AU2006202886B2 - A Transmitter and a Method for Transmitting Data - Google Patents

Description

1 A TRANSMITTER AND A METHOD FOR TRANSMITTING DATA FIELD OF INVENTION The invention relates to a transmitter and a method for transmitting data. The invention has been developed primarily for the field of radio frequency 5 identification (RFID), and more particularly to a method for transmitting data to a transponder with a single antenna, and will be described hereinafter with reference to that application. This invention has particular merit when applied to passive transponders where high speed data transmission is desirable. BACKGROUND OF THE INVENTION 10 Hitherto, high speed data has been transmitted to RFID transponders by modulation of the excitation field. Generally pulse position modulation with 100% depth amplitude modulation of the excitation field is used. The excitation field is turned off for short intervals which are detected by the transponder's processing circuitry. To achieve high data rates while maintaining the transmission of power 15 the intervals must be short and the duty cycle low. Typically a duty cycle of 10% is used and the intervals are I ps long and the average time between internals is 10ps. Short intervals such as these have a wide bandwidth. Accordingly, both the interrogator and the transponder require low Q factor, wide bandwidth antennae to transmit and receive the data. Low Q factor antennae are not energy 20 efficient and, as such, the interrogator antenna will consume more power than a high Q factor antenna. Moreover, for passive transponders a stronger excitation field is required to compensate for the less efficient means. Additionally, regulations governing the magnitude of electromagnetic emissions place upper limits on the strength of excitation fields that can be used 25 and the allowable bandwidth of an excitation field. The wide bandwidth of the prior art pulse, modulation data results in limitations being placed on the maximum excitation field strength. Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be 30 taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.

