Abstract:
A subsampling motion detector configured to detect motion information of an object under measurement receives a first wireless radio frequency (RF) signal and transmits a second wireless RF signal, the first wireless RF signal being generated by reflecting the second wireless RF signal from the object. The subsampling motion detector includes a controllable oscillator outputting an oscillation signal, wherein the first wireless RF signal is injected to the controllable oscillator for controlling the controllable oscillator through injecting locking. The subsampling motion detector further including a subsampling phase detector (SSPD) generating a control signal according to the oscillation signal generated by the controllable oscillator and a reference frequency, the SSPD outputting the control signal to the controllable oscillator for controlling the controllable oscillator, the oscillation signal of the controllable oscillator being locked to a multiple of the reference frequency and the control signal representing the motion information of the object.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a motion detector, and more particularly, to a subsampling motion detector for detecting motion of an object under measurement. 
       BACKGROUND 
       [0002]    In general, a motion detector detecting the status of displacement of an object by the Doppler Effect requires a high frequency signal output from an oscillation source operated at a high frequency to update the status of displacement of the object. Many of these motion detectors use a phase lock loop (PLL) to stabilize the high frequency oscillation signal. However, a PLL typically requires one of two implementations. In one method, a frequency divider is used for dividing the high frequency oscillation signal before the divided signal is then compared with a low frequency reference signal by a phase detector. Unfortunately, the frequency divider could consume a great deal of power during operation of the PLL. In an alternate method, a high frequency reference signal is provided for comparison with the high frequency oscillation signal by a phase detector. In this case, although the high frequency oscillation signal is not divided by a frequency divider, the phase detector has to perform phase detection at a fast rate, which could lead to increased power consumption. Therefore, a solution with lower power consumption is required in this field. 
       SUMMARY 
       [0003]    According to an exemplary embodiment of the claimed invention, a subsampling motion detector configured to detect motion information of an object under measurement is disclosed. The subsampling motion detector receives a first wireless radio frequency (RF) signal and transmits a second wireless RF signal, the first wireless RF signal being generated by reflecting the second wireless RF signal from the object. The subsampling motion detector includes a controllable oscillator outputting an oscillation signal, wherein the first wireless RF signal is injected to the controllable oscillator for controlling the controllable oscillator through injecting locking. The subsampling motion detector further including a subsampling phase detector (SSPD) generating a control signal according to the oscillation signal generated by the controllable oscillator and a reference frequency, the SSPD outputting the control signal to the controllable oscillator for controlling the controllable oscillator, the oscillation signal of the controllable oscillator being locked to a multiple or fractional multiple of the reference frequency and the control signal representing the motion information of the object, wherein the value of a multiple or the fractional multiplication factor is greater than 1. 
         [0004]    According to another exemplary embodiment of the claimed invention, a subsampling motion detector configured to detect motion information of an object under measurement is disclosed. The subsampling motion detector receives a first wireless radio frequency (RF) signal and transmits a second wireless RF signal, the first wireless RF signal being generated by reflecting the second wireless RF signal from the object. The subsampling motion detector includes a high frequency oscillator outputting a high frequency oscillation signal, wherein the first wireless RF signal is injected to the high frequency oscillator for controlling the high frequency oscillator through injecting locking, a low frequency controllable oscillator generating a low frequency oscillation signal according to a control signal, and a subsampling phase detector (SSPD) receiving the high frequency oscillation signal and the low frequency oscillation signal and detecting a phase difference between the high frequency oscillation signal and the low frequency oscillation signal at time periods indicated by the low frequency oscillation signal, the SSPD outputting a phase detection output signal according to the detected phase difference, the control signal being generated according to the phase detection output signal output from the SSPD, and the motion information of the object being calculated according to the phase detection output signal. 
         [0005]    According to yet another exemplary embodiment of the claimed invention, a subsampling motion detector configured to detect motion information of an object under measurement is disclosed. The subsampling motion detector receives a first wireless radio frequency (RF) signal and transmits a second wireless RF signal, the first wireless RF signal being generated by reflecting the second wireless RF signal from the object. The subsampling motion detector includes a high frequency oscillator outputting a high frequency oscillation signal, wherein the first wireless RF signal is injected to the high frequency oscillator for controlling the high frequency oscillator through injecting locking, a low frequency controllable oscillator generating a low frequency oscillation signal according to a control signal, and a subsampling analog-to-digital converter (SSADC) receiving the high frequency oscillation signal and the low frequency oscillation signal and detecting a phase difference between the high frequency oscillation signal and the low frequency oscillation signal at time periods indicated by the low frequency oscillation signal, the SSADC outputting a phase detection digital output signal according to the detected phase difference, the control signal being generated according to the phase detection digital output signal output from the SSADC, and the motion information of the object being calculated according to the phase detection digital output signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is illustrates a motion detector according to a first embodiment of the present invention. 
