UHF EPC Gen2 RFID-Reader Design

Portable Reader Design with Carrier Suppression

In UHF (Ultra High Frequency) RFID (Radio Frequency Identification) systems Rx (Receiver) is usually isolated from Tx (Transmitter) by a circulator or a directional coupler. Since tags become more sensitive in order to improve reading distance, the relatively poor isolation of 20dB – 30dB limits the tag to reader link. A way to improve a directional coupler’s isolation is to mismatch the unused port to generate a carrier cancelling signal. An impedance network using digitally tunable capacitors was developed. The advantage of this solution is that changes in the system – like antenna characteristics or cable length - can be recalibrated at any time by the controller. Finally an isolation of minimum 47dB for any antenna characteristic (minimum return loss of 18dB) and any cable length was reached. Also an algorithm was implemented on a simple MSP430 controller to automatically find the optimum impedance to minimize carrier leakage.

Principle of the Mismatched Coupler generating the cancelling signal (yellow)
Block diagram of UHF Handheld Reader

The reader design is based on AS3992 (former IDS R902) extended with amplifiers and completely controlled by a MSP430 controller.  For the mismatch load, which allows digitally controlled carrier suppression, only RLC elements are used. Thereof two integrated capacitors with very high linearity (third order input intercept point of 65dBm) are digitally tunable.

Specification

RFID Frequency Band
UHF 868 MHz ETSI Channels
Transmitter Power
500 mW
EIRP
up to 30 dBm with internal antenna
Reading Distance
typical 4 m for passive ID, 1 m for passive sensors
RFID Protocol
EPC Gen2, ISO 18000-6C
Reader Antenna
Quadrifilar Antenna from PHYCHIPS
Digital Tunable Load
PE64904 from Peregrine Semiconductor
Reader Chip
AS3992 from AMS
Protocol Controller
MSP430 from Texas Instruments