2 DISCLOSURE OF THE INVENTION It is an object of the invention, at least in the preferred embodiment, to overcome or substantially ameliorate at least one of the disadvantages of the prior art. 5 According to an aspect of invention, there is provided a method of and/or device for transmitting data for radio frequency identification from a first antenna, the method including the steps of: providing a carrier signal adapted to power the radio frequency identification device; 10 imposing a phase modulation on the carrier signal to create a modulated signal; providing the modulated signal to the antenna; characterised in that the phase modulation on the carrier signal is in accordance with a phase deviation where the phase deviation is in accordance 15 with a data signal. According to another aspect of invention, there is provided a method of and/or device adapted to interrogate a radio frequency identification device, including: a first antenna; 20 oscillator means for providing a carrier signal adapted to power the radio frequency identification device; modulation means for imposing phase modulation on the carrier signal to create a modulated signal; and providing the modulated signal to the first antenna; 25 characterized in that the modulation means imposes phase modulation on the carrier signal in accordance with a phase deviation where the phase deviation is in accordance with a data signal. According to a further aspect of invention, there is provided a method of and/or device adapted to receiving a modulated signal and derive therefrom a 30 data signal, the device including: an antenna adapted to receive the modulated signal and, in response thereto, produce a first signal, 2A means from deriving from the first signal, a local oscillator signal used to demodulate the first signal and obtain an indicative data signal. According to a still further aspect of invention, there is provided a method of and/or device for demodulating a modulated signal received by a device and 5 deriving therefrom a data signal, the method including the steps of: receiving the modulated signal, producing a first signal being a local oscillator signal, demodulating the modulated signal using the local oscillator signal to obtain an indicative data signal. 10 According to a yet further aspect of invention, there is provided a method of and/or device for encoding a data signal for transmission by a device, the method including the steps of: providing an excitation reference signal, passing the excitation reference signal through a 90 degree splitter, 15 combining one output from the splitter with a data signal, and adding another output from the splitter to the combined data signal for transmission. Other aspects and preferred aspects are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the 20 invention. DESCRIPTION OF THE DRAWINGS Further disclosure, objects, advantages and aspects of the present application may be better understood by those skilled in the relevant art by reference to the following description of preferred embodiments taken in 3 conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which: Figure 1 is a schematic illustration of a prior art transponder circuit; Figure 2 illustrates representative waveforms associated with the prior art 5 circuit of Figure 1; Figures 3(a) to 3(c) are frequency spectra associated with the waveforms of the prior art circuit of Figure 1; Figures 4(a) and 4(b) are phasor diagrams for waveforms produced in accordance with the invention; 10 Figures 5(a) to 5(c) are frequency spectra associated with the invention; Figures 6(a) and 6(b) respectively, illustrate methods of encoding and decoding data in accordance with the invention; Figure 7 is a schematic illustration of a preferred circuit for encoding the data signal for transmission; and 15 Figure 8 is a schematic illustration of a preferred circuit for decoding the data signal in the transponder. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Passive RFID transponders that incorporate a single antenna are 20 interrogated by an interrogator using an excitation field. This field is received by the transponder's antenna and the voltage induced on the antenna is rectified and used to power the transponder. Often it is necessary for the transponder to receive data transmitted from its interrogator. For single antenna transponders the received message must be received by the same antenna that is used to 25 receive the excitation signal used to power the transponder. In prior arty systems the excitation signal is amplitude modulated to convey messages from the interrogator to the transponder. Figure 1 shows a prior art transponder by amplitude modulation. The capacitor C and data is transmitted to the transponder by amplitude modulation. 30 The voltage V1 induced in the transponder's antenna coil is magnified by the antenna's tuning, rectified by the rectifiers and stored on the DC storage capacitor Cdc for use by the transponder's electronic circuits. The antenna voltage is peak 4 level detected by the diode envelope detector D1, C1 and RI to give the envelope voltage V2. Figures 2(a) and 2(b) illustrate waveforms associated with the prior art circuit of Figure 1. More particularly, Figure 2(a) shows the excitation voltage V1 5 with amplitude intervals to giving pulse position modulation. To deliver the maximum power to the transponder, a low duty cycle is used, typically 10:1. Figure 2(b) shows the envelope of the voltage V2 induced in the antenna. The antenna's transient response results in a finite rise and fall time for V2. The transient time of the antenna must be sufficiently short to allow narrow pulses to 10 pass without significant distortion. The antenna's transient response time constant Ts and bandwidth BW are related by T 8 =1(BW.nr). Accordingly, to pass short pulses the bandwidth of the antenna must be broad. For example, to pass 1 ps pulses a bandwidth of at least 1 MHz is required. Figures 3(a) to 3(c) are frequency spectra associated with the prior art 15 circuit of Figure 1. Figure 3(a) shows a typical data spectrum. For data at 100 kbps the first zero of the frequency spectrum occurs at 100 kHz. Figure 3(b) shows the data spectrum when encoded as pulse position modulation PPM where narrow low duty cycle pulses are used. The spectrum for this type of encoding is much broader than the original data spectrum. For 1 ps pulses with a 20 10:1 duty cycle the first amplitude zero of the frequency spectrum occurs at 1 MHz. Figure 3(c) shows the spectrum of the excitation signal when modulated with the PPM signal whose spectrum is shown at Figure 3(b). The modulated spectrum is double sided and accordingly, for 1ps pulses with a 10:1 duty cycle the width of the main spectral lobe is 2 MHz. Clearly the bandwidth of the PPM 25 modulated excitation signal is much broader than the original data spectrum. To pass the inherently broad band PPM excitation signal both the interrogator and transponder antenna must have a wide bandwidth. Consequently the interrogator and transponder antennae must have a low Q and will operate with a low efficiency. In the interrogator the generation of 100% 30 amplitude modulated PPM requires that excitation