           [0007]      FIG. 2  illustrates relative frequencies of the carrier frequency f c  and the reference frequency f XTAL . 
           [0008]      FIG. 3  illustrates a motion detector according to a second embodiment of the present invention. 
           [0009]      FIG. 4  illustrates a motion detector depicting a generalized version of the first embodiment and the second embodiment of the present invention. 
           [0010]      FIG. 5  illustrates a motion detector according to a third embodiment of the present invention. 
           [0011]      FIG. 6  illustrates relative frequencies of the carrier frequency f c  and the low frequency oscillation signal f s . 
           [0012]      FIG. 7  illustrates a motion detector according to a fourth embodiment of the present invention. 
           [0013]      FIG. 8  illustrates a motion detector depicting a generalized version of the third embodiment and the fourth embodiment of the present invention. 
           [0014]      FIG. 9  illustrates a motion detector according to a fifth embodiment of the present invention. 
           [0015]      FIG. 10  illustrates a motion detector according to a sixth embodiment of the present invention. 
           [0016]      FIG. 11  illustrates a motion detector depicting a generalized version of the fifth embodiment and the sixth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Below, exemplary embodiments will be described in detail with reference to accompanying drawings so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout. 
         [0018]    Please refer to  FIG. 1 .  FIG. 1  illustrates a motion detector  10  according to a first embodiment of the present invention. The motion detector  10  is created using primarily analog components, and comprises a transceiver  20  that may optionally include one or more antennas, a subsampling phase detector (SSPD)  12 , a charge pump  14 , a loop filter  16 , a voltage-controlled oscillator (VCO)  18 , and a frequency lock loop (FLL)  22 . The transceiver  20  receives a first wireless radio frequency (RF) signal and thereby generates a corresponding first electrical signal. The transceiver  20  is coupled to an output end of the VCO  18  and generates a second wireless RF signal that is output from the transceiver  20  to an object  24  under measurement. When the second wireless RF signal contacts the object  24 , the object  24  reflects the first wireless RF signal back to the transceiver  20 . Due to the Doppler Effect, the frequency of the first wireless RF signal is different from that of the second wireless RF signal. When the transceiver  20  converts the received first wireless RF signal into the corresponding first electrical signal, the first electrical signal is then input into an injection end of the VCO  18 . Due to the phenomena of injection pulling and injection locking, an oscillation frequency of the VCO  18  varies due to influence of both environmental effects and a Doppler shift caused by the object  24 . In other words, the oscillation frequency of the VCO  18  varies along with variation of the first electrical signal produced by the transceiver  20 . 
         [0019]    A crystal oscillator or other such reference generator provides a reference frequency f XTAL , which is input into the SSPD  12 . Besides receiving an input of the reference frequency f XTAL  the SSPD  12  also has an input for receiving the oscillation signal from the VCO  18 . The SSPD  12  performs phase detection by comparing phase differences of the reference frequency f XTAL  and the oscillation signal from the VCO  18 . The SSPD  12  samples the oscillation signal at a frequency controlled by the reference frequency f XTAL  for performing the phase detection function. The motion detector  10  is designed such that a frequency of the oscillation signal output from the VCO  18  is an integer multiple or fractional multiple (such as 1.5) of the frequency of the reference frequency f XTAL , wherein the value of the multiple or a fractional multiplication factor is greater than 1. Therefore, the SSPD  12  in effect provides the function of dividing the frequency of the oscillation signal into a lower frequency signal. The SSPD  12  uses much less power than a traditional phase detector and frequency divider combination of a phase lock loop (PLL). 
