Abstract:
A method and apparatus to generate an OOK-type of signal for transmitting digital data without having to use conventional mixer and oscillator circuitry as the carrier source is disclosed. The method utilizes a circuit that has a transfer characteristic comprising alternating unstable and stable operating regions, which produce respectively non-oscillatory and oscillatory output. The circuit is further characterized by having an operating point that can drive the circuit into stable or unstable operation based on the digital data. The resulting output signal is an OOK-type of signal suitable for transmission.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/340,130, filed Dec. 13, 2001, entitled “METHOD AND APPARATUS TO GENERATE AMPLITUDE SHIFT KEYING SIGNAL.” 
         [0002]    This application is related to commonly owned U.S. Pat. No. 6,259,390. This application is further related to U.S. application Ser. No. 09/805,854, filed Mar. 13, 2001, entitled “Method and Apparatus to Recover Data From Pulses” which is hereby incorporated by reference for all purposes. 
     
    
     
       STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0003]    NOT APPLICABLE  
         REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.  
         [0004]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0005]    This invention relates generally to signal modulation and more specifically to generation of signals using an ON-OFF keying modulation technique.  
           [0006]    In a communication system, an analog or digital information can be modulated on a carrier signal before it is transmitted over a physical channel. OOK (on-off keying) is a common modulation method used in a digital communication systems. While it is generally accepted that it is not an ideal modulation method, it is a very simple method to implement and thus has application in appropriate situations.  
           [0007]    [0007]FIG. 9 shows a conventional technique of producing an OOK modulated signal. A mixer  906 , typically a nonlinear three-port device, is used as an OOK modulator. The mixer comprises three ports  901 ,  902  and  903 . A data signal  905  is provided to input port  901 . A carrier signal is generated by an oscillator  904  and is provided to input port  902  of the mixer. A resulting OOK modulated signal is produced at output port  903 .  
           [0008]    The current state of the art improves the cost performance of the OOK modulator  906 . For example, U.S. Pat. No. 6,087,904 discloses an OOK modulator that can be implemented on a chip. The transformers, which are usually required in the mixer, are removed thus reducing device cost. A further OOK modulator improvement is disclosed in U.S. Pat. No. 6,292,067. In this patent, the OOK modulator only needs a positive-voltage power source which reduces implementation costs further. However, the method to generate OOK modulated waveform is essentially the same.  
           [0009]    Although there has been significant progress in making the OOK modulator cheaper and smaller, the conventional method always requires a power consuming subsystem, such as oscillator  904  to provide the OOK modulator with the carrier. Furthermore, the oscillatory component can be an interference source in a transceiver because the power it generates is relatively high. For example, in a transceiver, the carrier signal generated by an oscillator used in the transmitter to up-convert data signal could interfere with the received signal that is much weaker and therefore reduce the sensitivity of the receiver.  
           [0010]    There remains room for improvement of OOK modulation-based communication systems.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    Transmission of digital data in accordance with embodiments of the invention include applying an input signal based on a digital data stream to a non-linear circuit configured to produce oscillatory signals and non-oscillatory signals based on the input signal. The non-linear circuit produces oscillatory signals when the input signal is at a first signal amplitude and produces a non-oscillatory signal when the input signal is at second signal amplitude. The resulting oscillatory and non-oscillatory signals are suitable for transmission.  