signal be completely quenched for each pulse. This requires a wide band low efficiency antenna. Narrow band antennae would operate with high efficiency but are unable to respond to the narrow amplitude pulses of PPM. Similarly the transponder antenna bandwidth 5 must be broad band enough to pass the modulated excitation signal. Broad band antennae are inherently low Q and are poor collectors of energy from an excitation field. In this preferred embodiment of the invention data is imposed as a low 5 level signal having a modulated quadrature component. Most preferably this modulation is phase modulation although in other embodiments use is made of amplitude modulation. In the present embodiment the low level signal appears as a tiny phase jitter in the excitation field. There is no change in the amplitude of the excitation field and hence the transmission of power to the transponder is 10 unaffected. This form of modulation will be termed phase jitter modulation or, for convenience, PJM. There are many methods of producing small modulated phase shifts. For example, by passing the signal through a phase shifter such as an RC or tuned circuit, or through a variable length delay line. 15 In this embodiment, to produce the signal at the interrogator, a small portion of the excitation signal is phase shifted 90 degrees to give a quadrature signal. This is then PRK modulated with the data signal and added back onto the original excitation signal before being transmitted to the transponder. The resultant signal can be amplitude limited to remove any residual amplitude 20 component. At the transponder these tiny phase shifts in the excitation induce corresponding antenna voltage phase shifts that are unaltered by any circuit impedances or power regulation circuitry connected to the transponder's antenna. Figure 4(a) is a phasor diagram of the excitation signal Fc and the modulated quadrature signal PRK. The amplitude of the respective signals are 25 given by their phasor lengths. The phase deviation THETA caused by the modulated quadrature signal is, for low level signals, extremely small and is given by: THETA= arctan (2xMag(PRK)/Mag(Fc)) For a 40 dB attenuated PRK signal THETA = 1.2 degrees and for a 60 dB 30 attenuated PRK signal THETA = 0.12 degrees. Both of these are extremely small phase deviations of the excitation signal. Phase quadrature modulation is recovered using a local oscillator (LO) signal, with a fixed phase with respect to the excitation signal, to down convert 6 the modulated data to baseband in a mixer or multiplier. In the transponder the LO signal must be derived from the modulated excitation signal. The preferred method of extracting a LO signal from the modulated excitation signal uses a Phase Locked Loop PLL in the transponder to generate the LO signal. The LO 5 signal is generated by a low loop bandwidth PLL which locks to the original excitation signal's phase but is unable to track the high speed modulated phase shifts. The quadrature data signal is down converted and detected in a mixer or multiplier driven with the LO signal. Depending upon the type of phase detector used in the PLL, and the propagation delays through the circuit, the phase of the 10 LO with respect to the excitation signal can be anywhere between 0* and 360". If a conventional XOR phase detector is used in the PLL then the output of the PLL oscillator will be at nominally 90 degrees to the excitation signal and will be in phase with the data modulated phase quadrature signal. A 90* phase between the LO and the excitation signal is not necessary for the effective detection of 15 quadrature phase modulation. An XOR mixer has a linear phase to voltage conversion characteristic from 00 to 180* and 180* to 360*. Hence it gives the same output amplitude irrespective of the phase angle except around 0* and18 0 * where there is a gain sign change. The average output voltage DC level from a mixer is a function of the 20 average phase difference between its inputs. It is more convenient for circuit operation for the average output to be around midspan and hence an LO with a phase angle of around 90* is more convenient. The phase of the LO signal can be simply adjusted using fixed phase delay elements. Hence a 0* or 1800 phase detector can be used and a further 90* (roughly) of phase shift can be achieved 25 with a fixed delay element. Figure 4(b) is a phasor diagram of the modulated excitation signal and a quadrature local oscillator signal in the transponder used to demodulate the data signal. The local oscillator signals phase is at 90 degrees with respect to the excitation signal's phase. 30 For phase modulation the data bandwidth is no broader than the original double sided data bandwidth. When attenuated the level of the modulated data spectrum is extremely low with respect to the excitation signal amplitude making 7 conformance to regulatory emission limits significantly easier than with the prior art. Figures 5(a) to 5(c) are representative frequency spectra that explain the operation of the invention, More particularly, Figure 5(a) is a typical data 5 spectrum. For data at 100 kbps the first zero of the frequency spectrum occurs at 100 kHz. Figure 5(b) is a representative frequency spectrum of the data when modulated onto a quadrature version of the excitation signal. The spectrum for this type of modulation is the same as the double sided spectrum of the original data spectrum. In the invention the modulated quadrature signal is attenuated 10 and added to the original excitation signal. Figure 5(c) shows the spectrum of the excitation signal Fc plus the attenuated modulated quadrature signal whose spectrum is shown in Figure 5(b). The attenuation level is given by the difference between the amplitude of the excitation signal and the amplitude of the data sidebands. 15 Since the spectrum of the transmitted excitation signal is equal to the original double sided data spectrum, narrow band high Q interrogator and transponder antennae are used to respectively transmit and receive the modulated excitation signal. Consequently, the interrogator's excitation antenna operates with high efficiency and the transponder's antenna likewise receives 20 energy with high efficiency. In other embodiments use is made of low Q antennae. Figures 6(a) and 6(b) show methods of modulating and demodulating according to this invention. Turning first to Figure 6(a), the portion of the main excitation signal is phase shifted 90 degrees to produce a quadrature signal. The 25 quadrature signal is then modulated with data. The preferred form of modulation is phase reverse keying PRK. The PRK modulated quadrature signal is attenuated and then added back to the main excitation signal. Although shown in a particular order the sequence phase shift, modulation and attenuation are done in other orders in alternative embodiments. This method of modulation produces 30 low level data side bands on the excitation signal where the sidebands are in phase quadrature to the excitation signal. The data signal appears as a low amplitude phase jitter on the excitation signal. In some embodiment the signal is further amplitude limited to remove any residual amplitude component.