         [0020]    When the SSPD  12  detects the phase difference between the oscillation signal and the reference frequency f XTAL  at time periods indicated by the reference frequency f XTAL , the SSPD  12  outputs a phase detection output signal to the charge pump  14  according to the detected phase difference. The charge pump  14  converts the phase detection output signal received from the SSPD  12  into an output current signal, and the loop filter  16  converts the output current signal into the control voltage used to control the VCO  18 . The oscillation signal of the VCO  18  is locked to a multiple or fractional multiple of the reference frequency f XTAL . The control voltage used to control the VCO  18  can be used to extract the motion information of the object  24 . When the object  24  is not moving, and there is no environmental interference, the oscillation frequency of the VCO  18  is a carrier frequency f c . However, due to injection pulling, the oscillation frequency of the VCO  18  can be pulled to f c +Δf, where Δf is a frequency difference that is equal to a sum of a Doppler shift f d  caused by the object  24  and an injection pulling frequency shift f b  caused by environmental interference. The control voltage used to control the VCO  18  can be represented as a tuning voltage V T , which can represent the frequency difference Δf. Therefore, by providing the tuning voltage V T  to a processing unit  26 , it is possible to extract the Doppler shift f d  caused by the object  24  while ignoring the effects of the injection pulling frequency shift f b . In order to prevent aliasing effect, the reference frequency f XTAL  should be greater than or equal to twice the frequency difference Δf. Also, in order to isolate the Doppler shift f d  from the injection pulling frequency shift f b , a bandwidth of the loop filter  16 , which may be implemented as a low-pass filter, should be small enough. 
         [0021]    The FLL  22  receives as inputs both the reference frequency f XTAL  and the oscillation signal of the VCO  18 . The FLL  22  can be controllably turned on and off, and the FLL  22  assists the SSPD  12  with detecting a phase difference between the oscillation signal and the reference frequency f XTAL  when the FLL  22  is turned on. The FLL  22  can modify the output current signal that is provided from the charge pump  14  into the loop filter  16  for helping to control the function of the loop filter  16  when the SSPD  12  is unable to easily detect the phase difference between the oscillation signal and the reference frequency f XTAL . By keeping the FLL  22  turned off when it is not needed, and by not using a frequency divider, the motion detector  10  is able to save significant amounts of power during normal operation. The processing unit  26  can determine when it is necessary to turn on and off the FLL  22 , the processing unit  26  outputs a control signal to the FLL  22  for controlling when to turn on and turn off the FLL  22 . A timer in the processing unit  26  can be used to periodically turn on the FLL  22  for several cycles to ensure that the SSPD  12  is properly detecting the phase difference between the oscillation signal and the reference frequency f XTAL . 
         [0022]    Please refer to  FIG. 2 .  FIG. 2  illustrates relative frequencies of the carrier frequency f c  of the VCO  18 , the reference frequency f XTAL  the Doppler shift f d , and the frequency difference Δf that is equal to the sum of the Doppler shift f d  and the injection pulling frequency shift f b  caused by environmental interference. A top half of  FIG. 2  illustrates the value of the reference frequency f XTAL  with respect to the carrier frequency f c . The carrier frequency f c  is indicated with line  30 . A sum of the carrier frequency f c  and the frequency difference Δf is indicated with line  32 . A sum of the carrier frequency f c  and the Doppler shift f d  is indicated with line  34 . 
         [0023]    A bottom half of  FIG. 2  illustrates the effects of subsampling, which effectively forms a relationship between the carrier frequency f c  and the reference frequency f XTAL  such that the carrier frequency f c  is a multiple or fractional multiple of the reference frequency f XTAL . The far left side of the bottom of  FIG. 2  isolates the Doppler shift f d  and the frequency difference Δf from the carrier frequency f c  in order to show more clearly how the Doppler shift f d  can be determined. The frequency difference Δf is indicated with line  42 , and the Doppler shift f d  is indicated with line  44 . Line  46  shows an example of a bandwidth of the loop filter  16  that can be selected for isolating the Doppler shift f d  from the frequency difference Δf. So long as the loop filter  16  has a bandwidth greater than the size of the Doppler shift f d  and less than the frequency difference Δf, the loop filter  16  can ensure that the Doppler shift f d  is properly determined. Since typically the value of the Doppler shift f d  is much less than that of both the injection pulling frequency shift f b  caused by environmental interference and the summed value of the frequency difference Δf, the Doppler shift f d  can be effectively isolated so long as the injection pulling frequency shift f b  caused by environmental interference does not have a value very close to that of the reference frequency f XTAL . As stated above, in order to prevent aliasing effect, the reference frequency f XTAL  should be greater than or equal to twice the frequency difference Δf. 
