WO2008002137A1

WO2008002137A1 - Transceiver device for responder signals

Info

Publication number: WO2008002137A1
Application number: PCT/NL2007/050313
Authority: WO
Inventors: Jan Cornelis Stekelenburg
Original assignee: Nedap Agri B.V.
Priority date: 2006-06-28
Filing date: 2007-06-27
Publication date: 2008-01-03
Also published as: EP2041692A1; NL1032074C2

Abstract

A transceiver (1) device for responder signals provided by responders of a radio-frequency identification system, comprising a transmitter (2) which generates a transmitter signal on the basis of a crystal oscillator (3) and supplies it to at least one antenna (4) for forming an interrogation field for responders, and a receiving device connected with the at least one antenna for receiving signals generated by a responder, provided with means for reconstructing the original transmitter signal from the antenna signal and with an adding device with a negative input, to which the reconstructed signal is supplied, and a positive input, to which the antenna signal is supplied, wherein the output of the adding device (16) is connected with an AM receiver (17), which provides a responder signal as an output signal, and a method for obtaining a responder signal from an antenna signal of a transceiver device (1).

Description

Transceiver device for responder signals

The invention relates to a transceiver device for responder signals provided by responders of a radio-frequency identification system, comprising a transmitter which generates a transmitter signal on the basis of a crystal oscillator and supplies it to at least one antenna for forming an interrogation field for responders, and a receiving device connected with the at least one antenna for receiving signals generated by a responder. Such identification systems are often shortly referred to as RFID systems and are, for instance, used for electronically identifying persons, animals, goods, etc. which are, to this end, provided with a responder. Such responders are also referred to by other terms, such as tag, detection tag, transponder, chip, etc.

The invention further relates to a method for detecting responder signals.

The responders contain an electronic circuit provided with a code, which generates a responder signal in an interrogation field generated by a transmitter (often combined with a receiver and then referred to as transceiver), which responder signal comprises the code of the responder and can be received and recognized by the receiver. To this end, the responder abstracts power supply energy for the electronic circuit from the interrogation field, which electronic circuit then modulates the interrogation field with the responder signal. In principle, the interrogation field is an electromagnetic AC field but, with the relatively low RFID frequencies used here, it is virtually exclusively magnetic in nature.

The receiver needs to be able to effectively separate the antenna signal of an identification system into two components. Upon the receipt of a signal from an FDX (Full DupleX) responder, the antenna signal of the receiver consists of the transmitter signal for the energy supply of the responder and the responder signal incorporating the responder code. The transmitter signal has an amplitude which can be a million times larger than the responder signal. A receiver needs to be able to separate the weak responder signal from this strong signal, which entails a number of limitations in the design. The most common method to separate the responder code from the strong transmitter signal is the use of the top detector. Here, the total antenna signal is first led through a filter, which separates the frequency band with the information of the code therein from most other radio signals. Then the top detector transforms this frequency band to the baseband, which in effect means that the two sidebands on both sides of the transmitter signal are shifted one over the other in the frequency range from 0 to a few kHz. In this operation, the transmitter signal is converted to a DC voltage. After this, with the aid of a so-called coupling capacitor, then the strong transmitter signal can be separated from the code information. There are a number of drawbacks to this method. For instance, no distinction can be made anymore between information from the so-called lower sideband and that from the upper sideband. Interference from only one of these sidebands will always disturb the end result. Usually, this problem can be prevented with a quadrature detector, but in this situation with a very strong transmitter signal, such a detector is not practically usable. In addition, with a change of the amplitude of the transmitter signal, great disturbance of the received code may occur, whereby code information may be lost. The invention contemplates preventing or at least reducing the above-mentioned problems. There is a need for a receiver which effectively separates the responder signals from any interference signals from the environment. As appears from the above, this does not work optimally if the transmitter signal is still added to this responder signal. According to the invention, a receiving device of the above- mentioned type is characterized by means for reconstructing the original transmitter signal from the antenna signal and by an adding device with a negative input and a positive input, with the reconstructed signal and the antenna signal being supplied to the positive input and the negative input, respectively, or being supplied to the negative input and the positive input, respectively, while the output of the adding device is connected with an AM receiver, which provides a responder signal as an output signal.

It is noted that it is known from EP-0649111 to first demodulate the transmitter signal, the information from the sidebands being separated from the carrier wave. So, the transmitter signal is stripped of its carrier wave. Further, the sum of the transmitter signal and the reception signal of the responder, i.e. the antenna signal, is demodulated and thus stripped of its carrier wave. Then the difference of the two demodulated signals is determined. However, it is not known from EP-0649111 to determine a difference between a reconstructed original transmitter signal and the antenna signal.