8 Figure 6(b) illustrates a method for demodulating the data modulated on to the excitation signal. A LO signal is generated by a low loop bandwidth phase lock loop PLL. The PLL locks on to the excitation signals phase and is unable to follow the high speed phase jitter caused by the data modulation. For the 5 standard PLL phase detector the PLL oscillator will lock at a fixed phase with respect to the excitation signal's phase. This oscillator signal is then used as a LO to demodulate the quadrature sideband data signal in the multiplier. A low pass filter LPF filters out high frequency mixer products and passes the demodulated data signal. 10 Figure 7 shows an example circuit for encoding the data signal for transmission. An excitation reference source Fc is split through a 90 degree splitter. One output from the splitter is fed to the LO port of a mixer. Data is fed to the mixer's IF port and causes PRK modulation of the LO port's signal. The output of the mixer at the RF port is a PRK modulated quadrature signal. This is 15 attenuated and added back onto the reference by a zero degree combiner ready for transmission to the transponder. Figure 8 shows an example circuit for decoding the data signal in the transponder. The transponder antenna voltage is squared up by a Schmitt trigger, the output of which feeds a type 3 PLL. A type 3 phase detector is a 20 positive edge triggered sequence phase detector which will drive the PLL oscillator to lock at 1800 with respect to the input phase. With a low loop bandwidth the PLL is able to easily filter off the sidebands on the input signal. The output of the Schmitt is passed through a chain of invertors designed to add a fixed delay to the input signal. The delay is approximately chosen so that the 25 phase of the output from the delay chain is not 00 or 1800 with respect to the LO. A preferred phase value is 90* for circuit convenience. The output of the VCO acts as the LO to demodulate the Phase Jitter Modulated data. The data is demodulated in an exclusive OR gate, the output of which is low pass filtered and detected with a floating comparator. 30 Although the invention has been described with reference to a specific example it will be appreciated by those skilled in the art that it may be embodied in many other forms.