         [0024]    Please refer to  FIG. 3 .  FIG. 3  illustrates a motion detector  50  according to a second embodiment of the present invention. The motion detector  50  of  FIG. 3  is similar to the motion detector  10  of  FIG. 1 , but is created using primarily digital components. Only the differences between the motion detector  50  and the motion detector  10  will be described below. The SSPD  12  and the VCO  18  of the motion detector  10  are now respectively replaced with a subsampling analog-to-digital converter (SSADC)  52  and a digitally controlled oscillator (DCO)  58 . The SSADC  52  receives the reference frequency f XTAL  and an oscillation signal output from the DCO  58  and detects a phase difference between the oscillation signal and the reference frequency f XTAL  at time periods indicated by the reference frequency f XTAL . The SSADC  52  outputs a phase detection digital output signal A[n] according to the detected phase difference, which is in turn input into a digital loop filter  54 . The digital loop filter  54  converts the phase detection digital output signal A[n] into a digital control signal B[n] that is used for controlling the DCO  58 . This digital control signal B[n] can be used to determine the frequency difference Δf by a processing unit  66 . Similar to the processing unit  26  of the motion detector  10 , the processing unit  66  can also determine when it is necessary to turn on and off an FLL  62  and outputs a control signal to the FLL  62  for controlling when to turn on and turn off the FLL  62 . 
         [0025]    Please refer to  FIG. 4 .  FIG. 4  illustrates a motion detector  75  depicting a generalized version of the first embodiment and the second embodiment of the present invention. The motion detector  75  contains an SSPD  80  and a controllable oscillator  90 . The SSPD  80  may be realized as either the SSPD  12  shown in  FIG. 1  or the SSADC  52  shown in  FIG. 3 . The controllable oscillator  90  may be realized as either the VCO  18  shown in  FIG. 1  or the DCO  58  shown in  FIG. 3 . Operation of the motion detector  75  is similar to that of the motion detectors  10  and  50  described in  FIG. 1  and  FIG. 3 , respectively, and will not be repeated for the sake of brevity. 
         [0026]    Please refer to  FIG. 5 .  FIG. 5  illustrates a motion detector  100  according to a third embodiment of the present invention. Similar to the motion detector  10  of  FIG. 1 , the motion detector  100  is created using primarily analog components. The main difference is the motion detector  100  contains no crystal oscillator or other reference generator for providing the reference frequency f XTAL . Instead, the motion detector  100  comprises two different oscillators, including a high frequency oscillator  110  and a low frequency VCO  118 . A high frequency oscillation signal of the high frequency oscillator  110  has a carrier frequency f c . However, due to injection pulling, the frequency of the high frequency oscillation signal can be pulled to f c +Δf, where Δf is a frequency difference that is equal to a sum of the Doppler shift f d  caused by the object  24  and an injection pulling frequency shift f b  caused by environmental interference. The low frequency VCO  118  generates a low frequency oscillation signal f s  according to a received control voltage. The high frequency oscillator  110  has better phase noise performance than that of the low frequency VCO  118 . The low frequency VCO  118  could be less inexpensive than a crystal oscillator, and the lower frequency of the low frequency oscillation signal f s  allows power savings. 
         [0027]    The motion detector  100  further comprises an SSPD  112  having a first input for receiving the high frequency oscillation signal output from the high frequency oscillator  110 , and a second input for receiving the low frequency oscillation signal f s . The SSPD  112  performs phase detection by comparing phase differences of the low frequency oscillation signal f s  and the high frequency oscillation signal. The SSPD  112  samples the high frequency oscillation signal at a frequency controlled by the low frequency oscillation signal f s  for performing the phase detection function. The motion detector  100  is designed such that a frequency of the high frequency oscillation signal is a multiple or fractional multiple of the frequency of the low frequency oscillation signal f s . Therefore, the SSPD  112  in effect provides the function of dividing the frequency of the high frequency oscillation signal into a lower frequency signal, and the SSPD  112  could use much less power than a traditional frequency divider of a PLL. 
         [0028]    When the SSPD  112  detects the phase difference between the high frequency oscillation signal and the low frequency oscillation signal f s  at time periods indicated by the low frequency oscillation signal f s , the SSPD  112  outputs a phase detection output signal to the charge pump  114  according to the detected phase difference. The charge pump  114  converts the phase detection output signal received from the SSPD  112  into an output current signal, and the loop filter  116  converts the output current signal into the control voltage used to control the low frequency VCO  118 . The control voltage used to control the low frequency VCO  118  can be represented as a tuning voltage V T , which represents the frequency difference Δf. As in the motion detector  10 , by providing the tuning voltage V T  to a processing unit  126 , it is possible to extract the Doppler shift f d  caused by the object  24  while ignoring the effects of the injection pulling frequency shift f b . 
