Patent Publication Number: US-2021181323-A1

Title: Phase noise shaping in a distance measurement system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This divisional application claims priority to U.S. patent application Ser. No. 15/796,930, filed Oct. 30, 2017, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Three-dimensional (3D) time-of-flight (ToF) camera systems work by transmitting light with periodically varying intensity and measuring the phase of the reflected light detected by photo sensors. The amount of phase delay between the transmitted and reflected light signals is proportional to the distance between the camera system and the 3-D object. Therefore, distance is calculated form the measured phase delay. Various sources of noise are present in the measurements made by a 3D ToF camera system. Examples of source of noise include photon shot noise, pixel thermal noise (kTC), noise in the analog-to-digital converter (ADC) which produces a digital value from the measurements, and ADC quantization noise. Noise in the measurements causes inaccuracies in the computed distances. 
     SUMMARY 
     In accordance with at least one embodiment of the invention, a distance measurement system includes a light transmitter to generate a modulated light signal, a light sensor to generate measurement signals from reflected light among multiple (e.g., four) quad phase angles with respect to a phase of the generated light signal, and a controller. The controller selects a first set of quad phase angles, and generates first measurement signals at the quad phase angles of the first set. Based on the first measurement signals, the controller computes a first phase angle between the generated light signal and the reflected light signal, generates a second set of quad phase angles based on the first phase angle, and generates second measurement signals at the quad phase angles of the second set. Further, based on the second measurement signals, the controller computes a second phase angle between the generated light signal and the reflected light signal and calculates a distance using the second phase angle. 
     In another embodiment, a system includes a light transmitter configured to generate a modulated light signal and a light sensor configured to receive a reflected light signal and to generate measurement signals among four quad phase angles with respect to a phase of the generated light signal. The system also includes a controller coupled to the light sensor. The controller is configured to dynamically vary the quad phase angles and to calculate distance using the dynamically varying quad phase angles. 
     In yet another embodiment, a method includes generating a modulated light signal and selecting a first set of quad phase angles. Based a reflected light signal, the method includes generating first measurement signals at the quad phase angles of the first set. Further, based on the first measurement signals, the method includes computing a first phase angle between the generated light signal and the reflected light signal. In addition, the method includes generating a second set of quad phase angles based on the first phase angle, generating second measurement signals at the quad phase angles of the second set, and based on the second measurement signals, computing a second phase angle between the generated light signal and the reflected light signal. A distance using the second phase angle is then calculated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of various examples, reference will now be made to the accompanying drawings in which: 
         FIG. 1  illustrates a distance measurement system in accordance with various examples; 
         FIG. 2  shows a set of waveforms illustrating operation of the distance measurement system in accordance with various examples; 
         FIG. 3  illustrates the relationship between a computed phase angle between transmitted and reflected light signals and the measurements using a set of quad phase angles in accordance with one example; 
         FIG. 4  shows another example of the relationship between a computed phase angle between transmitted and reflected light signals and the measurements using a different set of quad phase angles; 
         FIG. 5  shows another set of waveforms illustrating operation of the distance measurement system in accordance with various examples; 
         FIG. 6  includes a flowchart illustrating a method in accordance with an example; 
         FIG. 7  shows an example of a use of the distance measurement system; and 
         FIG. 8  shows an example including a time delay in the transmit signal path. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the disclosed embodiments, a distance measurement system (e.g., a 3D ToF camera) transmits a modulated light signal, and calculates the phase angle between the transmitted modulated light signal and the reflected light signal received at a receiver of the system. The calculation of the phase angle uses a set of time windows that are phase delayed relative to the phase of the transmitted modulated light signal. In the illustrative example herein, there are four time windows (referred to as “quads”) and each time window has a different phase delay relative to the transmitted modulated light signal. One illustrative set of quad phase angles includes 0 degrees, 90 degrees, 180 degrees, and 270 degrees. 
     During each of this phase delayed time windows, the distance measurement system takes a measurement indicative of the amount of reflected light received by the system during the respective time window. In one example, the reflected light signal impinges on a photo detector which then generates an electrical current. The generated electrical current then may be multiplied by a reference electrical signal (which may have the same frequency as the light signal) and the multiplied resulting signal is then used to charge a capacitor. The capacitor&#39;s voltage is related to the amount of reflected light received during the time window. Other techniques are possible as well for making the measurement such as digitizing the current and then integrating the current in the digital domain. The time windows may be implemented by, for example, transistor switches that cause the photo detector to be coupled to or decoupled from the circuit that makes the measurement. The timing (start and stop times) of time windows are controlled by a controller within the distance measurement device. 
