Abstract:
A memory tag responsive to a signal generated by a reader, the tag comprising a resonant circuit part having a resonant frequency, the resonant frequency of the resonant circuit part being variable in accordance with data to be transmitted being variable to transmit data to the reader.

Description:
FIELD OF THE INVENTION 
   This invention relates to a memory tag, and a reader. 
   BACKGROUND OF THE INVENTION 
   Memory tags in the form of Radio Frequency Identification (RFID) tags are well known in the prior art, and the technology is well established (see for example: RFID Handbook, Klaus Finkenzeller, 1999, John Wiley &amp; Sons). RFID tags come in many forms but all comprise an integrated circuit with information stored on it and a coil which enables it to be interrogated by a read/write device generally referred to as a reader. Until recently RFID tags have been quite large, due to the frequency they operate at (13.56 MHz) and the size of coil they thus require, and have had very small storage capacities. Such RFID tags have tended to be used in quite simple applications, such as for file tracking within offices or in place of or in addition to bar codes for product identification and supply chain management. 
   Much smaller RFID tags have also been developed, operating at various frequencies. For example Hitachi-Maxell have developed “coil-on-chip” technology in which the coil required for the inductive link is on the chip rather than attached to it. This results in a memory tag in the form of a chip of 2.5 mm square, which operates at 13.56 MHz. In addition Hitachi has developed a memory tag referred to as a “mu-chip” which is a chip of 0.4 mm square and operates at 2.45 GHz. These smaller memory tags can be used in a variety of different applications. Some are even available for the tagging of pets by implantation. 
   Although it is known to provide tags with their own power source, in many applications the tag is also powered by the radio frequency signal generated by the reader. Such a known system is shown in  FIG. 1  where a reader is indicated generally at  10  and a tag at  12 . The reader  10  comprises a radio frequency generator  13  and a resonant circuit part  11 , in the present example comprising an inductor  14  and a capacitor  15  connected in parallel. The inductor  14  comprises a antenna. The resonant circuit part will have a particular resonant frequency in accordance with the capacitance and inductance of the capacitor  15  and the inductor  14 , and the frequency generator  13  is operated to generate a signal at that resonant frequency. 
   The tag  12  similarly comprises a resonant circuit part generally illustrated at  16 , a rectifying circuit part generally indicated at  17  and a memory  18 . The resonant circuit part  16  comprises an inductor  19  which again comprises in this example a loop antenna, and a capacitor  20 . The resonant circuit part  16  will thus have a resonant frequency set by the inductor  19  and capacitor  20 . The resonant frequency of the resonant circuit part  16  is selected to be the same as that of the reader  10 . The rectifying part comprises a forward-biased diode  21  and a capacitor  22  and thus effectively acts as a half-ware rectifier. 
   When the reader  10  is brought sufficiently close to the tag  12 , a signal generated by the frequency generator  13  will cause the resonant circuit part  11  to generate a high frequency electromagnetic field. When the resonant circuit part  16  is moved within this field, a current will be caused to flow in the resonant circuit part  16 , drawing power from the time varying magnetic field generated by the reader. The rectifying circuit part  17  will then serve to smooth the voltage across the resonant frequency part and provide a DC power supply to the tag&#39;s memory  18 . The rectifying circuit part  17  is sufficient to supply a sufficiently stable voltage to the memory  18  for the memory to operate. 
   To transmit data from the tag to the reader, the resonant circuit part is also provided with a switch  23 , here comprising a field effect transistor (FET). The FET is connected to the memory by a control line  24 . When the switch  23  is closed, it causes an increased current to flow in the tag resonant circuit part  16 . This increase in current flow in the tag results in an increased current flow in the reader&#39;s resonant circuit part  11  which can be detected as a change in voltage drop across the reader inductor  14 . Thus, by controlling the switch  23 , data stored in the memory  18  of the tag  12  can be transmitted to the reader  10 . 
   A problem in transmitting data from the tag in this manner arises because the memory  18  is also powered by energy drawn from the electromagnetic field of the reader  10 . Thus, when the switch  23  is closed, the power source supplying the rectifying circuit  17  is effectively shorted out. Although variations in the voltage at the memory  18  will be to some extent be smoothed by the capacitor  22 , there will nevertheless be undesirable voltage changes at the memory  18 , necessitating the addition of power control circuitry. 
   An aim of the invention is to provide a tag which reduces or overcomes this problem. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, we provide a memory tag responsive to a signal generated by a reader, the tag comprising a resonant circuit part having a resonant frequency, the resonant frequency of the resonant circuit part being variable in accordance with data to be transmitted to transmit data to the reader. 
   The resonant circuit part may comprise a variable capacitance element, the capacitance of the variable capacitance element being controllable to vary the resonant frequency of the resonant circuit part. 
   The resonant circuit part may comprise an inductor and a first capacitor, and wherein the variable capacitance element comprises a second capacitor connected in parallel with the first capacitor and in series with a switch operable to switch the second capacitor element out of the circuit. 