         [0029]    The FLL  122  receives as inputs both the high frequency oscillation signal and the low frequency oscillation signal f s . The FLL  122  can be controllably turned on and off, and the FLL  122  assists the SSPD  112  with detecting a phase difference between the high frequency oscillation signal and the low frequency oscillation signal f s  when the FLL  122  is turned on. The FLL  122  can modify the output current signal that is provided from the charge pump  114  into the loop filter  116  for helping to control the function of the loop filter  116  when the SSPD  112  is unable to easily detect the phase difference between the high frequency oscillation signal and the low frequency oscillation signal f s . The processing unit  126  controls when to turn on and off the FLL  122 , and the FLL  122  is usually kept powered off for the purpose of power savings. The processing unit  126  should ensure that the frequency of the low frequency oscillation signal f s  is greater than or equal to twice the frequency difference Δf. 
         [0030]    Please refer to  FIG. 6 .  FIG. 6  illustrates relative frequencies of the carrier frequency f c  of the VCO  18 , the low frequency oscillation signal f s , the Doppler shift f d , and the frequency difference Δf that is equal to the sum of the Doppler shift f d  and the injection pulling frequency shift f b  caused by environmental interference.  FIG. 6  is similar to  FIG. 2 , but the reference frequency f XTAL  is replaced with the low frequency oscillation signal f s  since the low frequency oscillation signal f s  is used for sampling the high frequency oscillation signal in the motion detector  100 . A top half of  FIG. 6  illustrates the value of the low frequency oscillation signal f s  with respect to the carrier frequency f c . The carrier frequency f c  is indicated with line  130 . A sum of the carrier frequency f c  and the frequency difference Δf is indicated with line  132 . A sum of the carrier frequency f c  and the Doppler shift f d  is indicated with line  134 . 
         [0031]    A bottom half of  FIG. 6  illustrates the effects of subsampling, which effectively forms a relationship between the carrier frequency f c  and the low frequency oscillation signal f s  such that the carrier frequency f c  is a multiple or fractional multiple of the low frequency oscillation signal f s . The far left side of the bottom of  FIG. 6  isolates the Doppler shift f d  and the frequency difference Δf from the carrier frequency f c  in order to show more clearly how the Doppler shift f d  can be determined. The frequency difference Δf is indicated with line  142 , and the Doppler shift f d  is indicated with line  144 . Line  146  shows an example of a bandwidth of the loop filter  116  that can be selected for isolating the Doppler shift f d  from the frequency difference Δf. In order to prevent aliasing effect, the low frequency oscillation signal f s  should be greater than or equal to twice the frequency difference Δf. 
         [0032]    Please refer to  FIG. 7 .  FIG. 7  illustrates a motion detector  150  according to a fourth embodiment of the present invention. Similar to the motion detector  100  of  FIG. 5 , the motion detector  150  is created using primarily analog components. The main difference is the motion detector  150  contains an analog-to-digital converter (ADC)  152  and a digital low-pass filter (LPF)  154 . The ADC  152  receives a clock input from the low frequency oscillation signal f s , and converts the phase detection output signal output from the SSPD  112  into a phase detection digital output signal at time periods indicated by the low frequency oscillation signal f s . The digital LPF  154  then converts the phase detection digital output signal into a digitalized version of the Doppler shift f d , which is in turn transmitted to a processing unit  176 . The processing unit  176  also controls operation of the FLL  122 , as explained with respect to the motion detector  100 . 
         [0033]    Please refer to  FIG. 8 .  FIG. 8  illustrates a motion detector  190  depicting a generalized version of the third embodiment and the fourth embodiment of the present invention. The motion detector  190  contains the high frequency oscillator  110 , the SSPD  112 , and a low frequency controllable oscillator  192 . The low frequency controllable oscillator  192  can be realized as the low frequency VCO  118 . Operation of the motion detector  190  is similar to that of the motion detectors  100  and  150  described in  FIG. 5  and  FIG. 7 , respectively, and will not be repeated for the sake of brevity. 
