Electronics Guide

Operating Principles

Passive tags harvest energy from the reader's RF field through their antenna. In low-frequency (LF) and high-frequency (HF) systems, inductive coupling between the reader and tag antennas transfers energy like a loosely coupled transformer. In ultra-high frequency (UHF) systems, the tag antenna captures electromagnetic wave energy and rectifies it to power the tag circuitry.

Once powered, the tag modulates the reader field to transmit its stored data back to the reader. This backscatter modulation alters the load on the tag antenna, creating detectable changes in the reader's received signal. The tag requires no transmitter, dramatically simplifying its design and reducing cost.

Low Frequency (LF) Tags

Operating at 125-134 kHz, LF tags use large coiled antennas wound around ferrite cores. Despite their shorter read range (typically under 10 cm), they offer excellent performance near metals and liquids that absorb higher frequencies.

Key Characteristics:

• Frequency: 125 kHz or 134.2 kHz
• Read range: Up to 10 cm typical
• Data rate: Low (under 1 kbps)
• Memory: 64 bits to 2 kilobits typical
• Antenna: Multi-turn coil, often with ferrite core
• Environmental performance: Excellent near metals and liquids

Common Applications: Animal identification, access control, vehicle immobilizers, industrial automation, laundry tracking

High Frequency (HF) Tags

Operating at 13.56 MHz, HF tags strike a balance between range, data rate, and compatibility. The 13.56 MHz frequency is globally available without licensing, enabling worldwide interoperability for applications like NFC and smart cards.

Key Characteristics:

• Frequency: 13.56 MHz
• Read range: Up to 1 meter typical
• Data rate: Medium (up to 424 kbps for NFC)
• Memory: 256 bytes to 8 kilobytes typical
• Antenna: Planar coil, typically 2-6 turns
• Standards: ISO 14443, ISO 15693, NFC Forum specifications

Common Applications: Contactless smart cards, NFC devices, library systems, pharmaceutical tracking, ticketing

Ultra-High Frequency (UHF) Tags

UHF passive tags operating in the 860-960 MHz band offer the longest read ranges of any passive technology, making them the standard for supply chain and logistics applications.

Key Characteristics:

• Frequency: 860-960 MHz (region-specific allocations)
• Read range: Up to 12 meters typical, 20+ meters achievable
• Data rate: High (up to 640 kbps)
• Memory: 96-512 bits EPC, 32 bytes to 64 kilobytes user memory
• Antenna: Dipole or patch designs, application-specific variants
• Standards: EPCglobal Gen2, ISO 18000-6C

Common Applications: Supply chain management, inventory control, asset tracking, retail anti-theft, toll collection

Passive Tag Construction

A passive RFID tag consists of three main components integrated onto a substrate:

Tag Integrated Circuit (IC):

• Power harvesting and rectification circuitry
• Voltage regulation and power management
• Clock recovery from carrier signal
• Demodulator for reader commands
• Memory array (ROM, EEPROM, or FRAM)
• State machine or microcontroller for protocol handling
• Modulator for backscatter transmission
• Optional security and encryption logic

Antenna:

• Coiled wire (LF/HF) or printed/etched metal (UHF)
• Matched to tag IC input impedance
• Form factor optimized for application
• Special designs for metal or liquid proximity

Substrate:

• PET, paper, or specialized materials
• Inlay form for label conversion
• Encapsulated packages for durability
• Flexible or rigid depending on application

Active Tag Architectures

Active tags fall into two categories based on their transmission approach:

Beacon Tags:

• Periodically broadcast their identity at preset intervals
• Do not require reader interrogation to transmit
• Simpler protocol but higher power consumption
• Ideal for real-time location systems (RTLS)
• Beacon intervals typically 1 second to several minutes

Transponder Tags:

• Transmit only when queried by a reader
• Lower power consumption extends battery life
• More complex protocol requiring reader coordination
• Better suited for inventory and access applications
• Can include reader-like functionality for tag-to-tag communication

Active Tag Specifications

Key Characteristics:

• Frequency: 433 MHz, 915 MHz, 2.45 GHz common
• Read range: 30-100+ meters typical
• Data rate: High (up to several Mbps at 2.45 GHz)
• Memory: Kilobytes to megabytes available
• Battery life: 2-7 years typical
• Size: Larger than passive due to battery
• Cost: Significantly higher than passive tags

Advantages Over Passive:

• Much longer read ranges
• Reliable communication independent of reader field strength
• Can include sensors (temperature, shock, humidity)
• Support for data logging and time-stamping
• Better penetration through obstacles

Semi-Passive (BAP) Tags

Semi-passive or Battery-Assisted Passive (BAP) tags use a battery to power the tag IC but communicate using backscatter like passive tags. This hybrid approach offers extended range while maintaining compatibility with passive reader infrastructure.

