Demodulator circuit of RFID system

This application claims priority to and the benefit of Korean Patent Application Nos. 2005-118237, filed Dec. 6, 2005, and 2006-50836, filed Jun. 7, 2006, the disclosures of which are incorporated herein by reference in their entirety.

1. Field of the Invention

The present invention relates to a demodulator circuit used in a tag or transponder in a radio frequency identification (RFID) system, and more particularly, to a demodulator circuit capable of generating a clearly level-identified output signal upon receiving an input amplitude signal having several modulation depths.

2. Discussion of Related Art

In general, an RFID tag is used in wireless communication systems such as an RFID system, and operates by receiving an amplitude signal from a reader. A communication distance between an RFID tag and a reader is defined by power consumption of the RFID tag. Since such a communication distance is a very important factor in application fields as well as performance of a system, researches for improving the communication distance have been constantly in progress.

However, conventional RFID tags require extremely-limited available driving voltage, and amplitude signals input thereto have very low power levels. Thus, it is exceedingly difficult to embody a data recovery circuit capable of detecting a modulation signal at low power.

Methods for solving such a technical problem have been suggested in U.S. Patent Laid-Open Publication Nos. 20040056691A1 and 20050104573A1 in which a voltage multiplier for data recovery comprises two output capacitors having different capacitance connected to inputs of a voltage comparator, and the voltage comparator compares voltages of the two capacitors with each other whenever the level of a tag input signal is changed. In addition, a method for comparing an output voltage of a voltage multiplier for data recovery with a reference voltage has been suggested in U.S. Patent Laid-Open Publication No. 20020149482A1, and also a method for converting output voltage of a voltage multiplier for data recovery into current and recovering data from variation of the current has been suggested.

However, the above-mentioned methods have drawbacks in that the period of a signal to be recovered may be somewhat changed by an offset difference of a compared voltage due to use of a comparator, there are many current paths due to the same reason, and a large amount of power is consumed due to use of an inverter.

When a reader used in an RFID system outputs an amplitude signal, a tag converts the signal into a direct current (DC) signal and then uses the DC signal as driving power for an entire circuit in the tag while detecting amplitude variation of the signal and decoding command data. A small modulation depth of an input amplitude signal is favorable to obtain large driving power, but a large modulation depth is favorable to easily detect an envelope upon demodulation. Here, a modulation depth is defined by following Formula 1:

Modulation depth=(high−low)/(high+low)×100%.  Formula 1

Because of the above-described reason, most RFID systems comprise a demodulator circuit that utilizes 100% of the modulation depth in the relatively short duration of a low level by adopting an encoding method making the duration of a high level of an input amplitude signal longer than the duration of a low level. Here, a conventional RFID demodulator circuit compares a signal passing through an envelope detector using a comparator, and generates a detected signal that can be classified into a high level and a low level. However, when the reading range of an RFID system increases, it is hard to utilize 100% of a modulation depth, and only 10 to 30% of the modulation depth may be used.

When only a small modulation depth is used, the level of a data signal to be recovered by a tag does not have a large difference from the level of an amplitude signal for supplying driving power. The demodulator circuit of the tag should be able to recover small variation of the input signal into a logic signal of “0” or “1.” However, when the input signal is slightly changed, an amplifier should be additionally installed or the number of stages of a voltage multiplier should be increased so as to amplify the signal. Therefore, power consumption for data recovery increases, and desired impedance of a tag is not easily obtained or driving power for the tag needs to be reduced.

The present invention suggests a demodulator circuit capable of performing, at low power, recovery of data from a slightly varying input amplitude signal. It is an object of the present invention to provide a radio frequency identification (RFID) demodulator circuit for detecting output variation of a voltage multiplier for data recovery by capacitive coupling the output of the voltage multiplier to a node at which a P-channel metal oxide semiconductor (PMOS) transistor is connected to an N-channel metal oxide semiconductor (NMOS) transistor.

One aspect of the present invention provides a demodulator circuit of an RFID system, the demodulator circuit capacitively coupling a direct current (DC) output converted from an input amplitude signal in an envelope waveform corresponding to the input amplitude signal.