     As mentioned above, various sources of noise such as photon shot noise, pixel thermal noise, noise in the analog-to-digital converter (ADC) which produces a digital value from the measurements, and ADC quantization noise can infect the measurements. In accordance with the disclosed embodiments, the distance measurement system dynamically adjusts the quad phase angles to reduce the detrimental effects of such noise. Thus, during operation, the distance measurement system may change the quad phase angles from a first set of quad phase angles (e.g., 0 degrees, 90 degrees, 180 degrees, and 270 degrees) to a second set of quad phase angles that may include, for example, 0 degrees, 180 degrees, and two other angles different than 90 degrees and 270 degrees. The calculation of the new quad phase angles is based on a determination of the phase angle between the transmitted and reflected light signals, and example of the calculation is provided below. Once the new set of quad phase angles is determined, the new set of quad phase angles are used to compute a second phase angle between the transmitted and reflected light signals. The resulting new set of quad phase angles is such that the effects of noise on the second phase angle are less than would have been the case if the former set of quad angles were used. 
       FIG. 1  shows an example of a distance measurement system  100  usable to measure distance between the distance measurement system  100  and a 3D object  130 . In one example, the distance measurement system  100  is a 3D ToF camera. The distance measurement system  100  in this example includes a light transmitter  102 , a modulator  104 , optic lenses  106  and  110 , a receiver array  108 , an analog-to-digital converter (ADC)  112 , and a controller  114 . Alternative or additional components may be included as well. The transmitter  102  may include a solid-state laser or a light emitting diode (LED) operating at, for example, a near-infrared wavelength such as 850 nm, although different wavelengths are possible as well. The modulator  104  generates a modulation signal which is used by the transmitter to generate a modulated light signal to be transmitted through lens  106 . The modulation frequency of the transmitted light  120  sets the maximum distance that can be measured by the distance measurement system. As such, different modulation frequency can be selected for different distance applications. 
     Transmitted light  120  reflects off a surface of the 3D object  130  and reflected light  125  is received through lens  110  into the receiver array  108 . The receiver array  108  may comprise multiple photo detectors  109 . Each photo detector  109  generates a current based on the light received by the photo detector as well as a reference electrical signal  117  from the modulator  104  (as explained above), and the current can be used to make a measurement as noted above. For example, the current can be used to charge a capacitor whose resulting voltage is the measurement value used to compute distance. The various photo detectors  109  comprising the receiver array  108  may be used to compute distances to various points on the surface of the 3D object  130  thereby creating, for example, a 3D contour map of the object. 
     The receiver array  108  is coupled to the ADC  112 . Measurement signals  111  from the receiver array  108  are digitized by the ADC  112  and digital values  113  are provided to the controller  114  for further processing. The controller  114  may perform one or more of the operations described herein upon execution of machine instructions (e.g., firmware). In other embodiments, the controller  114  may comprise a programmable logic device, a discrete circuit or other type of circuit or device that can perform the operations described herein. 
     The controller  114  exposes the receiver array in distinct time windows (quads). Each quad has a different phase angle and the phase angle is the phase difference between the transmitted light signal and the reference electrical signal. The controller  114  computes the quad phase angles  115  and provides them to the receiver array  108  for implementation. The quad phase angles may be implemented, for example, as a start timing signal relative to an edge of the transmitted light signal  120 . The length of a given time window may be specified by the controller  114  as well as a time value relative to the start timing signal. 
       FIG. 2  shows an example set of waveforms. The transmitted light signal  120  is a modulated signal illustrated as a generally square wave with rising edges  121  and corresponding falling edges  122 . The reflected light signal  125  is modulated the same as the transmitted light signal  120  but is phase delayed by an angle θ from the transmitted light signal  120 . The edges  126  (and  127 ) of the reflected signal  125  correspond to edges  121  (and  122 ) of the transmitted light signal  120  but delayed due to the distance over which the light travels. The amount of the phase delay θ is a function of the distance between the distance measurement system  100  and the point on the surface of the 3D object  130  which reflects the transmitted light signal  120 . 
       FIG. 2  also illustrates a set of time windows C 1 , C 2 , C 3 , and C 4  corresponding to a set quad phase angles θ degrees, 90 degrees, 180 degrees, and 270 degrees. Time window C 1  (0 degrees) is aligned to the phase of the transmitted light signal  120  whereas time window C 2  (180 degrees) is 180 degrees out of phase with respect to the transmitted light signal as shown. Time windows C 3  and C 4  represent 90 degree and 270 degree phase shifts from the transmitted light signal  120 . The cross-hatching shown for the various time windows represents the time period during each such time window that the reflected light signal is impinging on the photo detector. The time periods in which the reflected light signal impinges on the photo detector during the various time windows is a function of the corresponding quad phase angle. The measurement signals recorded by the receiver array for the quad phase angles thus may vary from time window to time window and are used by the controller  114  to calculate the phase angle θ between the transmitted and reflected light signals  120  and  125 . 
     Equation (1) below represents an example of the relationship between the various quad phase angles and the measurements from the receiver array  108 . 
     