Key Characteristics:

• Battery powers IC, not transmitter
• Read range: 3-30 meters typical
• Backscatter communication maintains passive compatibility
• Extended sensor and logging capabilities
• Lower cost than full active tags
• Longer battery life than beacon-style active tags

Common Applications: Cold chain monitoring, high-value asset tracking, aerospace and defense, pharmaceutical logistics

Reader Architecture

A typical RFID reader includes the following subsystems:

RF Front End:

• Local oscillator and frequency synthesis
• Power amplifier for field generation
• Transmit/receive switch or circulator
• Low-noise amplifier for backscatter reception
• Mixer and demodulator circuits
• Antenna matching network

Digital Subsystem:

• Baseband signal processing
• Protocol state machine
• Anti-collision algorithm implementation
• Host interface (USB, Ethernet, serial, GPIO)
• Configuration and management firmware

Reader Types

Fixed Readers:

• Permanently installed at portals, conveyor lines, or storage areas
• Multiple antenna ports (typically 4-8)
• Network connectivity for enterprise integration
• High power output for maximum range
• Continuous operation capability

Handheld Readers:

• Portable units for mobile inventory operations
• Integrated antenna and display
• Battery-powered operation
• Wireless connectivity (WiFi, Bluetooth, cellular)
• Often integrated with barcode scanning

Embedded Reader Modules:

• Compact modules for OEM integration
• Reduced feature set optimized for specific applications
• Lower power consumption
• Serial or SPI host interfaces
• Certification pre-completed for faster time-to-market

Reader Antennas

The antenna is critical to reader performance, determining read zone shape, range, and directivity:

LF/HF Antennas:

• Loop antennas creating magnetic field
• Tuned to resonance at operating frequency
• Q factor balances range and bandwidth
• Near-field coupling limits range
• Large loops for portal applications

UHF Antennas:

• Patch, dipole, or Yagi designs
• Linear or circular polarization
• Gain typically 6-12 dBi
• Beamwidth determines coverage pattern
• VSWR critical for power transfer efficiency

Antenna Arrays:

• Multiple elements for beam steering
• Phased arrays for dynamic coverage
• MIMO configurations for localization
• Switched antenna multiplexing

Reader Performance Parameters

• Transmit power: Regulated by region (typically 1-4W EIRP for UHF)
• Receiver sensitivity: Determines minimum readable backscatter level
• Read rate: Tags per second capability
• Dense reader mode: Coordination for multi-reader environments
• Protocol support: Gen2, ISO standards, proprietary

NFC Operating Modes

NFC defines three operating modes enabling flexible device interaction:

Reader/Writer Mode:

• NFC device acts as reader for passive tags
• Compatible with ISO 14443 and ISO 15693 tags
• Reads NFC Forum tag types (Type 1-5)
• Enables smart poster and pairing applications

Card Emulation Mode:

• NFC device appears as contactless smart card
• Compatible with existing payment infrastructure
• Host Card Emulation (HCE) or Secure Element based
• Enables mobile payment and access control

Peer-to-Peer Mode:

• Bidirectional communication between NFC devices
• LLCP (Logical Link Control Protocol) foundation
• SNEP (Simple NDEF Exchange Protocol) for data transfer
• Enables Bluetooth/WiFi handover initiation

NFC Tag Types

The NFC Forum defines five tag types with varying capabilities:

Type 1 Tags:

• Based on ISO 14443A
• Memory: 96 bytes to 2 kilobytes
• Read/write or read-only configuration
• 106 kbps data rate
• No anti-collision support

Type 2 Tags:

• Based on ISO 14443A (similar to MIFARE Ultralight)
• Memory: 48 bytes to 2 kilobytes
• Read/write with lock capability
• 106 kbps data rate
• Anti-collision supported

Type 3 Tags:

• Based on Sony FeliCa
• Memory: Up to 1 megabyte
• Read/write or read-only
• 212 or 424 kbps data rate
• Widely used in Japan for transit

Type 4 Tags:

• Based on ISO 14443A/B
• Memory: Up to 32 kilobytes typical
• Read/write with optional lock
• Up to 424 kbps data rate
• File system structure

Type 5 Tags:

• Based on ISO 15693 (vicinity cards)
• Extended read range compared to other types
• Memory: Up to 64 kilobytes
• 26 kbps data rate
• Industrial and logistics applications

NFC Controllers and Readers

NFC controller ICs integrate the RF front end, protocol processor, and host interface for complete NFC functionality:

Integrated NFC Controllers:

• Complete analog front end with antenna matching
• Multi-protocol support (all NFC modes)
• Secure Element interface (SWP, I2C)
• Host processor interface (I2C, SPI, UART)
• Low power modes for mobile applications
• Examples: NXP PN532/PN7150, ST ST25R series

NFC Reader ICs:

• Reader/writer mode only
• Simplified design for embedded applications
• Lower cost than full controllers
• GPIO and interrupt outputs
• Examples: MFRC522, ST25R3911

Coil Design Parameters

Inductance:

• Determines resonant frequency with tuning capacitor
• Calculated from coil geometry (turns, diameter, spacing)
• LF tags: Typically 1-10 mH
• HF tags: Typically 1-5 microhenries

Quality Factor (Q):

• Ratio of stored energy to dissipated energy
• Higher Q increases voltage at resonance
• Trade-off with bandwidth and detuning sensitivity
• Typical values: 20-80 for tag coils

Coil Resistance:

• DC resistance limits Q factor
• AC resistance increases at high frequency (skin effect)
• Minimized by conductor width and thickness

Coil Construction Methods

Wire-Wound Coils:

• Enameled copper wire on bobbin or core
• High inductance in compact form (LF tags)
• Ferrite cores enhance field coupling
• Used in glass capsule tags and LF access cards

Etched Coils:

• Copper traces on PCB or flexible substrate
• Precise, repeatable geometry
• Standard manufacturing processes
• Multi-layer designs increase inductance

Printed Coils:

• Conductive ink on paper or plastic
• Low cost for high volume
• Lower conductivity requires wider traces
• Compatible with roll-to-roll manufacturing

Aluminum Coils:

• Lower cost than copper
• Higher resistance requires design compensation
• Common in UHF tag antennas
• Etched from aluminum PET laminate

Coil Tuning and Matching

Proper tuning maximizes energy transfer between reader and tag:

Resonant Tuning:

• Parallel capacitor tunes coil to operating frequency
• On-chip tuning capacitors in modern tag ICs
• External capacitors for precise tuning
• Temperature compensation for stable performance

Impedance Matching:

• Match coil impedance to tag IC input
• Maximize power transfer to rectifier
• Shunt and series matching networks
• Accounts for IC input impedance variation with power level

Collision Problem

Multiple tags responding simultaneously create signal collisions that prevent successful communication. Anti-collision algorithms resolve this by:

• Serializing tag responses in time
• Identifying unique tag identifiers
• Allowing selected communication with individual tags
• Maximizing throughput while ensuring all tags are read

ALOHA-Based Protocols

Probabilistic protocols where tags respond at random times:

Pure ALOHA:

• Tags respond immediately when powered
• Simple implementation
• Maximum throughput approximately 18%
• Suitable for few tags, short transactions

Slotted ALOHA:

• Reader defines time slots
• Tags respond in randomly selected slots
• Maximum throughput approximately 37%
• Used in ISO 18000-6C (Gen2) standard

Dynamic Frame Slotted ALOHA:

• Reader adjusts frame size based on collision rate
• Optimizes throughput for tag population
• Q parameter in Gen2 controls slot count
• Adapts to varying tag densities

Binary Tree Protocols

Deterministic protocols that systematically eliminate collisions:

Binary Search:

• Reader queries tag ID ranges
• Collision indicates multiple tags in range
• Range recursively subdivided until unique tag found
• Guaranteed to identify all tags

Bit-by-Bit Arbitration:

• Tags transmit ID one bit at a time
• Reader detects collisions at each bit position
• Used in ISO 14443A (NFC Type A)
• Efficient for small tag populations

Gen2 Anti-Collision

EPCglobal Gen2 (ISO 18000-6C) uses an advanced slotted ALOHA implementation:

• Q algorithm dynamically adjusts slot count
• 16-bit random number generator in tags
• Session and target flags prevent re-inventorying
• Can inventory hundreds of tags per second
• Dense reader mode coordinates multiple readers

Read-Only Memory (ROM)

Factory-programmed memory that cannot be modified:

• Unique tag identifier (TID) programmed at manufacture
• Guaranteed uniqueness and authenticity
• Cannot be cloned or modified
• Zero write cycles required
• Lowest per-tag cost

One-Time Programmable (OTP)

Memory that can be written once, then becomes permanent:

• Fuse-based or antifuse technology
• User programs unique data after manufacture
• Data becomes permanent after programming
• Cannot be erased or modified
• Used for EPC encoding in some applications

EEPROM

Electrically Erasable Programmable Read-Only Memory enables multiple write cycles:

• Non-volatile data retention
• Byte or page erasure and programming
• Limited write endurance (10,000-100,000 cycles typical)
• Slower write times than RAM
• Most common rewritable tag memory

FRAM (Ferroelectric RAM)

Advanced memory offering fast, low-power writes with high endurance:

• Non-volatile storage
• Fast write speeds approaching RAM
• Virtually unlimited write endurance (10^14 cycles)
• Lower write power than EEPROM
• Higher cost limits adoption
• Ideal for data logging applications

Memory Organization

Gen2 UHF tags organize memory into four banks:

• Bank 0 (Reserved): Kill password and access password
• Bank 1 (EPC): Electronic Product Code and protocol control
• Bank 2 (TID): Tag and chip identifier (often read-only)
• Bank 3 (User): User-programmable data storage

Authentication Mechanisms

Password Protection:

• Access and kill passwords in Gen2 tags
• 32-bit passwords limit brute-force attempts
• Password required for memory modification
• Lock bits prevent password changes

Challenge-Response Authentication:

• Reader sends random challenge
• Tag computes response using secret key
• Prevents replay attacks
• Requires cryptographic capability in tag

Mutual Authentication:

• Both reader and tag verify each other
• Prevents rogue reader attacks
• Used in payment and high-security applications
• Based on symmetric or asymmetric cryptography

Cryptographic Implementations

Symmetric Cryptography:

• AES-128/256 for high security
• DES/3DES in legacy systems
• Proprietary algorithms (CRYPTO1 in MIFARE Classic)
• Key management challenges at scale

Asymmetric Cryptography:

• Elliptic Curve Cryptography (ECC) for constrained devices
• Public key eliminates key distribution problem
• Digital signatures for authenticity
• Higher computational requirements

Privacy Protection

Measures to prevent unauthorized tracking and data collection:

Kill Command:

• Permanently disables tag
• Requires 32-bit kill password
• Protects consumer privacy post-purchase
• Cannot be reversed

Privacy Mode:

• Reduces tag response to authorized readers only
• Password required to exit privacy mode
• Reversible alternative to kill command

Untraceable Mode:

• Gen2v2 feature for range and data hiding
• Limits response range when enabled
• Hides EPC until authenticated
• Configurable assertion and de-assertion

Secure Elements

Dedicated security ICs for high-assurance applications:

• Tamper-resistant hardware
• Secure key storage
• Certified to Common Criteria standards
• Used in payment cards and government ID
• Examples: NXP SmartMX, Infineon SLE series

RF Energy Harvesting Principles

The tag antenna couples energy from the reader field, which must be converted to DC power for the tag IC:

Inductive Coupling (LF/HF):

• Magnetic field induces voltage in tag coil
• Voltage proportional to field strength and coil parameters
• Coupling coefficient decreases rapidly with distance
• Read range limited by near-field boundary

Far-Field Capture (UHF):

• Antenna captures electromagnetic wave energy
• Received power follows inverse square law
• Polarization matching critical for efficiency
• Path loss determines practical range limits

