Transponder demodulator for a low antenna limiter threshold

An RFID transponder having an antenna for receiving an RF signal including an amplitude modulated downlink data signal (DDS), and a demodulating stage (2) coupled to the antenna for receiving a derived RF signal (DRF) which is derived from the received RF signal, the demodulating stage (2) comprising:

• a first filter (4) for extracting a field strength signal component (FSS) from the derived RF signal (DRF);
• a second filter (3) for extracting the modulated downlink data signal component (DDS) from the derived RF signal (DRF);
• and
• a demodulator coupled to the second filter (3) to receive the modulated downlink signal for demodulating the modulated downlink data signal component (DDS) and coupled to the first filter (4) to receive the field strength signal (FSS) such that the demodulator is adapted to vary a demodulation sensitivity parameter in response to the field strength signal (FSS), wherein a limiter (1) is coupled between the antenna and the demodulating stage (2), the limiter (1) being adapted to act fast enough to suppress amplitude variations due to the modulated downlink data signal included in the received RF signal in a limited internal supply signal (Vcc), and wherein the first filter (4) and the second filter (3) are coupled with an input to the limiter (1) for receiving as the derived RF signal an internal control signal of the limiter, which reflects the amplitude variations that are to be prevented by the limiter.

A method of controlling a demodulator of an RFID transponder, the method comprising:

• receiving an RF signal, which includes an amplitude modulated downlink data signal, with an antenna of the transponder;
• deriving a derived RF signal (DRF) from the received RF signal, which is indicative of amplitude variations of the received RF signal;
• splitting the received RF signal into a first signal component (FSS) indicative of the field strength of the RF signal, and a second signal component (DDS) indicative of the amplitude modulated downlink data signal;
• and demodulating the second signal component (DDS) with a demodulator, the demodulation sensitivity of which is controlled by use of the first signal component (FSS).