In the following, the invention will be explained in more detail with reference to the appended drawing. The single Figure shows a block diagram of an example of a part of a transceiver device 1 of an RFID system. The transceiver device 1 shown comprises a transmitter 2, which generates a transmitter signal of a suitable frequency, for instance 134 kHz, on the basis of a crystal oscillator 3. The transmitter signal is emitted with the aid of at least one antenna 4 for forming an interrogation field in a detection zone near the antenna.

If a responder associated with the RFID system is present in the interrogation field, the responder is activated by the interrogation field and the responder will generate a responder signal. The responder signal modulates the amplitude of the interrogation field. In the case of an FDX responder, this results in an antenna signal at the antenna which is composed of the transmitter signal and the responder signal. The antenna signal is a signal modulated in amplitude, with the amplitude modulation representing the responder signal.

As already noted in the above, while it is possible to transform the antenna signal in a conventional manner with a top detector in order to obtain the responder signal containing the responder code, this known method does not lead to optimal results.

According to the invention, therefore, the responder signal is recovered from the antenna signal in a different manner, namely by reconstructing the original transmitter signal and then subtracting it from the antenna signal.

To this end, the antenna signal, which is the sum of the transmitter signal and the responder signal, is supplied to a limiter circuit 5. The limiter circuit removes all amplitude information from the signal, so that a block-shaped signal 6 is created. The block-shaped signal is in phase with the transmitter signal presented to the antenna, but there may be phase variations with respect to the original signal of the crystal oscillator 3, which has an extremely stable phase. These possible phase differences may, for instance, have arisen in an antenna filter (not shown) and are indicated in the block-shaped signal 6 by the vertical sides of the block-shaped signal drawn in thickened lines.

The block-shaped signal 6 is therefore supplied to a phase compensation device 25, which comprises, in this example, a phase modulator 7, a phase detector 8, a high-pass filter 10 and a comparing circuit 11. Via the phase modulator 7, the block-shaped signal is supplied to the phase detector 8, which compares the phase of the block-shaped signal 6 with the phase of the signal 9 of the crystal oscillator. The result of the comparison is supplied to the comparing circuit 11 via the high-pass filter 10.

At the one input, the comparing circuit receives the output signal of the high-pass filter 10, representing the phase difference between the crystal oscillator signal and the block-shaped signal 6. At the other input of the comparing circuit, a suitable reference voltage is supplied. With the aid of the output signal of the comparing circuit, the phase modulator 7 is controlled such that the phase detector 8 no longer supplies an output signal. This means that the phase modulator 7 then supplies a copy 12 of the transmitter signal, with the same phase and without amplitude modulation. This copy signal 12 is then supplied to an amplitude reconstruction device 26, which comprises, in this example, an amplitude detector 13, a low-pass filter 14, and an amplitude modulator 15. The signal 12 is supplied to the amplitude detector 13, which also receives the antenna signal. The amplitude detector continuously determines the momentary amplitude of the antenna signal and generates, with the aid of the copy signal 12, a signal which is equal to the amplitude of the antenna signal. The low-pass filter 14 receives this amplitude signal and forms the average value thereof. Thereby the modulation caused by the responder is removed. With the average value signal, with the aid of the amplitude modulator 15, to which also the copy signal provided by the phase modulator 7 is supplied, the copy signal is modulated. The thus obtained reconstructed signal now has exactly the same frequency, phase and amplitude as the original transmitter signal.

The reconstructed signal is supplied to the negative input of an adding device 16, while the antenna signal is supplied to the positive input of the adding device 16. Thus, the adding device provides an output signal which consists of the antenna signal minus the reconstructed original transmitter signal. This output signal therefore consists exclusively of the responder signal and any external interferences. The output signal of the adding device 16 is supplied to an AM receiver 17, which in turn supplies the responder code, at the output 18, to a device not shown in detail for further processing of the responder code. The AM receiver further detects any remainders of the transmitter signal and supplies these to an offset setting input 20 of the amplitude detector via an amplitude offset control device 19, and to an offset setting input 22 of the comparing circuit 11 via a phase offset control device 21. A great advantage of the use of the circuit described is that it allows different types of receivers to be used. Both classic analogous receivers and digital receivers with a digital signal processor are usable.