       
         
           
             
               
                 
                   
                     
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     where S 1 -S 4  represent the measurements (e.g., voltage measurements) during the four time windows corresponding to the four quad phase angles. Equation (1) is valid in the example in which transmitted and reflected modulated light signals are sinusoids and the photo detector current is a multiplication of the received light signal with the electrical reference signal. The arguments of the cosine and sine values represent the four quad phase angles in radians. Thus, quad phase angles 0 rad, π/2 rad, π rad, and 3π/2 rad corresponding to the angles 0 degrees, 90 degrees, 180 degrees and 270 degrees, respectively. Equation (1) can be solved by the controller  114  for I and Q, which can then be used to calculate the phase angle θ between the transmitted and reflected light signals  120  and  125 : 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
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                         Q 
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     The distance between the distance measurement system  100  and the 3D object then can be calculated as: 
     
       
         
           
             
               
                 
                   d 
                   = 
                   
                     θ 
                      
                     
                       ( 
                       
                         c 
                         
                           4 
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     where c is the speed of light and f is the modulation frequency. 
       FIG. 3  illustrates vectors corresponding to each of the four measurement values S 1 -S 4 . An illustrative calculated phase angle θ also is shown. The magnitude of the vector  140  defining the phase angle θ is the calculated distance d. The dashed region  150  represents the uncertainty in the calculation of the phase angle θ and thus the distance d. In this example, the noise region  150  is approximately circular and the actual distance d may vary from that shown bounded by the noise region  150 . 
     In accordance with illustrative embodiments, the controller  114  may modify the quad phase angles to be used to make the various measurements S 1 -S 4  to thereby change the shape of the noise region. The shape can be changed to compress it in the direction of the distance vector.  FIG. 4 , for example, illustrates an example in which the set of quad phase angles include 0 rad, π/4 rad, π rad, and 7π/4 rad. The 0 rad and π rad quad phase angles are retained but the π/2 rad and π/2 quad phase angles have been changed to π/4 rad and 7π/4 rad. The π/4 rad (S 2 ) and 7π/4 rad (S 4 ) quad phase angles have been calculated by the controller  114  to define an axis  160  such that the vector  140  defined by angle θ bisects the angle defined by S 1  and S 2 , and thus 8 equals al in  FIG. 4 . With the quad phase angles recalculated in this manner for phase angle θ, a new phase angle θ′ is calculated. The shape of the noise region  155  for the new phase angle θ′ is compressed for the range of possible phase angle calculations as shown thereby reducing the uncertainty range of the phase angle. With a tighter possible range of possible phase angles, the calculated distance d′ advantageously has a narrower range as well. 
     In another example, the new set of quad phase angles can all be different than the former set of quad phase angles. In one such example, the new set of quad phase angles are calculated by the controller  114  such that the vector defined by phase angle θ is perpendicular to the average of the set of quad phase angles. That is, 
     
       
         
           
             
               
                 
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     where n represents the number of quads ( 4  in the disclosed examples). In some embodiments, the controller  114  determines the quads to satisfy equation (4) and to minimize 
     
       
         
           
             
               
                 
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     The generalized form of equation (1) is given as follows: 
     
       
         
           
             
               
                 
                   
                     