9 While this invention has been described in connection with specific embodiments thereof, it will be understood that it- is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention 5 and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth. As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should 10 be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the 15 specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures. For example, although a nail and a screw may not be 20 structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface to secure wooden parts together, in the environment of fastening wooden parts, a nail and a screw are equivalent structures. "Comprises/comprising" when used in this specification is taken to specify 25 the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof." Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to 30 an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims (45)

1. A method of and/or device for transmitting data for radio frequency identification from a first antenna, the method including the steps of: 5 providing a carrier signal adapted to power the radio frequency identification device; imposing a phase modulation on the carrier signal to create a modulated signal providing the modulated signal to the antenna; 10 characterized in that the phase modulation on the carrier signal is in accordance with a phase deviation where the phase deviation is in accordance with a data signal.

2. A method as claimed in claim 1, wherein the modulated signal appears as a 15 phase jitter in an excitation field transmitted by the antenna.

3. A method as claimed in claim 1 or 2, wherein a quadrature of the carrier is modulated by the data. 20 4. A method as claimed in claim 1, 2 or 3, wherein the phase modulation is attenuated.

5. A method as claimed in any one of claims I to 4, wherein the signal provided to the first antenna is amplitude limited. 25

6. A method as claimed in any one of claims 1 to 5, wherein the phase deviation is provided by the equation: THETA = arctan (2xMag(PRK)/Mag(Fc)), where Fc is the carrier signal and PRK is the quadrature component. 30

7. A method as claimed in any one of claims 3 to 6, wherein the quadrature component is derived from a portion of the carrier signal, which is phase shifted 90 degrees to create a first signal. 11

8. A method as claimed in claim 7, wherein the first signal is PRK modulated with the data signal. 5 9. A method as claimed in any one of claims I to 8, wherein a phase shifter controlled by the data signal is used to provide the phase deviation.

10. A method as claimed in claim 9, wherein the phase shifter is a delay line. 10 11. A method as claimed in claim 9, wherein the phase shifter is a tuned circuit.

12. A method as claimed in claim 9, wherein the phase shifter is an RC circuit.

13. A method as claimed in any one of claims 1 to 12, wherein the phase 15 modulation is selected such that the sidebands are greater than 10 dB below the carrier amplitude.

14. A method as claimed in any one of claims 1 to 12, wherein the phase modulation is selected such that the sidebands are greater than 40 dB below the 20 carrier amplitude.

15. A method as claimed in any one of claims 1 to 12, wherein the modulated signal is created such that sidebands of the modulated signal are less than -15 dB below and preferably between -40 dB and -60 dB below the amplitude of the carrier 25 signal.

16. A method according to any one of claims 1 to 12, wherein the phase modulation is selected such that the sidebands are greater than 60 dB below the carrier amplitude. 30

17. A method as claimed in any one of claims 1 to 16, further including the step of receiving the modulated signal with a second antenna which, in response thereto, produces a second signal indicative of the data signal. 12

18. A method as claimed in any one of claims 7 to 17, wherein the first signal is used to power a receiver means. 5 19. A method of and/or device adapted to interrogate a radio frequency identification device, including: a first antenna; oscillator means for providing a carrier signal adapted to power the radio frequency identification device; 10 modulation means for imposing phase modulation on the carrier signal to create a modulated signal; and providing the modulated signal to the first antenna; characterized in that the modulation means imposes phase modulation on the carrier signal in accordance with a phase deviation where the phase deviation is in 15 accordance with a data signal.

20. A device as claimed in claim 19, wherein the modulated signal includes a sum of the carrier signal and an attenuated quadrature carrier signal which is modulated with the data signal. 20

21. A device as claimed in claim 19 or 20, wherein the antenna is a tunable coil.

22. A device as claimed in claim 19, 20 or 21, wherein the modulation means is a phase shifter. 25

23. A device as claimed in claim 22, wherein the phase shifter is a delay line.

24. A device as claimed in claim 22, wherein the phase shifter is a tuned circuit. 30 25. A device as claimed in claim 22, wherein the phase shifter is an RC circuit.

26. A device as claimed in any one of claims 19 to 25, wherein the modulated signal is created such that sidebands of the modulated signal are less than -15 dB 13 below and preferably between -40 dB and -60 dB below the amplitude of the carrier signal.

27. A radio frequency identification device (RFID) adapted for interrogation by the 5 device as claimed in any one of claims 19 to 26, the RFID including: a second antenna, and receiver means, adapted to derive a second signal indicative of a data signal from a first signal provided by the second antenna in response to receiving the modulated signal. 10

28. A RFID device as claimed in claim 27, wherein the first signal is used to power the receiver means.

29. A device as claimed in claim 27 or 28, wherein the receiver is a passive 15 device.

30. A device as claimed in claim 27, 28 or 29, wherein the receiver is a transponder. 20 31. An identification system including a device as claimed in any one of claims 15 to 30.