   The switch may comprise a field effect transistor. 
   The resonant circuit part may comprise an inductor, and the variable capacitance element may comprise a varactor diode connected in parallel with the inductor and wherein a control line is connected to the cathode of the varactor diode to vary the reverse bias voltage of the varactor diode. 
   The resonant circuit part may further comprise a first capacitor connected in parallel with the inductor. 
   The controllable capacitance element may be set to have a first capacitance corresponding to a binary “one” and a second capacitance corresponding to a binary “zero”. 
   A rectifying circuit part may be operable to rectify a signal received from the resonant circuit part to supply power to a memory. 
   The memory tag may comprise an integrated circuit. 
   The inductor may comprise an antenna. 
   According to a second aspect of the invention, we provide a reader for reading a tag, the reader comprising a frequency source to generate a driving signal and a resonant circuit part connected to the frequency source operable to provide inductive coupling to a tag, the reader being operable to receive information from a tag via the resonant circuit part, the reader comprising a demodulator operable to compare a reference signal generated by the frequency source and a reflected signal from the resonant circuit part and generate an output depending on the relative phase of the reference signal and the reflected signal. 
   The demodulator may comprise a multiplier operable to multiply the reference signal and the reflected signal and a low pass filter to pass a signal corresponding to the relative phase. 
   According to a third aspect of the invention, we provide a method of transmitting data from a memory tag to a reader, wherein the tag comprises a resonant circuit part having a resonant frequency, the method comprising the steps of varying the resonant frequency of the resonant circuit part to transmit data to the reader. 
   The resonant circuit part may comprise a variable capacitance element, and the step of varying the resonant frequency of the resonant circuit part may comprise the step of varying the capacitance of the variable capacitance element. 
   According to a fourth aspect of the invention, we provide a method of reading data from a memory tag, the method comprising the step of supplying a driving signal to a resonant circuit part of a reader, comparing a reference signal corresponding to the driving signal and a reflected signal reflected from the resonant circuit part, and detecting the relative phase of the reference signal and the reflected signal. 
   The step of comparing the reflected signal and the reference signal may comprise multiplying the reflected signal and the reference signal, and passing the resulting signal through a low pass filter, wherein the output of the load pass filter is dependent on the relative phase. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein; 
       FIG. 1  is a schematic circuit diagram of a tag and reader of known type, 
       FIG. 2  is a diagrammatic illustration of a circuit for a tag and reader embodying the present invention, 
       FIG. 3   a  is a diagrammatic circuit diagram of a further tag embodying the present invention, 
       FIG. 3   b  is a diagrammatic illustration of a yet further tag embodying the present invention, 
       FIG. 4   a  is a graph showing variation in the amplitude of a reflected signal detected by the reader, 
       FIG. 4   b  is a graph showing variation in the phase of a reflected signal detected by the reader, and 
       FIG. 5  is a graph showing data transmitted by the tag of  FIG. 2  and an output voltage requested by a rectifying circuit of the tag of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 2 , a tag embodying the present invention is shown at  30  and a reader shown at  31 . The tag  30  comprises a resonant circuit part  32  and a rectifying circuit part  33 , together with a memory  34 . The resonant circuit part  32  comprises an inductor L 2  shown at  35  and a capacitor L 2  shown at  36  connected in parallel in like manner to the tag  12  of  FIG. 1 . The resonant circuit part  32  further comprises a controllable capacitive element generally indicated at  37 , in the example of  FIG. 2  comprising a capacitor C 3  shown at  38  and a switch S 1  shown at  39 . The rectifying circuit part  33  comprises a diode D 1  shown at  40  connected to the resonant circuit part  32  in a forward biased direction and a capacitor C 4  shown at  41  connected in parallel with the components of the resonant circuit part  32 . The rectifying circuit part  33  operates in like manner to the rectifying circuit part  17  of  FIG. 1  as a half-wave rectifier to provide power to the memory  34 . 
   The reader  31  comprises a resonant circuit part  42  which comprises an inductor shown at  43  and a capacitor C 1  shown at  44  connected in parallel. A frequency generator  45  is connected to the resonant circuit part  42  to provide a driving signal. 
   The reader  31  further comprises a demodulator, generally shown at  46 . The demodulator  46  comprises a power splitter  47  connected between the frequency generator  45  and the resonant circuit part  42  to split off a part of the driving signal to provide a reference signal. A coupler  48  is provided to split off a reflected signal reflected back from the resonant circuit part  42 , and pass the reflected signal to a multiplier indicated at  49 . The multiplier  49  multiplies the reflected signal received from the coupler  48  and the reference signal received from the splitter  47  and passes the output to a low pass filter  50 . The low pass filter  50  passes the signal corresponding the phase difference between the reference signal and the reflected signal to an output  51 . 
   The inductor L 1   43  comprises an antenna of the reader  31 , and the inductor  35  comprises an antenna of the tag  30 . 
   The reference signal from the splitter  47  will be of the form
 
