Abstract:
A spatial filter includes a number of photodetectors that generate individual signals in the time domain. The individual signals are subsequently divided and grouped to create the I and Q channels output from the spatial filter. Prior to the formation of the I and Q channels, a window function is used to shape the spatial response of the spatial filter.

Description:
BACKGROUND  
       [0001]     A window function is commonly applied to a signal in order to change the spectrum of the signal. A window function may be used, for example, to suppress side lobes in the spectrum of a signal. Some window functions, such as Blackman, Hanning, and Hamming, have predetermined shapes. Other window functions, including Kaiser, have adjustable or user-defined shapes.  
         [0002]      FIG. 1  is a diagrammatic illustration of windowing system according to the prior art. Windowing system  100  includes spatial filter  102  and window function  104 . Spatial filter  102  includes a repeating pattern of photodetectors  106 ,  108 ,  110 ,  112 . Photodetectors  106 ,  108 ,  110 ,  112  generate individual signals that are grouped and summed  114 ,  116  to create signals  118 ,  120 , respectively. Signal  118  is typically known as the in-phase (I) channel and signal  120  as the quadrature (Q) channel.  
         [0003]     The I and Q channels may have indistinct frequencies, noise, and drop-outs. These factors can make it difficult to interpret the information contained in the I and Q channels. Window function  104  is therefore applied to the channels in order to modify the spectrum of the signals and improve their resolution.  
       SUMMARY  
       [0004]     In accordance with the invention, a method and system for spatial windowing are provided. A spatial filter includes a number of photodetectors that generate individual signals in the time domain. The individual signals are subsequently divided and grouped to create the I and Q channels output from the spatial filter. Prior to the formation of the I and Q channels, a window function is used to shape the spatial response of the spatial filter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention will best be understood by reference to the following detailed description of embodiments in accordance with the invention when read in conjunction with the accompanying drawings, wherein:  
         [0006]      FIG. 1  is a diagrammatic illustration of windowing system according to the prior art;  
         [0007]      FIG. 2  is a graph of a window function in an embodiment in accordance with the invention;  
         [0008]      FIG. 3  is a diagrammatic illustration of a series of detectors in an embodiment in accordance with  FIG. 2 ;  
         [0009]      FIG. 4  is a graph of a spatial response of a spatial filter without windowing and a graph of a spatial response of a spatial filter with windowing in an embodiment in accordance with the invention;  
         [0010]      FIG. 5  is a block diagram of a first spatial filter construction in an embodiment in accordance with the invention;  
         [0011]      FIG. 6  is a diagrammatic illustration of a second spatial filter construction in an embodiment in accordance with the invention;  
         [0012]      FIG. 7  is a diagrammatic illustration of a third spatial filter construction in an embodiment in accordance with the invention;  
         [0013]      FIG. 8  is a block diagram of a first programmable windowing system in accordance with the embodiments of  FIG. 6  and  FIG. 7 ; and  
         [0014]      FIG. 9  is a block diagram of second programmable windowing system in an embodiment in accordance with the invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]     The following description is presented to enable one skilled in the art to make and use embodiments in accordance with the invention, and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the appended claims and with the principles and features described herein.  
         [0016]     With reference to the figures and in particular with reference to  FIG. 2 , there is shown a graph of a window function in an embodiment in accordance with the invention. The shape of window function  200  is created by a series of photodetectors designed to approximate window function  200  and to shape a spatial response of a spatial filter. Techniques for shaping the spatial response of one or more spatial filters are described in more detail in conjunction with  FIGS. 3 and 4 - 8 .  
         [0017]      FIG. 3  is a diagrammatic illustration of a series of photodetectors in an embodiment in accordance with  FIG. 2 . The series of photodetectors  300  approximate the window function of  FIG. 2 . A spatial filter that is designed with photodetectors  300  has N= 2 , where N represents the number of repetitions for a series of four photodetectors. Although  FIG. 3  depicts the photodetectors in a rectangular shape, other embodiments in accordance with the invention are not limited to this configuration. The photodetectors in a spatial filter may be implemented with any shape, such as, for example, a square or oval.  
