Patent ID: 7619466

Claim:
A quadrature demodulator comprising reception means for receiving a signal transmitted by an RFID tag and containing a specific pattern and data following the specific pattern, multiplying the reception signal by a local signal to generate an I-signal, and multiplying the reception signal by the local signal shifted in phase by 90 degrees to generate a Q-signal, first demodulating means, and second demodulating means, the first demodulating means comprising an I-signal squaring section which squares the I-signal, a Q-signal squaring section which squares the Q-signal, an adding section which adds the output from the I-signal squaring section to the output from a Q-signal squaring section, an automatic gain control section which controls gain of the I- and Q-signals on the basis of a value from the adding section, and a data generating section which generates data by inverting a signal level of a signal output by the adding section every time the value of the signal becomes equal to or greater than a predetermined threshold or smaller than a predetermined threshold, the second demodulating means comprising I-signal specific-pattern detecting means for detecting the specific pattern in the I-signal, Q-signal specific-pattern detecting means for detecting the specific pattern in the Q-signal, and decoding means for decoding the data following the specific pattern detected by one of the specific-pattern detecting means, the quadrature demodulator being configured to select one of the first demodulating means and the second demodulating means in accordance with a gain of the automatic gain control section.