Preferably, the demodulator circuit comprises an NMOS transistor and a PMOS transistor biased by a constant gate voltage, wherein the DC output from a voltage multiplier or rectifier is capacitively coupled to a node at which drains of the P-channel metal oxide semiconductor (PMOS) and N-channel metal oxide semiconductor (NMOS) transistors biased by a constant gate voltage are connected to each other.

The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred exemplary embodiments thereof with reference to the attached drawings in which:

FIG. 1 is a block diagram of a wireless communication device recovering data of an input amplitude signal according to an exemplary embodiment of the present invention;

FIG. 2 is a circuit diagram of a voltage multiplier for data recovery according to an exemplary embodiment of the present invention; and

FIG. 3 is a circuit diagram of a demodulator circuit according to an exemplary embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below and can be implemented in various forms. Therefore, the present exemplary embodiments are provided for complete disclosure of the present invention and to fully inform the scope of the present invention to those of ordinary skill in the art.

FIG. 1 is a block diagram of a wireless communication device recovering data of an input amplitude signal according to an exemplary embodiment of the present invention.

Referring to FIG. 1 , the wireless communication device according to the present invention comprises voltage multipliers 102 and 103 for data recovery, a demodulator circuit 105 , and a trigger circuit 106 . Variation of an amplitude signal received at antennas 100 and 101 is first detected at the voltage multipliers 102 and 103 for data recovery. Here, an output signal 104 of the voltage multipliers 102 and 103 is an envelope of the signal applied to the antennas 100 and 101 . The demodulator circuit 105 converts the output of the voltage multiplier 102 into a logic signal to be transmitted to a decoder, and the trigger circuit 106 waveform-shapes the converted logic signal.

Meanwhile, the amplitude signal received at the antennas 100 and 101 may be transmitted to a voltage multiplier (not shown) for tag driving power generation in parallel with the voltage multipliers 102 and 103 for data recovery. The voltage multiplier for tag driving power generation includes more diodes and capacitors than the general voltage multipliers 102 and 103 for data recovery so as to use most of the received amplitude signal for driving voltage generation.

FIG. 2 is a circuit diagram of a voltage multiplier for data recovery according to an exemplary embodiment of the present invention.

Referring to FIG. 2 , the voltage multiplier for data recovery is configured of a circuit for converting a received amplitude signal into a direct current (DC) signal. The amplitude signal is input through a first capacitor C 1 from an input node 110 connected to an antenna. When the input signal is positive, a second diode D 2 is turned on and charges are accumulated in a second capacitor C 2 . When the input signal is negative, the second diode D 2 is turned off, a first diode D 1 is turned on, and charges are accumulated in a specific node 111 . In this manner, DC voltage corresponding to the amplitude signal is generated at an output node 112 and input to a demodulator circuit.

FIG. 3 is a circuit diagram of a demodulator circuit according to an exemplary embodiment of the present invention.

Referring to FIG. 3 , the demodulator circuit according to the present invention comprises seven transistors M 1 to M 7 each having a first electrode, a second electrode and a control electrode, one capacitor C, and two current sources I 1 and I 2 . In this embodiment, the first, second, fourth and sixth transistors M 1 , M 2 , M 4 and M 6 are P-channel metal oxide semiconductor (PMOS) transistors, and the third, fifth and seventh transistors M 3 , M 5 and M 7 are N-channel metal oxide semiconductor (NMOS) transistors. Here, the first and second electrodes of the transistors may respectively correspond to a drain and a source, and the control electrode may correspond to a gate.

One electrode of the capacitor C is connected to an input node 120 of the demodulator circuit, and the other electrode of the capacitor C is connected to the control electrode of the seventh transistor M 7 . The first current source I 1 is connected between the one electrode of the capacitor C and a ground terminal.

