Method for reducing a noise in a signal received in a contactless-card interrogator and a circuit to perform said method

Abstract

A calibrating output signal of the transmitter is generated in an interrogator in that a first output signal of a local oscillator is shallowly amplitude-modulated with a pilot signal having a frequency at which a contactless-card encodes data. A receiver reference signal is generated by combining the calibrating output signal of the transmitter and a signal whose carrier signal has a frequency equaling the frequency of the local oscillator signals, conducting the combined signal through a band-pass filter and amplifying it. A first and a second receiver output signals are cleared by subtracting the receiver reference signal, which has been attenuated by a calibrated factor and has a calibrated polarity, from the first and the second receiver output signal, respectively. The attenuation factor and the polarity of the receiver reference signal are sporadically calibrated for each of both receiver output signals by determining and setting the attenuation factor and the polarity of the receiver reference signal as a pair of values for each of both receiver output signals, at which pair of values the first and the second cleared receiver output signals have the lowest amplitude. The signal-to-noise ratio for the interrogator receiver output signal is improved, because this signal no longer contains an amplitude noise of the interrogator transmitter.