         [0034]    Please refer to  FIG. 9 .  FIG. 9  illustrates a motion detector  200  according to a fifth embodiment of the present invention. Similar to the motion detector  100  of  FIG. 5 , the motion detector  200  contains the high frequency oscillator  110  and another low frequency oscillator. However, the motion detector  200  is created using primarily digital components. The low frequency VCO  118  of the motion detector  100  is replaced with a low frequency DCO  208  that is controlled with a digital control signal C[n]. The low frequency DCO  208  generates a low frequency oscillation signal f s  according to the digital control signal C[n]. The SSPD  112  of the motion detector  100  is replaced with an SSADC  202 . The SSADC  202  receives the low frequency oscillation signal f s  and the high frequency oscillation signal output from the high frequency oscillator  110 , and detects a phase difference between the high frequency oscillation signal and the low frequency oscillation signal f s  at time periods indicated by the low frequency oscillation signal f s . The SSADC  202  outputs a phase detection digital output signal A[n] according to the detected phase difference, which is in turn input into a both a digital LPF  204  with low bandwidth and a digital LPF  206  with high bandwidth. The digital LPF  206  receives a clock input from the low frequency oscillation signal f s , and converts the phase detection digital output signal A[n] into the digital control signal C[n] that is used for controlling the DCO  208 . The digital LPF  204  converts the phase detection digital output signal A[n] into a motion output signal B[n] representing a digitalized version of the Doppler shift f d , which is in turn transmitted to a processing unit  216 . The processing unit  216  also controls operation of an FLL  212  by determining when it is necessary to turn on and off an FLL  212 , and outputs a control signal to the FLL  212  for controlling when to turn on and turn off the FLL  212 . The output of the FLL  212  is input into the digital LPF  206  for helping to adjust an output of the digital LPF  206 , and the output of the FLL  212  is not input into the digital LPF  204 . 
         [0035]    Please refer to  FIG. 10 .  FIG. 10  illustrates a motion detector  250  according to a sixth embodiment of the present invention. Similar to the motion detector  200  of  FIG. 9 , the motion detector  250  differs in that the digital LPF  204  and the digital LPF  206  are replaced with a single digital LPF  254  and in that an FLL  262  receives a reference frequency f XTAL  from a crystal oscillator or other reference generator. The SSADC  202  outputs the phase detection digital output signal A[n] according to the detected phase difference between the high frequency oscillation signal and the low frequency oscillation signal f s  at time periods indicated by the low frequency oscillation signal f s . This phase detection digital output signal A[n] is input into the digital LPF  254 . The digital LPF  254  converts the phase detection digital output signal A[n] into the digital control signal C[n] that is used for controlling the DCO  208 , and also converts the phase detection digital output signal A[n] into a motion output signal B[n] representing a digitalized version of the Doppler shift f d , which is in turn transmitted to the processing unit  216 . The relationship between the reference frequency f XTAL  the low frequency oscillation signal f s , and the carrier frequency f c  output by the high frequency oscillator  110  is explained as follows. The low frequency oscillation signal f s  is a first multiple or fractional multiple M of the reference frequency f XTAL . The carrier frequency f c  is a second multiple N of the low frequency oscillation signal f s . Thus, the carrier frequency f c  is equal to M*N the reference frequency f XTAL , where M is either an integer or a fractional value greater than 1, and N is either an integer or a fractional value greater than 1 that may be equal to or different to the value of M. In this embodiment, a lower cost and lower frequency (such as 32.768 kHz) crystal oscillator is enough to calibrate both the low frequency oscillation signal f s  and the carrier frequency f c . As in other embodiments, in order to prevent aliasing effect, the low frequency oscillation signal f s  should be greater than or equal to twice the frequency difference Δf. 
         [0036]    Please refer to  FIG. 11 .  FIG. 11  illustrates a motion detector  290  depicting a generalized version of the fifth embodiment and the sixth embodiment of the present invention. The motion detector  290  contains the high frequency oscillator  110 , the SSADC  202 , and a low frequency controllable oscillator  292 . The low frequency controllable oscillator  292  can be realized as the low frequency DCO  208 . Operation of the motion detector  290  is similar to that of the motion detectors  200  and  250  described in  FIG. 9  and  FIG. 10 , respectively, and will not be repeated for the sake of brevity. 
         [0037]    In summary, the embodiments of present invention try to avoid the need for a frequency divider that is common in other motion detectors. By lowering the sampling rate using subsampling, the overall power consumption of the motion detector could be lowered considerably without sacrificing the accuracy of detection. 
         [0038]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.