           [0012]    In one embodiment of the invention, the input signal is the digital data stream itself. In another embodiment of the invention, the input signal is a pulse code modulated signal representative of the digital data stream.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings:  
         [0014]    [0014]FIG. 1 is a high level architectural diagram illustrating an OOK modulator according to the present invention;  
         [0015]    [0015]FIG. 2 shows a non-linear circuit as used in an OOK modulator according to the present invention;  
         [0016]    [0016]FIG. 3 shows alternating stable and unstable operating regions in a transfer function of the non-linear circuit shown in FIG. 2;  
         [0017]    [0017]FIG. 4 shows an illustrative embodiment of a non-linear circuit according to the present invention;  
         [0018]    [0018]FIG. 5 is a signal trace of signals produced in accordance with the present invention;  
         [0019]    [0019]FIG. 6 shows a typical communication system component adapted in accordance with the present invention;  
         [0020]    [0020]FIG. 7 shows another typical communication system component adapted in accordance with the present invention;  
         [0021]    [0021]FIG. 8 is a block diagram of a radio frequency identification (RFID) technique adapted in accordance with the present invention; and  
         [0022]    [0022]FIG. 9 shows a prior art signal generating system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    [0023]FIG. 1 is a generalized diagram illustrating an OOK modulator  102  according to the present invention. Digital data feeds into the modulator to produce an OOK-like signal representative of the digital data. An output of the modulator is a modulated signal suitable for transmission, and in an illustrated embodiment serves as the transmission signal itself.  
         [0024]    [0024]FIG. 2 is a high level function diagram of a circuit  201  comprising the OOK modulator  102  in accordance with an illustrative embodiment of the invention. The embodiment shows a circuit element  201  which has an N-shaped I-V transfer characteristic. A data signal  206  can be provided to an input port  204  of the circuit. An inductor  202  is coupled at an output  205  of the circuit.  
         [0025]    As will be explained shortly, the circuit  201  produces a modulated signal representative of the data signal  206 , which can be obtained from output port  205 . If needed, a capacitor (not shown) can be connected across the inductor  202  to remove sharp edges or other high frequency components of the modulated signal. Similarly, a filter (not shown) can be connected to port  205  to remove the high frequency components of the waveforms generated. The data signal  206  includes two information regions  206   a ,  206   b  encoded in the amplitudes of the data signal.  
         [0026]    [0026]FIG. 4 shows an example of an implementation of the circuit  201 . In this particular example, a tunnel diode  401  (e.g., part number is MP1605) serves as the circuit element  201 . The value of the inductor  402  is about 10 μH. As noted above, a capacitor  403  can be added across the inductor  402  to smooth out the waveform generated. The capacitor value for this example is about 10 pF. The data signal  206  is applied to data input port  404 . An OOK-type of modulated signal suitable for transmission can be tapped out from signal output port  405 .  
         [0027]    [0027]FIG. 3 shows a transfer function  301 , I=Ψ(V), of the circuit  201  as implemented in FIG. 4. For the purposes of the present invention, the “transfer function” (characteristic) of a circuit refers to the relationship between any two state variables of a circuit. Electronic circuits are typically characterized by their I-V curves, relating the two state variables of current and voltage. Such curves indicate how one state variable (e.g., current) changes as the other state variable (voltage) varies. As can be seen in FIG. 3, the transfer function for the circuit of FIG. 4 includes a portion which lies within a region  307 , referred to herein as an “unstable” region. The unstable region is bounded on either side by regions  306  and  308 , each of which is herein referred to as a “stable” region.  
         [0028]    The circuit of FIG. 4 has an associated “operating point”  303  which is a location on the transfer function  301 . The nature of the output  405  of the circuit depends on the location of its operating point. If the operating point is positioned along the portion of the transfer function that lies within region  307 , the output of the circuit will exhibit an oscillatory behavior. It is for this reason that the region  307  is referred to as an unstable operating region. If the operating point is positioned along the portions of the transfer function that lie within either of regions  306  and  308 , the output of the circuit will exhibit a generally time-varying but otherwise non-oscillatory behavior. It is for this reason that regions  306  and  308  are referred to as stable operating regions.  
         [0029]    The operating point  303  of the circuit is a function of the signal supplied to the input  404  of the circuit. FIG. 3 furthers shows such a control signal  305 , having a first region  305   a  and a second region  305   b . A line  302  is drawn to illustrate the relation of the amplitude of the control signal  305  V s  to the transfer function  301 . The intersection of line  302  and transfer function  301  sets the operating point  303  of the circuit  201 . Thus, as the control signal amplitude varies between amplitudes  305   a  and  305   b , it can be seen that the operating point of the circuit of FIG. 4 moves between its stable and unstable operating regions, with corresponding changes in the behavior of the circuit output  405 . Additional discussion of this and other circuits is provided in U.S. Pat. No. 6,259,390.  