         [0018]     Each photodetector has been formed with a physical area designed to produce a signal level proportional to a respective window coefficient. In the embodiment of  FIG. 3 , the height  302  of the photodetectors is fixed while the width of each photodetector is adjusted to produce the desired physical area. In another embodiment in accordance with the invention, the width of the photodetectors is fixed while the height of one or more photodetectors is adjusted. In yet another embodiment in accordance with the invention, both the height and width of one or more photodetectors are adjusted pursuant to a particular window function. And finally, in another embodiment in accordance with the invention, the physical area of one or more photodetectors is adjusted independent of the other detectors such that the height of some or all of the detectors is different from the others, the width of some or all of the detectors is different from the others, or both the height and width of some or all of the detectors are adjusted independently from the others.  
         [0019]     The width  304  of photodetector  306  is determined by a(x 1 )W, where a(x 1 ) represents a window coefficient and W a fixed baseline width. Multiplying width  304  by the window coefficient a(x 1 ) adjusts the physical area of photodetector  306  such that photodetector  306  generates the spatial window function at point x 1  (see  FIG. 2 ). The width  308  of photodetector  310  is defined by a(x 2 )W, where a(x 2 ) represents a different window coefficient from a(x 1 ). The value a(x 2 )W determines the physical area of photodetector  310  such that detector  310  produces the spatial window function at point x 2 .  
         [0020]     The width  312  of photodetector  314  is determined by a(x 3 )W, where a(x 3 ) represents another window coefficient. Again, the window coefficient a(x 3 ) produces an area for photodetector  314  that generates the spatial window function at point x 3 . Similarly, widths  316 ,  320 ,  324 ,  328 ,  332  of photodetectors  318 ,  322 ,  326 ,  330 ,  334 , respectively, are governed by the equations a(x 4 )W, a(x 5 )W, a(x 6 )W, a(x 7 )W, a(x 8 )W. The window coefficients a(x 4 ), a(x 5 ), a(x 6 ), a(x 7 ), a(x 8 ) all produce physical areas that generate the spatial window function at points x 4  through x 8 , respectively. Thus, by varying the physical areas of photodetectors  306 ,  310 ,  314 ,  318 ,  322 ,  326 ,  330 ,  334 , the detectors collectively produce window function  200 .  
         [0021]     The distances  336 ,  338 ,  340 ,  342 ,  344 ,  346 ,  348  between photodetectors  306 ,  310 ,  314 ,  318 ,  322 ,  326 ,  330 ,  334 , respectively, are equal in the embodiment of  FIG. 3 . In other embodiments in accordance with the invention some or all of distances  336 ,  338 ,  340 ,  342 ,  344 ,  346 ,  348  are not equal. Thus, the physical area of photodetectors  306 ,  310 ,  314 ,  318 ,  322 ,  326 ,  330 ,  334 , distances  336 ,  338 ,  340 ,  342 ,  344 ,  346 ,  348 , or both may be determined by a particular window function designed to produce a desired spatial response from a spatial filter. The areas of detectors  306 ,  310 ,  314 ,  318 ,  322 ,  326 ,  330 ,  334  and distances  336 ,  338 ,  340 ,  342 ,  344 ,  346 ,  348  are formed using, for example, complementary metal oxide semiconductor (CMOS) fabrication techniques.  
         [0022]     Referring to  FIG. 4 , there is shown a graph of a frequency response curve of a spatial filter without windowing and a graph of a frequency response curve of a spatial filter with windowing in an embodiment in accordance with the invention. Frequency response curve  400  was generated without spatial windowing. Frequency response curve  402  was generated by a spatial filter having photodetector areas proportional to a Hamming window function. As shown in  FIG. 4 , frequency response curve  402  has a broader main lobe and suppressed side lobes  404  compared with curve  400 . The reduced side lobes of frequency response curve  402  suppress noise and make detection of a sinusoidal component in the I and Q channels easier.  
         [0023]     A spatial filter designed with the series of photodetectors shown in  FIG. 3  has the largest physical detector areas in the center of the filter (detectors  318 ,  322 ). The widths of adjacent detectors  314 ,  326 ,  310 ,  330 ,  306 ,  334  become smaller as the detectors move away from the center, thereby tapering the fall off of the frequency response of the filter. In another embodiment in accordance with the invention, the fall off may be tapered differently from that of  FIG. 3 . Moreover, the largest detector areas may include any number of detectors positioned anywhere within the spatial filter.  
         [0024]     Thus, in other embodiments in accordance with the invention, the physical area of one or more photodetectors may be determined pursuant to different types of window functions. When designing a spatial filter, the desired shape of a spatial response for the spatial filter is determined by the selection of a window function. For example, window functions such as Blackman, Hanning, and user-defined window functions may be used in other embodiments in accordance with the invention.  