Rectifier Circuits

Rectifiers convert AC energy from the antenna to DC power for the tag IC:

Diode Rectifiers:

• Half-wave or full-wave configurations
• Schottky diodes minimize forward voltage drop
• Diode-connected transistors in integrated designs
• Charge pump topologies multiply voltage

Voltage Multipliers:

• Dickson charge pump most common
• Multi-stage multiplication increases DC voltage
• Trade-off between voltage and current capability
• Essential for UHF tags with low antenna voltages

Power Management

Efficient power management maximizes operating range:

Voltage Regulation:

• Shunt regulators clamp excess voltage
• Series regulators provide stable supply
• Prevents IC damage from close-range high fields
• Maintains consistent operation across range

Power-On Reset:

• Detects sufficient power for operation
• Initializes tag state machine
• Prevents partial operation at field edge

Low-Power Design:

• Subthreshold circuit operation
• Clock gating and power gating
• Minimal transistor counts
• Optimized protocol timing

Tag Sensitivity

Tag sensitivity defines the minimum power required for operation:

• Modern UHF tags: -20 to -25 dBm typical
• State-of-art designs approaching -30 dBm
• Lower sensitivity enables longer read range
• Trade-off with functionality and security features
• Measured at IC input, not free-space field

Frequency Selection

Choose operating frequency based on application requirements:

• LF (125-134 kHz): Excellent for metal/liquid environments, animal ID, access control
• HF (13.56 MHz): Good balance for smart cards, NFC, item-level tracking
• UHF (860-960 MHz): Long range for supply chain, inventory, asset tracking
• Microwave (2.45 GHz): High data rate, precision location, toll collection

Tag Selection Criteria

• Read range: Match to application requirements
• Memory capacity: Sufficient for data storage needs
• Environmental rating: Temperature, humidity, chemical exposure
• Form factor: Compatible with attachment method
• Security level: Appropriate for data sensitivity
• Standards compliance: Ensure interoperability
• Cost: Balance performance with budget constraints

Reader Deployment

• Position antennas to cover required read zones
• Account for multipath and interference
• Implement dense reader protocols where needed
• Consider regulatory power limits
• Plan for network integration and data management
• Test with representative tag populations

Metal and Liquid Considerations

Metals and liquids significantly impact RFID performance:

• Metals reflect and detune UHF antennas
• Liquids absorb UHF energy
• Special on-metal tag designs available
• LF less affected due to magnetic coupling
• Test in actual deployment environment

Supply Chain and Logistics

• Case and pallet tracking through distribution
• Receiving and shipping verification
• Warehouse inventory management
• Cross-docking automation
• Returns processing

Retail

• Item-level inventory visibility
• Electronic article surveillance (EAS)
• Self-checkout enablement
• Omnichannel fulfillment
• Customer experience enhancement

Healthcare

• Patient identification and tracking
• Medication verification
• Surgical instrument tracking
• Blood product management
• Equipment location and maintenance

Access Control and Security

• Building and room access
• Time and attendance tracking
• Vehicle access and parking
• Event ticketing
• Government ID and e-passports

Contactless Payment

• Credit and debit card transactions
• Mobile wallet payments (Apple Pay, Google Pay)
• Transit fare collection
• Closed-loop payment systems
• Micropayment applications

Industrial and Manufacturing

• Work-in-process tracking
• Tool and equipment management
• Quality control and traceability
• Returnable transport item (RTI) tracking
• Maintenance scheduling

RFID Standards

• ISO 11784/11785: Animal identification
• ISO 14443: Proximity cards (HF contactless smart cards)
• ISO 15693: Vicinity cards (HF longer range)
• ISO 18000-6C: UHF air interface (EPCglobal Gen2)
• EPCglobal standards: Supply chain RFID specifications

NFC Standards

• ISO 18092: NFC interface and protocol
• NFC Forum specifications: Tag types, NDEF, protocols
• EMVCo specifications: Contactless payment
• GlobalPlatform: Secure element management

Regulatory Compliance

• FCC Part 15 (United States)
• ETSI EN 302 208 (Europe)
• Regional frequency allocations
• Maximum transmit power limits
• Spurious emissions requirements