Radio frequency voltage limiter for an rfid transponder

A radio frequency (RF) voltage limiter for a radio frequency identification (RFID) transponder, the RF voltage limiter comprising:

• an envelope detector having an input coupled to an antenna, the envelope detector configured to track an envelope of a signal received at the antenna, the envelope detector having an output for providing an envelope signal;
• a peak detector having an input coupled to the output of the envelope detector. wherein the peak detector comprises a current mirror having an input coupled to the output of the envelope detector, and an output configured to provide a control signal;
• and
• a shunt circuit coupled to provide an impedance to the antenna in response to the control signal, wherein the impedance decreases with an increasing voltage magnitude of the control signal;
• a first transistor having a first current electrode and a control electrode both coupled to receive a reference current, and a second current electrode coupled to the output of the envelope detector;
• a second transistor having a first current electrode, a control electrode coupled to the control electrode of the first transistor, and a second current electrode coupled to provide the control signal to the shunt circuit;
• and
• wherein the peak detector further comprises a first current source coupled between the first current electrode of the first transistor and a power supply voltage terminal, the first current source for providing the reference current.

A radio frequency identification (RFID) transponder comprising a radio frequency (RF) voltage limiter, the RF voltage limiter comprising:

• an envelope detector having an input coupled to an antenna, the envelope detector configured to track an envelope of a signal received at the antenna, and the envelope detector having an output for providing an envelope signal;
• a peak detector having an input coupled to the output of the envelope detector, wherein the peak detector comprises a current mirror having an input coupled to the output of the envelope detector, and an output configured to provide a control signal, wherein the control signal is configured to have a rising edge that rises with a rising edge of the envelope signal, and wherein the peak detector is configured to provide the control signal having a falling edge that more slowly follows a falling edge of the envelope signal;
• a shunt circuit coupled to provide an impedance to the antenna in response to the control signal, wherein the impedance decreases with an increasing voltage magnitude of the control signal;
• a first transistor having a first current electrode and a control electrode coupled to receive a first reference current, and a second current electrode coupled to the output of the envelope detector;
• and
• a second transistor having a first current electrode, a control electrode coupled to the control electrode of the first transistor, and a second current electrode coupled to provide the control signal to the shunt circuit.

A method for limiting a radio frequency (RF) voltage at an antenna, the method comprising:

• tracking an envelope of an RF signal at the antenna to produce an envelope signal;
• detecting a voltage level of the envelope signal using a current mirror, and providing a control signal, using the current mirror, that has a rising edge that rises with a rising edge of the envelope signal, and the control signal having a falling edge that follows more slowly than a falling edge of the envelope signal wherein the current mirror comprises:
• a first transistor having a first current electrode and a control electrode coupled to receive a first reference current, and a second current electrode coupled to an output of an envelope detector;
• and
• a second transistor having a first current electrode, a control electrode coupled to the control electrode of the first transistor, and a second current electrode coupled to provide the control signal to a shunt circuit;
• and
• providing an impedance to the antenna in response to the control signal, wherein the impedance decreases with an increasing control signal.