         [0025]     Referring to  FIG. 5 , there is shown a block diagram of a first spatial filter construction in an embodiment in accordance with the invention. Spatial filter  500  includes detector block  502  and blocking pattern  504 . Detector block  402  includes four photodetectors in the embodiment of  FIG. 5 .  
         [0026]     Blocking pattern  504  prevents light from striking some of the surface, or the entire surface, of one or more photodetectors in detector block  502 . Blocking all or some of the surface reduces the amount of signal generated by a photodetector. The blocking pattern is designed such that the amount of signal generated by each photodetector is proportional to a particular window coefficient for that photodetector. The shape of the spatial response of spatial filter  500  is determined with the signals generated by the combination of detector block  502  and blocking pattern  504 . Blocking pattern  504  includes a metal plate fabricated onto a portion of a surface, or the entire surface, of one or more photodetectors in one embodiment in accordance with the invention. In other embodiments in accordance with the invention, blocking pattern  504  includes any type of opaque mask overlying a portion of the surface, or the entire surface, of one or more photodetectors.  
         [0027]     The signals generated by each photodetector in detector block  502  are transmitted to summing block  506  via signal lines  508 . Summing block  506  groups the individual signals and generates the I and Q signals on lines  512 ,  514 , respectively. Although  FIG. 5  has been described with four photodetectors and one set of I and Q channels, other embodiments in accordance with the invention may include any number of photodetectors that generate the desired number of I and Q channels.  
         [0028]      FIG. 6  is a diagrammatic illustration of a second spatial filter construction in an embodiment in accordance with the invention. Spatial filter  600  includes photodetectors  602 ,  604 ,  606 ,  608  and summing circuits  114 ,  116  from  FIG. 1 . Photodetectors  602 ,  604 ,  606 ,  608  may have the same physical area for detecting light or have areas that are sized independent of each other. And the physical shape of each photodetector  602 ,  604 ,  606 ,  608  may assume any desired shape, including, but not limited to, a square, rectangle, and oval.  
         [0029]     Each signal generated by photodetectors  602 ,  604 ,  606 ,  608  is multiplied by a particular window coefficient via analog multiplying circuits  610 ,  612 ,  614 ,  616 , respectively. The windowed signals are then grouped and summed by summing circuits  114 ,  116  to generate the I and Q channels on lines  618 ,  620  respectively. The values of the window coefficients associated with multiplying circuits  610 ,  612 ,  614 ,  616  depend upon the desired shape of the spatial response of spatial filter  600  and the type of window function used to shape the spatial response.  
         [0030]     Referring to  FIG. 7 , there is shown a diagrammatic illustration of a third spatial filter construction in an embodiment in accordance with the invention. Spatial filter  700  includes photodetectors  702 ,  704 ,  706 ,  708  and summing circuits  114 ,  116  from  FIG. 1 . Photodetectors  702 ,  704 ,  706 ,  708  may have the same physical area for detecting light or have areas that are sized independent of each other. And the physical shape of each photodetector  702 ,  704 ,  706 ,  708  may assume any desired shape, including, but not limited to, a square, rectangle, and oval.  
         [0031]     Each signal generated by photodetectors  702 ,  704 ,  706 ,  708  is converted to a digital signal by analog-to-digital converters  710 ,  712 ,  714 ,  716 , respectively. The digital signals are then multiplied by a particular window coefficient via digital multiplying circuits  718 ,  720 ,  722 ,  724 . The values of the window coefficients associated with multiplying circuits  718 ,  720 ,  722 ,  724  depend upon the desired shape of the spatial response of spatial filter  700  and the type of window function used to shape the spatial response. The windowed signals are then grouped and summed by summing circuits  114 ,  116  to generate the I and Q channels on lines  726 ,  728  respectively.  
         [0032]      FIG. 8  is a block diagram of a first programmable windowing system that may be used in accordance with the embodiments of  FIG. 6  and  FIG. 7 . Programmable windowing system  800  includes photodetector block  802 , multiplying block  804 , programming block  806 , memory  808 , and summing block  810 . Photodetector block  802 , multiplying block  804 , programming block  806 , memory  808 , and summing block  810  are fabricated together in one embodiment in accordance with the invention. One or more of the blocks, however, may be fabricated separately or as discrete components in other embodiments in accordance with the invention.  