32. A system as claimed in claim 31 for identifying luggage. 25 33. A method of and/or device adapted to receiving a modulated signal and derive therefrom a data signal, the device including: an antenna adapted to receive the modulated signal and, in response thereto, produce a first signal, means from deriving from the first signal, a local oscillator signal used to 30 demodulate the first signal and obtain an indicative data signal.

34. A device as claimed in claim 33, wherein the deriving means includes a phase locked loop to derive the local oscillator signal. 14

35. A device as claimed in claim 34, wherein the phase locked loop is a low loop bandwidth phase locked loop. 5 36. A device as claimed in claim 33, wherein the deriving means includes a mixer in demodulating the first signal.

37. A device as claimed in claim 33, wherein the deriving means includes a multiplier in demodulating the first signal. 10

38. A device as claimed in any one of claims 33 to 37, wherein the first signal is used as a source of power for the device means.

39. A device according to any one of claims 33 to 38, wherein the antenna has a 15 high Q factor.

40. A device as claimed in any one of claims 33 to 39, wherein the device is a Radio Frequency Identification Device. 20 41. A device as claimed in any one of claims 33 to 40, wherein the device is a passive device.

42. A device as claimed in any one of claims 33 to 41, wherein the device is a transponder. 25

43. A method of and/or device for demodulating a modulated signal received by a device and deriving therefrom a data signal, the method including the steps of: receiving the modulated signal, producing a first signal being a local oscillator signal, 30 demodulating the modulated signal using the local oscillator signal to obtain an indicative data signal. 15

44. A method as claimed in claim 43, wherein a phase locked loop is used to derive the local oscillator signal.

45. A method as claimed in claim 44, wherein the phase locked loop is a low loop 5 bandwidth phase locked loop.

46. A method as claimed in claim 43, 44 or 45, wherein a mixer is used in demodulating the first signal. 10 47. - A method as claimed in claim 43, 44 or 45, wherein a multiplier is used in demodulating the first signal.

48. A method as claimed in any one of claims 43 to 47, wherein a low pass filter is used to filter out high frequency signal components and pass the demodulated data 15 signal.

49. A method as claimed in any one of claims 43 to 48, wherein the first signal is used as a source of power for the device. 20 50. A method of and/or device for encoding a data signal for transmission by a device, the method including the steps of: providing an excitation reference signal, passing the excitation reference signal through a 90 degree splitter, combining one output from the splitter with a data signal, and 25 adding another output from the splitter to the combined data signal for transmission.

51. A method as claimed in claim 50, wherein the data signal is PRK modulated. 30 52. A method as claimed in claim 50 or 51, wherein the combined data signal is PRK quadrature modulated. 16

53. A method as claimed in claim 50, 51 or 52, wherein the combined data signal is attenuated.

54. A device as claimed in any one of claims 19 to 30 or 33 to 42, wherein the 5 modulated signal has a phase jitter in an excitation field transmitted by the antenna.

55. A device as claimed in any one of claims 19 to 30 or 33 to 42, wherein the modulated signal has a quadrature component. 10 56. A device as claimed in any one of claims 19 to 30 or 33 to 42, wherein the phase deviation is provided by the equation: THETA = arctan (2xMag(PRK)/Mag(Fc)), where Fc is the carrier signal and PRK is the quadrature component. 15 57. A device as claimed in claim 55, wherein: the quadrature component is derived from a portion of the carrier signal; and the portion is phase shifted 90 degrees to create a first signal.

58. A method as claimed in claim 1, 43 or 50, substantially as herein described 20 with reference to the accompanying drawings.

59. A device as claimed in claim 19 or 33, substantially as herein described with reference to the accompanying drawings. 25 60. A system as claimed in claim 31 or 32, substantially as herein described with reference to the accompanying drawings.