Intermodulation mitigation technique in an RFID system
View Patent ↗When multiple readers for RF transponders have to be placed in close proximity, such as in adjacent lanes of a highway toll barrier, they can be set to operate at different frequencies. When signals from two adjacent ones of the readers interfere, the resulting signal includes interference terms whose frequencies equal the sum of the reader frequencies and the difference between the reader frequencies. To remove such interference terms while passing the desired terms, a tag includes a low-pass or other frequency-selective filter.
1. A transponder for receiving an interrogation signal from a reader at a first frequency and sending a response signal from the transponder at a third frequency, while mitigating interference effects from an interfering signal at a second frequency, the transponder comprising:
an antenna for receiving the interrogation signal from the reader;
a detector in communication with the antenna, said detector detecting the interrogation signal and outputting a baseband analog signal representing the interrogation signal;
a frequency-selective filter comprising a low-pass filter in communication with the detector to receive the baseband analog signal output by the detector, said frequency-selective filter performing frequency-selective filtering of the baseband analog signal to output an analog signal with reduced interference; and
a signal processor in communication with the frequency-selective filter to receive the analog signal with the reduced interference, said signal processor processing the analog signal with the reduced interference to extract information from the analog signal with the reduced interference;
wherein the low-pass filter has a cutoff frequency which is less than a difference between the first frequency and the second frequency.
2. The transponder of claim 1 , wherein the low-pass filter comprises an input, an output, a resistor coupled between the input and the output, and a capacitor coupled between the output and a ground.
3. The transponder of claim 1 , wherein the signal processor comprises:
a limiter for receiving the analog signal with reduced interference from the frequency-selective filter and generating a digital signal; and
a baseband decoder for receiving the digital signal from the limiter.
4. A transponder for receiving an interrogation signal from a reader at a first frequency and sending a response signal from the transponder at the first frequency, while mitigating interference effects from an interfering signal at a second frequency, the transponder comprising:
an antenna for receiving the interrogation signal from the reader and the interfering signal at the second frequency;
a detector in communication with the antenna, said detector detecting the interrogation signal and the interfering signal and outputting a baseband analog signal representing the interrogation signal, the interfering signal, and a combination of the interrogation signal and the interfering signal;
a frequency-selective filter comprising a high-pass filter in communication with the detector to receive the baseband analog signal output by the detector, said frequency-selective filter performing frequency-selective filtering of the baseband analog signal to output an analog signal, wherein the interfering signal is reduced by an amount proportional to a square of the interfering signal; and
a signal processor in communication with the frequency-selective filter to receive the analog signal output by the frequency-selective filter, said signal processor processing the analog signal output by the frequency-selective filter to extract information from the analog signal output by the frequency-selective filter;
wherein the high-pass filter has a cutoff frequency which is greater than a difference between the first frequency and the second frequency and less than a frequency of modulation of the interrogation signal from the reader.
5. The transponder of claim 4 , wherein the signal processor comprises:
a limiter for receiving the analog signal with reduced interference from the frequency-selective filter and generating a digital signal; and
a baseband decoder for receiving the digital signal from the limiter.
6. A transponder for receiving an interrogation signal from a reader at a first frequency and sending a response signal from the transponder at a third frequency, while mitigating interference effects from an interfering signal at a second frequency, the transponder comprising:
an antenna for receiving the interrogation signal from the reader and the interfering signal at the second frequency;
a detector in communication with the antenna, said detector detecting the interrogation signal and the interfering signal and outputting a baseband analog signal representing the interrogation signal, the interfering signal, and a combination of the interrogation signal and the interfering signal;
a frequency-selective filter comprising a high-pass filter in communication with the detector to receive the baseband analog signal output by the detector, said frequency-selective filter performing frequency-selective filtering of the baseband analog signal to output an analog signal, wherein the interfering signal is reduced by an amount proportional to a square of the interfering signal; and
a signal processor in communication with the frequency-selective filter to receive the analog signal output by the frequency-selective filter, said signal processor processing the analog signal with the reduced interference to extract information from the analog signal output by the frequency-selective filter;
wherein the high-pass filter has a cutoff frequency which is greater than a difference between the first frequency and the second frequency and less than a frequency of modulation of the interrogation signal from the reader.
7. The transponder of claim 6 , wherein the signal processor comprises:
a limiter for receiving the analog signal with reduced interference from the frequency-selective filter and generating a digital signal; and
a baseband decoder for receiving the digital signal from the limiter.