UHF RFID Guide: EPC Gen2, Inventory & Diagnostics

Core Technical Specifications

center Freq EU: 865–868 MHz / US: 902–928 MHz

modulation ASK / DSB-ASK / PR-ASK

data Encoding FM0 or Miller (M=2/4/8) subcarrier

operating Range 1–12 m

antenna Circularly polarised patch, dipole, or near-field

tx Power EU: 2W ERP / US: 4W EIRP (36 dBm)

bandwidth 200–500 kHz per channel

channel Spacing EU: 200 kHz / US: 500 kHz, FHSS

Real-World Devices & Applications

Supply Chain & Retail

Warehouse inventory tags (pallets, cartons)
Retail item-level tagging (apparel, electronics)
Anti-theft EAS tags with EPC
Library book tags

Transport & Access

Vehicle toll tags (E-ZPass, TollTag)
Parking access tags
Race timing chips
Airline baggage tags

Reference Circuits & Schematics

UHF RFID Reader — Impinj R2000

The Impinj R2000 is a high-performance UHF RFID reader IC used in commercial fixed and handheld readers. Supports 4 antenna ports with 30+ dBm TX power.

Pin	Description
`TX Antenna`	50Ω antenna port(s) with circular polarisation
`RX Path`	Low-noise receiver with −82 dBm sensitivity
`GPIO`	General purpose I/O for triggers and indicators
`Network`	Ethernet/USB for LLRP protocol communication

Circuit Walk-Through

The reader transmits a CW (continuous wave) signal at 865–928 MHz. Tags harvest energy from this field and respond by modulating their antenna impedance (backscatter). The reader demodulates the backscattered signal to recover the EPC (Electronic Product Code). Anti-collision uses Q-algorithm to inventory hundreds of tags per second.

Signal Structure & Waveform Analysis

EPC Gen2 Inventory Round

The reader manages tag populations using a slotted ALOHA anti-collision protocol.

Query

22 bits

Reader initiates inventory round with Q-value (slot count)

RN16

16-bit random

Tag responds with random number in its slot

ACK

18 bits

Reader acknowledges with RN16 echo

EPC + CRC

96–496 bits

Tag transmits full EPC, PC, and CRC-16

Timing Notes: Typical inventory rate: 200–1000 tags/second. EPC: 96 bits standard (12 bytes), expandable to 496 bits. Tag sensitivity: −18 to −22 dBm. Reader→tag link is ASK modulated; tag→reader is backscatter.

Analysis & Capture Tips

Use Wireshark with LLRP dissector for reader protocol analysis
Check antenna VSWR < 1.5:1 at operating frequency
Monitor tag read rates — collisions reduce throughput
Verify regional frequency settings (EU vs US channel plan)

Bench Troubleshooting & Repair Guide

1. Low Read Rate

Reader inventory log / RSSI analysis

Check antenna alignment and polarisation match. Circular-pol antennas work with random tag orientations. Increase Q-value for larger tag populations. Check for metal/liquid interference near tags.

2. Antenna VSWR

NanoVNA / SWR meter

High VSWR reflects power back to the reader PA, reducing effective range and potentially damaging the reader. Check cable connectors and antenna with NanoVNA.

3. Dense Tag Environments

Reader configuration / LLRP

Many tags in a small area cause collisions. Use session flags (S0–S3) to manage populations. Adjust Q-value to match expected tag count. Consider using select commands to filter.

RFID UHF (860–960 MHz)