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     where φ 1  through φ 4  represent the four quad phase angles. 
       FIG. 5  shows the waveforms of  FIG. 2  but with the time windows C 3  and C 4  recalculated to have different quad phase angles (120 degrees which is 3π/4, and 300 degrees which is 7π/4). As a result, the amount (time exposure) of reflected light received by the receiver array during the C 3  time window is increased relative to that of  FIG. 2 . 
       FIG. 6  illustrates a method in accordance with various embodiments. The operations may be performed in the order shown, or in a different order. Further, the operations may be performed sequentially, or the two or more of the operations may be performed concurrently. The operations may be performed by, or under the control of, the controller  114  of  FIG. 1 . 
     At  208 , the method includes transmitting a modulated light signal with, for example, an average modulation frequency f. The modulated light signal may be continuously transmitted as soon as the distance measurement system  100  is turned on, or may be transmitted upon a user activating a control (e.g., a button) on the system. The transmitted light signal may be in the near-infrared part of the electromagnetic spectrum, although other wavelengths may be implemented as well for the transmitter. 
     At  210 , the method includes selecting a first set of quad phase angles. In one example, the first set of quad phase angles includes 0 degrees, 180 degrees, 90 degrees and 270 degrees, although a different set of quad phase angles may be selected. At  212 , the method includes generating a first set of measurement signals at the quad angles of the first set. For example, the voltage on a capacitor may be monitored during each of the time windows corresponding to each of the quad phase angles of the first set. 
     At  214 , based on the first set of measurement signals, the method includes computing a first phase angle between the transmitted light signal and the reflected light signal. In one embodiment, this computation may comprise solving equation (5) above for I and Q and then computing the phase angle using equation (2). The phase angle calculated in operation  214  is an estimate of the phase angle between the transmitted and reflected light signals. 
     At  216 , the illustrative method includes generating a second set of quad phase angles based on the phase angle calculated in  214 . An example of this operation is to retain the quad phase angles θ and 180 degrees (0 and π rad) and compute an additional two quad phase angles as explained above. If the phase angle computed at  214  is less than the second lowest quad phase angle (e.g., less than 90 degrees if 0, 90, 180, and 270 degree quad phase angles are used), then one quad phase angle defines an axis that bisects 0 and the second lowest quad phase angle. However, if the phase angle computed at  214  is greater than the second lowest quad phase angle (e.g., greater than 90 degrees if 0, 90, 180, and 270 degree quad phase angles are used), then one quad phase angle defines an axis that bisects axis defined by the second lowest quad phase angle and 180 degrees. In either, case, a fourth quad phase angle is computed to be supplementary the newly calculated quad phase angle. Another example is provided above in which a second set of quad phase angles is computed so that the phase angle θ is perpendicular to the average of the set of quad phase angles (see eq. (4) above). 
     At  218 , the method includes generating second measurement signals at the quad phase angles of the second set. This operation is similar to that of operation  212  but with a set of quad phase angles that is different than that used in operation  212 . At  214 , based on the second set of measurement signals, the method includes computing a second phase angle between the transmitted light signal and the reflected light signal. As explained above, this computation may comprise solving equation (4) above for I and Q and then computing the phase angle using equation (2). At  222 , the method includes calculating a distance using the second phase angle. 
     As explained above, distance can be calculated from a calculation of the phase angle θ between the transmitted and reflected light signals. The size of the phase angle θ is a function of the time it takes for the light to travel to, and reflect off of, the 3D object and be received by the receiver array  108 . Thus, the phase angle θ will be different for light that is reflected off different surface points of the 3D object. Surface points that are farther from the distance measurement system  100  will have larger phase delays than for surface points that are closer. In general, the signal-to-noise ratio (SNR) is smaller for more distant points than for closer points. Accordingly, in some embodiments, the controller  114  may dynamically compute quad phase angles using the largest phase angle θ computed from the receiver array  108 . That is, the controller calculates the phase angles θ for each of the photo detectors  109  in the receiver array and thus to various points on a 3D object or scene. The largest calculated phase angle corresponds to the most distant point and the quad phase angles are dynamically determined for the subsequent assessment of distances using that particular phase angle. 
     One application of the distance measurement system  100  is to compute a 3D distance map to a time varying scene (e.g., a live scene for which distances to the various surfaces in the scene vary with respect to the distance measurement system). The distance measurement system  100  may calculate a set of distances for each of the photo detectors  109  of the receiver array  108  at various time intervals (e.g., periodic time intervals) such as 30 times per second ( 30  “frames” per second). The quad phase angles thus can be adjusted between successive frames. For a given frame, the phase angles are computed for the various photo detectors and for the next frame, the quad phase angles are adjusted based on the largest phase angle computed from the previous frame. 
       FIG. 7  illustrates another use of the distance measurement system  100  and how the quad phase angles can be dynamically changed. In this example, a 3D scene  300  includes a person holding up his hand. As such, the person&#39;s hand is closer to the distance measurement system  100  than the person&#39;s head and the rest of the background. In some embodiments, a portion of a scene may be of greater interest for distance tracking than other portions of the scene. In the example of  FIG. 7 , the person&#39;s hand is of greater interest. In such embodiments, the controller  114  may compute the quad angles based on the calculated phase angle θ to the scene portion of interest. A bounding box  310  is shown encompassing the person&#39;s hand (and the portion of interest in general). The system  100  could decide the portion of interest differently depending on the application for which it is being used. For obstacle avoidance, it would look at a rectangle of a fixed size with pixels which show the smallest (nearest) phase value. If it is a gesture recognition system, it would look for specific patterns like a hand, arm, etc. Such determinations of the portion of interest may be performed by a processor separate from the controller  114  of the distance measurement system  100 . 
     The distance measurement system  100  calculates the phase angles θ for different points within the bounding box  310 , and then assigns each computed phase angle to one of multiple bins. Each bin defines a relatively small range of phase angles (or a single phase angle). By assigning each calculated phase angle to its corresponding bin, a count of the number of phase angles within each bin can be determined. The controller  114  determines ( 312 ) the “mode” of the bounding box to be the phase angle θ of the bin with the largest number of phase angles. If the bin is mapped to a range of phase angles, the mode of the bin may be computed as the average phase angle or may be selected to be one of the phase angles in the range. At  314 , the controller calculates the quad phase angles based on the determined mode&#39;s phase angle θ. 
     Distance is calculated as explained herein based on phase angle, which wraps around every 2π radians, resulting in an aliasing distance. The distance at which aliasing occurs is referred to as the ambiguity distance, damb, and is inversely related to the modulation frequency, f, as: 
     