 S ( t )= A  cos(ω t )
 
and the reflected signal R(t) tag will be of the form
 
 R ( t )= a  cos(ω t +φ( t ))
 
   where
         A=amplitude of the reference signal,   a=amplitude of the reflected signal   φ(t)=the relative phase and   ω=the frequency of the signal generated by the frequency source  45 .       

   R(t) is multiplied by the carrier reference signal S(t) at the multiplier  49 , producing a resulting signal 
                 a   ⁢           ⁢   A     2     ⁢     cos   ⁡     (       2   ⁢           ⁢   ω   ⁢           ⁢   t     +     φ   ⁡     (   t   )         )         +         a   ⁢           ⁢   A     2     ⁢     cos   ⁡     (     φ   ⁡     (   t   )       )               
The first of these terms, the second harmonic, is simply filtered by the low pass filter  50  leaving the second term that comprises the phase difference between the reference and reflected signals. It is a known effect of resonant circuits that when the circuit passes a signal which has a frequency less than the resonant frequency of the resonant circuit, a phase lag is introduced to the passed signal frequency, whilst when the frequency is greater than that of the resonant circuit, a phase lead is induced. Thus, by modulating the frequency of the reflected signal by changing the resonant frequency of the resonant circuit part of the tag  32 , the reflected signal will have a phase difference relative to the reference signal from the frequency source  45  which may easily be measured by the demodulator as discussed above.
 
   The effects of opening or closing the switch S 1  are shown in the graphs of  FIGS. 4   a  and  4   b . The graph of  FIG. 4   a  is a plot of the amplitude of the signal reflected back from the tag as measured at the connection between the coupler  48  and the multiplier  49 . The minimum of each plot represents the maximum power transfer, when the resonant frequency of the resonant current part  32  matches the frequency of the signal from the frequency source  45 . The change in the resonant frequency when S 1  is closed and when S 1  is open is apparent from the two plots on the graphs. It will be apparent by selecting the resonant frequencies of the resonant circuit  32  when S 1  is closed and S 1  is open to lie either side of the resonant frequency of the resonant circuit part  42 , F ref , that the curves for S 1  closed and S 1  open intersect as near to F ref  as possible and so there is no or minimal change in the transferred power when S 1  is closed and when S 1  is open. There is of course some loss in coupling efficiency, in that because the resonant circuit part  32  is slightly de-tuned from the resonant frequency of the resonant circuit  42 , maximum power transfer will not occur, however, the relative loss in the power transfer is balanced against the constant of power transfer achieved. From the phase plot of  FIG. 4   b , the relatively small change in resonant frequency leads to a relatively large change in phase with reference to the reference frequency. 
   Where, for example, where S 1  is closed, to represent digital “one”, and where S 1  is open, to represent a digital “0”, the relative phase of the reflected signal will vary between one of two values and the output of the demodulator  46  will be a train of pulses as shown in the lower plot of  FIG. 5   c . Meanwhile, because the transferred power is generally the same when S 1  is opened and closed as shown in  FIG. 3   a , the DC supply generated in the tag will be generally constant and stable, as shown in the top plot of  FIG. 5   c.    
   Two possible implementations of the variable capacitance device  37  are shown in  FIGS. 3   a  and  3   b . In  FIG. 3   a , the switch  37  is provided by an transistor  37 ′, in this case a field effect transistor (FET) controlled by a control line  34   a  to the memory  34  and connected to the gate of the FET. In the alternative of  FIG. 4   b , the switch  37  is provided by a varactor diode  37 ″ connected with its cathode connected to the memory  34  via control line  34   a . The control line  34   a ′ is provided with a resistor R 1   34   b . Since the varactor diode  37 ″ is only required to be reverse-biased, the resistor R 1   34   b  can be of a relatively high resistance. This high resistance then prevents any RF energy at the varactor cathode entering the memory  34 . A characteristic of a varactor diode is that the capacitance falls with increasing reverse bias. Thus when a relatively high voltage is supplied to the cathode of the varactor diode  37 ″ via the control line  34   a ′, its capacitance will be relatively low and thus the resonant frequency of the resonant circuit part will be relatively high. When no voltage is applied to the cathode of the varactor diode  37 ″, its capacitance will be relatively high and the resonant frequency of the resonant circuit part will be relatively low. Of course, it will be apparent that the varactor diode  37 ″ may be used to modulate the resonant frequency of the resonant circuit of  32  and thus the detected phase between multiple levels, or even in an analogue fashion as required. 
   In a preferred embodiment, the resonant frequency of the resonant circuit part  42 , and hence the frequency of the signal generated by the frequency source  45  is about 2.45 GHz, and the resonant frequency of the resonant circuit part  32  is modulated by about 0.05 GHz either side of this reference frequency. At this frequency, component values for the inductors and the capacitors are small, allowing easy integration of the circuit and require relatively small areas of silicon on an integrated circuit. It is particularly desirable that the tag  30  be provided as a integrated circuit, for example as a CMOS integrated circuit. 
   Although the embodiments described herein use a variable capacitance element to vary the resonant frequency of the tag resonant circuit part, it will be apparent that the resonant frequency may be varied by other means as desired. For example, a variable inductive element may be provided, or a second inductor may be switched in and out of the resonant circuit part. 
   In the present specification “comprises” means “includes or consists of” and “comprising” means “including or consisting of”. 
   The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.