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This paper presents a ground switching load modulation scheme for passive HF RFID transponders. The proposed modulation scheme allows HF transponders to communicate with the reader in the strong field with a higher modulation index using simple RF clamps, compared with conventional resistive and capacitive load modulation schemes. Also presented is a ground-isolated voltage doubler rectifier to eliminate effects of parasitic diodes in CMOS processes, thereby increasing the communication distance. A 13.56-MHz transponder prototype was implemented in a standard CMOS 0.35-process. Measurement results show that the proposed transponder achieves over 35-mV modulation depth for the magnetic field strength from 0.15 to 7.5 A/m.