Method for reducing amplitude noise in a received signal within an RFID interrogator

1. Method for reducing an amplitude noise AN originating from an RFID interrogator transmitter and appearing in a signal RS received by said RFID interrogator, said method being carried out when a signal TS transmitted by the RFID interrogator is a radiofrequency electromagnetic sine wave, characterized in that a contribution RSd AN of said amplitude noise AN to a demodulated received signal RSd within a frequency interval of a low-pass filter at an interrogator receiver output is determined by means of a measured demodulated transmitted signal TSd as

RSd AN =k ·TSd

in that a direct-current component DC RS in the demodulated received signal RSd and a direct-current component DC TS in the demodulated transmitted signal TSd are measured and a conversion factor k is determined as

k =DC RS /DC TS

and that a receiver output signal O is continuously generated out of the received signal RS by subtracting said contribution RSd AN of the amplitude noise from the demodulated received signal RSd and filtering the signal RSd−k·TSd cleaned in said way by the low-pass filter and amplifying it.

2. Method as recited in claim 1 , characterized in that said contribution RSd AN of the amplitude noise AN is determined in a normal rhythm, whose period is at least twice a period corresponding to a medium frequency of said low-pass filter, by measuring, in the normal rhythm, the direct-current component DC RS in the demodulated received signal RSd and the direct-current component DC TS in the demodulated transmitted signal TSd and determining the conversion factor k each time as k=DC RS /DC TS and that the receiver output signal O is continuously generated.

3. Method as recited in claim 2 , characterized in that the conversion factor k is determined after the RFID interrogator has been turned on in that the demodulated transmitted signal TSd being reduced by an attenuation factor AF, which is stepwise increased in the normal rhythm, is subtracted from the demodulated received signal RSd in the normal rhythm until a direct-current component of said difference moves closest to zero and that last attenuation factor AF is thereafter retained as an approximation of the conversion factor k.

4. Method as recited in claim 3 , characterized in that said attenuation factor AF is stepwise increased by 0.1 from an initial value 0.

5. Method as recited in any of claims 2 to 4 , characterized in that the conversion factor k is determined in a rhythm being slower than the normal rhythm and the receiver output signal O is continuously generated, whereas the last value of the conversion factor k is used in the meantime.

6. Method as recited in any of claims 1 to 5 , characterized in that the received signal RS is demodulated in two receiving channels in that it is mixed with a first output signal I of a local oscillator and with its second output signal Q being in quadrature to the first one, that the conversion factor k 1 , k 2 for either channel is determined independently of the conversion factor for the other channel and that the stronger one of the two output signals O 1 , O 2 in said two channels is digitized and conducted to a communication controller circuit.

7. Method as recited in claim 6 , characterized in that the transmitted signal TS is demodulated in that it is mixed with itself.

8. Method as recited in claim 6 , characterized in that the transmitted signal TS is demodulated in that it is mixed with a phase shifted output signal of the local oscillator.

9. Method as recited in any of claim 7 or 8 , characterized in that the transmitted signal TS is attenuated before the demodulation.

10. Method as recited in any of claims 1 to 9 , characterized in that a new interrogator receiving after the RFID interrogator has stopped transmitting data to a label is resumed with the last determined conversion factors k 1 , k 2 for the first and second receiving channel, respectively.