         [0030]    Thus, if the control signal  305  is replaced with the data signal  206 , the operating point  303  of the circuit  201  will vary according to amplitude of the first information region  206   a  and the second information region  206   b . An OOK-type of modulated signal then can be produced when the circuit is driven into the stable and unstable operating regions to produce non-oscillatory output and oscillatory output according to the data signal  206 . The output of the circuit is an OOK signal if the pulse duty cycle of the data signal is 50%.  
         [0031]    [0031]FIG. 5 are signal traces of an input data signal  500   a  and an output OOK modulated signal  500   b . The input signal contains information region  502  (e.g., binary 0) and information region  503  (e.g., binary 1). Information region  502  places the operating point of the circuit  401  in the stable region ( 306  or  308 , FIG. 3), while information region  503  places the operating point of the circuit  401  in the unstable region  307 . When the operating point is in the stable region, a silent period  504  is observed at the output  500   b . When the operating point is the unstable region, oscillations  505  are observed at the output. If the capacitor  403  is not present, the oscillation frequency is primarily determined by the value of inductor  402 . However, if the capacitor  403  is present, the oscillation frequency is related by the expression f osc =(2π) −1 (LC) −1/2 . L and C correspond to values of inductor  402  and capacitor  403  respectively. Thus, the oscillation frequency can be tuned as needed to be suitable for use as a transmitted signal.  
         [0032]    [0032]FIG. 6 is a high level block diagram of an ultra wideband (UWB) transmitter system adapted in accordance with the modulation technique of the present invention. A digital source  606  provides a serial digital data stream  601  that constitutes digital information to be transmitted. The digital data is encoded by a pulse coded modulator  606   a . For example, the pulse code modulator might use a pulse position modulation (PPM) technique. Another pulse coding technique is pulse amplitude modulation (PAM). Still another commonly used pulse code modulation technique that can be used is pulse width modulation (PWM). In addition, the pulse coded modulator  606   a  may have spread spectrum capability such as Direct Sequence Spread Spectrum (DSSS).  
         [0033]    The system  600  includes an OOK modulator  602  according to various embodiments of the present invention. The pulse encoded output  603  of the pulse coded modulator  606   a  is delivered to the OOK modulator  602 . Typically, the pulse encoded output  603  will contain first and second information regions. The OOK modulator  602  produces an OOK-type of signal  605  in response to receiving the pulse encoded signal having portions which correspond to the first and second information regions of the pulse encoded output. The OOK-type of signal can then be transmitted to the air channel through an antenna  604  using conventional and known transmission techniques. As can be appreciated from the discussion above, the OOK-type of signal can generated without the use of a combined free running oscillator subsystem and mixer subsystem.  
         [0034]    The transmitter embodiment illustrated in FIG. 6 can be used in conjunction with a UWB receiver such as disclosed in commonly owned, co-pending U.S. application Ser. No. 09/847,777 or as disclosed in U.S. application Ser. No. 09/970,385 to form a transceiver pair. To be compatible with the UWB transmitter shown in FIG. 6, an envelope detector should be used as the wave-shaper circuit shown in FIG. 1 of U.S. application Ser. No. 09/847,777.  
         [0035]    [0035]FIG. 7 shows a block diagram of an amplitude shift keying (ASK) transceiver adapted in accordance with the present invention. At the transmitter side, a digital source  706  produces the digital serial data  701  which constitutes the digital information to be transmitted. The digital serial data is fed to an input to the OOK modulator  702 . As in FIG. 6, a modulated signal  705  is produced at the output of the OOK modulator. The modulated signal is transmitted through the antenna  704   a  to the air channel to the receiver side. Optionally, an amplifier (not shown) may be inserted in between the OOK modulator  702  and the antenna  704   a  to amplify the modulated signal before transmission.  
