Method for reducing a noise in a signal received in a contactless-card interrogator and a circuit to perform said method
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.
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.
2. The method as recited in claim 1 , characterized in that a pilot signal frequency equals a frequency, at which the contactless-card encodes data.
3. The method as recited in claim 2 , characterized in that the attenuation factor and the polarity of the receiver reference signal are recalibrated for each of both receiver output signals, whenever conditions on an antenna or in a space around the antenna have considerably changed.
4. The method as recited in claim 3 , characterized in that the receiver reference signal is generated by combining the transmitter output signal and a fourth output signal of the local oscillator.
5. The method as recited in claim 3 , characterized in that the receiver reference signal is generated by combining the transmitter output signal and the transmitter output signal with a changed amplitude.
6. The method as recited in claim 4 , characterized in that a phase of the fourth output signal of the local oscillator is set.
7. The method as recited in claim 4 , characterized in that the transmitter-receiver module comprises a coupling circuit and a transmitting and receiving antenna.
8. The method as recited in claim 4 , characterized in that the transmitter-receiver module comprises a transmitting antenna and a receiving antenna.
9. A circuit for reducing a noise in a signal received in a contactless-card interrogator,
in which a transmitter-receiver module is connected both
to an output of an interrogator transmitter amplitude modulator
whereto a first output signal of a local oscillator as a carrier signal of a transmitter output signal is conducted and to a first input of a first and a second receiver mixer stages
whereat a second and a third output signal of the local oscillator being in quadrature to each other are conducted to a second input of said first and second receiver mixer stages, respectively, and
all output signals of the local oscillator have a same frequency, and output signals of said first and second receiver mixer stage are conducted
through a first and a second band-pass filter and an amplifier, respectively,
to a first and a second input of a selection-digitalization circuit, respectively,
which selection-digitalization circuit is connected to a communication controller
which is connected to a digital-to-analogue converter, whose output is connected to a first input of the interrogator transmitter amplitude modulator
said first input being provided for a modulating signal to be combined with the first output signal of the local oscillator,
characterized in
that a transmitter output signal is conducted to a first input of an auxiliary mixer stage to whose second input a signal is conducted,
whose carrier signal has a frequency of the local oscillator output signal, and
whose output is connected to an input of an auxiliary band-pass filter and an amplifier,
that an output of the auxiliary band-pass filter and amplifier is connected through a first and a second controlled attenuating and polarity setting circuit to a second input of a first and a second subtractor, respectively,
to whose first input an output of the first and the second band-pass filter and amplifier, respectively, is connected and
whose output is connected to the first and the second input of the selection-digitalization circuit, respectively,
that outputs of the first and the second subtractors are connected to a first and a second input of a correction circuit, respectively,
which controls the first and the second controlled attenuating and polarity setting circuits and a pilot-signal generator
whose output is connected to
a second input of the interrogator transmitter amplitude modulator, said second input being provided for a modulating signal to be combined with the first output signal of the local oscillator
and that the correction circuit is connected to the communication controller,
which controls the interrogator transmitter amplitude modulator.
10. The circuit as recited in claim 9 , characterized in that a pilot signal frequency equals a frequency, at which the contactless-card encodes data.
11. The circuit as recited in claim 10 , characterized in that a fourth output signal of the local oscillator is conducted to the second input of the auxiliary mixer stage.
12. The circuit as recited in claim 10 , characterized in that the transmitter output signal is conducted through a signal-amplitude setting circuit to the second input of the auxiliary mixer stage.
13. The circuit as recited in claim 11 , characterized in that the fourth output signal of the local oscillator is conducted through a phase shifter to the second input of the auxiliary mixer stage.
14. The circuit as recited in claim 11 , characterized in that the transmitter-receiver module comprises a coupling circuit and a transmitting and receiving antenna.
15. The circuit as recited in claim 11 , characterized in that the transmitter-receiver module comprises a transmitting antenna and a receiving antenna.
16. The circuit as recited in claim 14 , characterized in that the coupling circuit is a directional coupler.
17. The circuit as recited in claim 14 , characterized in that the coupling circuit is a circulator.
18. The circuit as recited in claim 14 , characterized in that the coupling circuit is a discrete circuit provided for separating the received signal and the transmitter carrier signal.