military radar systems.

Certification Strategies

Several approaches simplify regulatory certification:

Pre-certified modules: Using modules with existing certifications can enable certification by reference, avoiding full testing. Conditions typically include using approved antennas and following integration guidelines.

Modular approval: Designing custom modules meeting modular approval requirements enables separate module certification and simplified final product certification.

Full testing: Custom designs or modified modules require complete testing. Working with experienced test laboratories streamlines the process.