         [0033]     The individual signals generated by each photodetector in detector block  802  are transmitted to multiplying block  804  via lines  812 . Multiplying block  804  multiplies one or more of the signals by a respective window coefficient. The signals may be converted to digital signals prior to the application of the window coefficients.  
         [0034]     The window coefficients are input into multiplying block  804  by programming block  806 . In one embodiment in accordance with the invention, one or more window coefficients are input into programming block  806  via signal line  814 . In another embodiment in accordance with the invention, window coefficients are stored in memory  808 . Programming block  806  reads one or more of the window coefficients out of memory  808  and programs multiplying block  804 . Signal line  814  and memory  808  allow the window coefficients in multiplying block  804  to be programmed and reprogrammed with different values during the operation of windowing system  800 .  
         [0035]     The windowed signals are then input into summing block  810  via lines  816 . Summing block  810  groups and sums the signals in order to generate the I and Q channels on lines  818 ,  820 . As with the embodiments of  FIG. 6  and  FIG. 7 , the values of window coefficients depend upon the desired shape of the spatial response and the type of window function used to shape the spatial response.  
         [0036]      FIGS. 6-8  use four photodetectors to generate one set of I and Q channels. Embodiments in accordance with the invention, however, are not limited to this configuration. One or more spatial filters may include any number of photodetectors that generate a desired number of I and Q channels.  
         [0037]     Referring to  FIG. 9 , there is shown a block diagram of second programmable windowing system in an embodiment in accordance with the invention. Programmable windowing system  900  includes an array of discrete photodetectors  902 ,  904 ,  906 ,  908 , a second array of discrete photodetectors  910 ,  912 ,  914 ,  916 , a third array of discrete photodetectors  918 ,  920 ,  922 ,  924 , and a fourth array of discrete photodetectors  926 ,  928 ,  930 ,  932 . The four photodetector arrays form a spatial filter that produces an I channel on line  118  and a Q channel on line  120 .  
         [0038]     An array of switches  934  is connected to photodetectors  902 ,  904 ,  906 ,  908  while a series of switches  936  is connected to photodetectors  910 ,  912 ,  914 ,  916 . Similarly, arrays of switches  938 ,  940  are connected to photodetectors  918 ,  920 ,  922 ,  924  and photodetectors  926 ,  928 ,  930 ,  932 , respectively. Controller  942  outputs control signals  944  that control the position of each switch connected to discrete photodetectors  902 - 932 . Although only four control signals  944  are shown in  FIG. 9 , controller  942  is capable of generating and transmitting a control signal to each switch connected to a discrete photodetector in an embodiment in accordance with the invention.  
         [0039]     By opening or closing particular switches, the number of discrete photodetectors used to generate signals  946 ,  948 ,  950 ,  952  is programmable and may be modified during operation of the spatial filter. In the embodiment of  FIG. 9 , the switches in switch array  934  that are connected to discrete photodetectors  904 ,  906  are closed. Thus, discrete photodetectors  904 ,  906  create “effective” photodetector  954  that produces signal  946 . The switches in switch array  936 ,  938  are all closed, thereby allowing discrete photodetectors  910 ,  912 ,  914 ,  916  and discrete photodetectors  918 ,  920 ,  922 ,  924  to form “effective” photodetectors  956 ,  958 , respectively. And effective photodetectors  956 ,  958  generate signals  948 ,  950 , respectively. And finally, signal  952  is produced by “effective” photodetector  960  that is constructed with discrete photodetectors  930 ,  932 . The number of discrete photodetectors used to create each effective photodetector is determined by a particular window function, where the amount of signal generated by each effective photodetector is proportional to a particular window coefficient.  
         [0040]     Signals  944 ,  948  are input into summing block  114  to produce the I channel on line  962  while signals  946 ,  950  are input into summing block  116  to produce the Q channel on line  964 . Thus, the spatial response of the spatial filter is shaped by the signals generated by particular combinations of discrete photodetectors that are used to create effective photodetectors  954 ,  956 ,  958 ,  960 .  
         [0041]     Although  FIG. 9  depicts four discrete photodetectors in each array, other embodiments in accordance with the invention may include any number of discrete photodetectors in an array. Moreover, each discrete photodetector  902 - 932  may have the same physical area for detecting light or have areas that are sized independent of each other. And the physical shape of each discrete photodetector included in a spatial filter may assume any desired shape, including, but not limited to, a square, rectangle, and oval.