It is noted that the invention is very suitable for use with ISO transceivers. Such transceivers can act as an ASK (Amplitude Shift Keying) receiver and as an FSK (Frequency Shift Keying) receiver and can also recognize HDX (half duplex) responders. With HDX responders, the transmitter is switched on and off a number of times per second, for instance 50 ms on and 20 ms off. During the transmission (The "on" phase), the HDX responder is provided with energy and, during the "off' phase, the code of the HDX responder is read by the receiver. An ISO transceiver device according to the invention is transparent to HDX responders. The invention is by no means limited to the above-described embodiment. Thus, the reconstructed signal may also be supplied to the positive input of the adding device, while the antenna signal is supplied to the negative input of the adding device 16. The output signal of the adding device 16 then has an opposite sign compared to the output signal as discussed hereinabove, but this makes no difference because this signal is supplied to an AM receiver 17.

Claims

1. A transceiver device for responder signals provided by responders of a radio-frequency identification system, comprising a transmitter which generates a transmitter signal on the basis of a crystal oscillator and supplies it to at least one antenna for forming an interrogation field for responders, and a receiving device connected with the at least one antenna for receiving signals generated by a responder, characterized by means for reconstructing the original transmitter signal from the antenna signal and by an adding device with a negative input and a positive input, wherein the reconstructed signal and the antenna signal are supplied to the positive input and the negative input, respectively, or are supplied to the negative input and the positive input, respectively, wherein the output of the adding device is connected with an AM receiver, which provides a responder signal as an output signal.

2. A transceiver device according to claim 1, characterized in that the means for reconstructing the original transmitter signal comprise a limiter circuit, which operatively receives the antenna signal and removes all amplitude information therefrom for forming a block-shaped signal.

3. A transceiver device according to claim 2, characterized in that the means for reconstructing the original transmitter signal further comprise a phase compensation device, which compares the phase of the block-shaped signal with that of the signal of the crystal oscillator and removes phase variations.

4. A transceiver device according to claim 3, characterized in that the phase compensation device comprises a phase modulator, which is connected with a phase detector, which also receives the signal of the crystal oscillator and which provides an output signal respecting the phase difference between the signal of the phase modulator and the signal of the crystal oscillator to a comparing circuit, which further receives a reference signal and which provides an output signal, which controls the phase modulator such that the phase detector no longer supplies an output signal.

5. A transceiver device according to claim 4, characterized in that a high-pass filter is connected between the phase detector and the comparing circuit.

6. A transceiver device according to any one of claims 3-5, characterized in that the phase compensation device has an output which is connected with an amplitude reconstruction device 26 for reconstructing the amplitude of the original transmitter signal.

7. A transceiver device according to claim 6, characterized in that the amplitude reconstruction device comprises an amplitude detector, which receives the antenna signal and the output signal of the phase compensation device and generates an output signal which is equal to the amplitude of the antenna signal.

8. A transceiver device according to claim 7, characterized in that, via a low-pass filter, which removes the amplitude modulation caused by the responder signal, the output signal of the amplitude detector is supplied to an amplitude modulator, which also comprises the output signal of the phase compensation device, and which modulates this in amplitude with the signal provided by the low-pass filter for forming a reconstructed signal with the same frequency, phase and amplitude as the original transmitter signal.

9. A transceiver device according to any one of the preceding claims, characterized in that the AM receiver is arranged to detect any remainders of the transmitter signal in the output signal of the adding device and to supply, on the basis thereof, phase and/or amplitude offset signals via suitable control devices to the means for reconstructing the original transmitter signal.

10. A transceiver device according to claim 9 and any one of claims 4-8, characterized in that the AM receiver is connected, via a phase offset control device, with an offset setting input of the comparing device.

11. A transceiver device according to claim 9 and any one of claims 7 or 8, characterized in that the AM receiver is connected, via an amplitude offset control device, with an offset setting input of the amplitude modulator.

12. A transceiver device according to any one of the preceding claims, characterized in that the output of the adding device supplies a difference between the reconstructed transmitter signal and the antenna signal.

13. A method for detecting responder signals in a transceiver device of a radio-frequency identification system, which radio-frequency identification system comprises a transmitter, which generates a transmitter signal and supplies it to at least one antenna for forming an interrogation field for responders, and a receiving device connected with the at least one antenna for receiving signals generated by a responder, wherein, from the antenna signal operatively prevalent at the antenna, the original transmitter signal is reconstructed with the aid of a phase compensation device and an amplitude reconstruction device, and that, for obtaining the responder signal, the reconstructed signal is subtracted from the antenna signal or the antenna signal is subtracted from the reconstructed signal.

14. A method according to claim 13, characterized in that the transceiver device is a transceiver device according to the ISO standard, which can also recognize half duplex responders.