         [0036]    At the receiver side, the modulated signal  705  from the air, combined with noise and other interference signals, are received through the antenna  704   b . The received signal may be amplified through an optional amplifier not shown) before it is inputted into an envelope detector  722 . The envelope detector  722  will remove the carrier from the modulated signal  705  to produce an analog waveform  715 . The analog signal, because of the noise and other interference effects of the transmission medium, resembles the original digital serial data  701 , but with distortions.  
         [0037]    The waveform  715  is then fed to a pulse generator  724  that has N-Shape I-V transfer characteristics. For example, such a circuit is described in U.S. Pat. No. 6,259,390. The output of the pulse generator  724  is a signal comprising groups of spikes  713  that are correlated with the analog waveform  715 . These groups of spikes can be decoded by a counter or other decision device  726  to regenerate the digital information  711 . Digital information  711  is identical to the digital serial data  701  when perfect transmission is successful. Examples of the algorithm used in the decision device  726  are more fully disclosed in U.S. application Ser. No. 09/805,854.  
         [0038]    Alternatively, the digital information  711  can be recovered by performing hard decision analysis on analog waveform  715 ; for example by using a comparator. While this approach obviates the pulse generator  724  and decision device  726 , it might not be suitable for all applications for reasons such as system performance, system robustness, and so on.  
         [0039]    [0039]FIG. 8 shows a high level block diagram of another transmission system adapted in accordance with the present invention. A Radio Frequency Identification (RFID) system is shown. RFIDs employ the use of passive tags which are electronic devices tags that do not need a battery or like power source to operate. Instead, an RFID tag derives its power from a received signal transmitted to the tag.  
         [0040]    In this system, a reader  8010  (also referred to as an interrogator) transmits an interrogator signal through antenna  8020  at frequency F T  to identify tags ID that are within its range. Each tag, Tag  1  to Tag N, will receive the interrogator signal and process it in the following manner. The tag will receive this signal through its antenna  8030 . This signal will be converted to produce DC power by the rectenna (rectifying antenna) circuit  8040 . The DC power can provide power to the microcontroller  8050  and to the OOK modulator. The microcontroller  8050  generates a digital bit stream containing first and second information regions. These first and second information regions are inputted into the OOK modulator  8060  to generate an OOK modulated signal. The OOK modulator  8060  for each tag may oscillate at a different frequency. This can be achieved, for example, by setting different values for inductor  402  (see FIG. 4) in each tag. In such a the case, the OOK modulated signal for each tag has different center frequency. Tag  1  will use center frequency F, (for example), and Tag N will use center frequency F N . The advantage of using a different frequency for each tag is that there will not be information collision in the air.  
         [0041]    The modulated signal transmitted from the OOK modulator  8060  can be transmitted through antenna  8070 . In an embodiment of the invention, the antennae  8070  and  8030  can be combined into a single dual band antenna. The reader  8010  will be able to receive the signals transmitted from the tags via its antenna  8020 . The antenna  8020  is appropriately configured to receive signals F 1  to F N . The reader  8010  can post-process the signal F 1 +. . . +F N  to identify which tag is present and what information is contained in each tag. Thus, for example, if F 3  is identified, then Tag  3  must be present and the information carried in the center frequency F 3  corresponds to the information contained in Tag  3 .  
         [0042]    A simpler version of an RFID system can be developed by removing the microcontroller  8050  in each tag. In this version, the rectenna  8040  is connected to the OOK modulator  8060 . This alternate connection is illustrated in the figure by the dashed line  8050 ′. When the signal F T  is received, the rectenna  8040  converts the signal to produce DC and thus energize the OOK modulator  8060 . The operating point  303  (FIG. 3) of the OOK modulator  8060  in this configuration is fixed to lie in the unstable region  307 . The modulator will then simply oscillate at its oscillation frequency and transmit through its antenna  8070 . The reader will be able to identify which tag is present by identifying the oscillation frequencies present in the air. This variation of RFID tags might suitable in an application where only simple identification is needed.