Rfid tag having parallel resonant circuit for magnetically decoupling

Abstract

An RFID tag circuit provides magnetic decoupling and amplitude modulation. The circuit is made up of a first inductor and a second inductor connected in series, a first capacitor, a second capacitor, and a switch. The circuit includes a first resonant circuit formed from a parallel connection of the series connected first and second inductors, and the first capacitor. The first resonant circuit has a primary resonant frequency. The circuit also includes a second circuit formed from a series connection of the second capacitor and the switch. The series connection of the second capacitor and the switch are connected in parallel to the second inductor. One end of the series connected second capacitor and switch is connected to the common connection between the series connected first and second inductors. When the switch is open, the second circuit has a minimal or no effect on the tag and the first resonant circuit resonates at the primary resonant frequency when the tag is exposed to an external field at or near the primary resonant frequency. When the switch is closed, the second circuit defines a high impedance parallel resonant circuit which functions to block or minimize current flow at the primary resonant frequency, thereby preventing the tag from drawing any significant amount of power from the external field and from resonating at the primary resonant frequency. The tag is thereby decoupled from its environment.