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

摘要

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.

主权项

1. A method for reducing a noise in a signal received in a contactless-card interrogator
in which a transmitter-receiver module is connected both to an interrogator transmitter and an interrogator receiver, and a carrier signal of a transmitter output signal is a first output signal of a local oscillator, all output signals of the local oscillator having a same frequency, and a first receiver output signal is generated by combining a receiver input signal and a second output signal of the local oscillator, conducting the combined signal through a band-pass filter and amplifying it, and a second receiver output signal is generated by combining the receiver input signal and a third output signal of the local oscillator being in quadrature to the second output signal of the local oscillator, conducting the combined signal through a band-pass filter and amplifying it, and a stronger one of said receiver output signals is selected, digitized and conducted to a communication controller, characterized in that a calibrating output signal of the interrogator transmitter is generated in that the first output signal of the local oscillator is shallowly amplitude-modulated with a pilot signal, that a receiver reference signal is generated by combining the calibrating output signal of the interrogator 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, that the first and the 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 signals, respectively, and that an attenuation factor and a 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 as well as the second cleared receiver output signals have a lowest amplitude.