The first electrode of the first transistor M 1 is connected to a supply voltage terminal Vdd, the second electrode is connected to one terminal of the second current source I 2 , and the control electrode is diode-connected to the second electrode. The second current source I 2 is connected between the second electrode of the first transistor M 1 and the ground terminal. The first electrode of the second transistor M 2 is connected to the supply voltage terminal Vdd, the second electrode is connected to the first electrode of the third transistor M 3 , and the control electrode is connected with the control electrode of the first transistor M 1 , the control electrode of the fourth transistor M 4 and the control electrode of the sixth transistor M 6 . The second electrode of the third transistor M 3 is connected to ground, and the control electrode is diode-connected to the first electrode. The first electrode of the fourth transistor M 4 is connected to the supply voltage terminal Vdd, and the second electrode is connected to the first electrode of the fifth transistor M 5 , the other electrode of the capacitor C and the control electrode of the seventh transistor M 7 . The second electrode of the fifth transistor M 5 is connected to the ground terminal, and the control electrode is connected to the control electrode of the third transistor M 3 . The first electrode of the sixth transistor M 6 is connected to the supply voltage terminal Vdd, and the second electrode is connected to the first electrode of the seventh transistor M 7 . The second electrode of the seventh transistor M 7 is connected to the ground terminal. Here, the sixth and seventh transistors M 6 and M 7 function as first and second output transistors in an output stage, respectively.

Operation of the demodulator circuit will be described below.

DC current generated from the voltage multiplier flows to the ground terminal through the first current source I 1 . When the first current source I 1 for flowing DC voltage stored in the second capacitor C 2 of the voltage multiplier to the ground terminal is not connected, the demodulator circuit cannot make the same voltage waveform as the envelope of an input amplitude signal and makes a waveform of a voltage which decreases according to a time constant based on capacitor capacity. Therefore, the first current source I 1 is required to be connected to the input node 120 . In this case, the amount of the current flowing to the ground terminal via the first current source I 1 is very small, and thus hardly affects operation of a circuit for data recovery and a tag. The envelope waveform of the thus generated input amplitude signal is applied to an intermediate node 121 between the fourth and fifth transistors M 4 and M 5 through the capacitor C.

A node 122 at which the gate and drain of the first transistor M 1 are connected is connected to the second current source 12 , and provides the second, fourth and sixth PMOS transistors M 2 , M 4 and M 6 with a constant gate bias. Since the gates of the third and fifth NMOS transistors M 3 and M 5 are connected to each other and a node 123 between the gates is connected to the drain of the third transistor M 3 , the amount of current flowing through the second and third transistors M 2 and M 3 is the same as that flowing through the fourth and fifth transistors M 4 and M 5 . In other words, the second and fourth PMOS transistors M 2 and M 4 and the third and fifth NMOS transistors M 3 and M 5 form a current mirror, and operate to make the current I 3 flowing through the second and third transistors M 2 and M 3 identical to the current I 4 flowing through the fourth and fifth transistors M 4 and M 5 . Here, the amount of the current I 3 flowing through the second and third transistors M 2 and M 3 is uniform. Therefore, when the amount of the current I 3 is very small, the amount of the current I 4 also becomes very small.

The fourth and fifth transistors M 4 and M 5 are equivalent to two resistors having great resistance connected in serial between the supply voltage terminal Vdd and the ground terminal. The signal applied through the input node 120 passes through a high pass filter (HPF) consisting of the capacitor C and the equivalent resistors of the fourth and fifth transistors M 4 and M 5 , and then is applied to the intermediate node 121 . Since the equivalent resistor of the fourth and fifth transistors M 4 and M 5 has very great resistance, the cutoff frequency of the HPF is several tens of kHz. The copied current I 4 is very small, about 0.1 μA or less, and thus the voltage at the intermediate node 121 easily varies according to minute variation of the envelope waveform applied through the capacitor C. The varying voltage is applied to the gate of the seventh transistor M 7 , turns on/off the seventh transistor M 7 , and connects an output node 124 to the supply voltage terminal Vdd or the ground terminal, thereby converting the variation of the input amplitude signal into a logic signal, “0” or “1.” The output 124 is waveform-shaped by the trigger circuit 106 of FIG. 1 , and then transmitted to a decoder.

As described above, the demodulator circuit of an RFID system according to the present invention can recover data from an input amplitude signal having a small modulation depth, at low power.

According to the present invention, the output of a voltage multiplier is connected to a low current path by capacitive coupling so that data is recovered. Thus, data can be recovered from a signal having a small modulation depth in a tag of an RFID system, which makes more remote communication with a reader possible. In addition, operation at low power is possible and a chip size is reduced.

While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.