       
         
           
             
               
                 
                   damb 
                   = 
                   
                     c 
                     
                       2 
                        
                       f 
                     
                   
                 
               
               
                 
                   ( 
                   6 
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     The ambiguity distance, damb, is the maximum measurable distance for a given frequency. To increase the measurable distance, the frequency f can be lowered but at the cost of reduced accuracy. 
     In accordance with the disclosed embodiments, multiple (e.g., 2) different frequencies are used concurrently. The light transmitter  102  in  FIG. 1  can transmit multiple light signals with different modulation frequencies. Measuring the same object with two (or more) different frequencies produces two different phase angles between the transmitted and reflected light signals. The combination of two phase angles is unambiguous for a distance that is longer than for either of the frequencies individually. For example, the unambiguous distance for each of the frequencies 60 MHz and 80 MHz is 2.498 m and 1.875 m, respectively. However, concurrently using both 60 MHz and 80 MHz together results in an unambiguous distance of 7.5 m. In such embodiments, the system  100  performs measurements with both frequencies concurrently. 
     For the individual frequency measurements to be discernible, the cos/sin matrix of equation (5) should be invertible. The modified version of equation (5) to accommodate two different frequencies may be: 
     
       
         
           
             
               
                 
                   
                     
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                                 θ2 
                                 ) 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                 ( 
                                 θ2 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   ϕ 
                                    
                                   
                                       
                                   
                                    
                                   3 
                                 
                                 ) 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   ϕ 
                                    
                                   
                                       
                                   
                                    
                                   3 
                                 
                                 ) 
                               
                             
                           
                           
                             
                               cos 
                                
                               
                                 ( 
                                 θ3 
                                 ) 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                 ( 
                                 θ3 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   ϕ 
                                    
                                   
                                       
                                   
                                    
                                   4 
                                 
                                 ) 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   ϕ 
                                    
                                   
                                       
                                   
                                    
                                   4 
                                 
                                 ) 
                               
                             
                           
                           
                             
                               cos 
                                
                               
                                 ( 
                                 θ4 
                                 ) 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                 ( 
                                 θ4 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                      
                     
                       [ 
                       
                         
                           
                             
                               I 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         
                           
                             
                               Q 
                                
                               1 
                             
                           
                         
                         
                           
                             
                               I 
                                
                               
                                   
                               
                                
                               2 
                             
                           
                         
                         
                           
                             
                               Q 
                                
                               2 
                             
                           
                         
                       
                       ] 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             S 
                              
                             1 
                           
                         
                       
                       
                         
                           
                             S 
                              
                             2 
                           
                         
                       
                       
                         
                           
                             S 
                              
                             3 
                           
                         
                       
                       
                         
                           
                             S 
                              
                             4 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     I 1  and Q 1  are the I, Q components of the first frequency&#39;s vector. I 2  and Q 2  are the I, Q components of the second frequency&#39;s vector. On (n is 1, 2, 3, 4) are the quad phase angles of the first frequency, and θn are the quad phase angles for the second frequency. Each frequency&#39;s quad phase angles can be optimized to obtain the noise shaping advantage as is described above. Although two different frequencies are noted above, the technique can be extended to any number of frequencies. 
       FIG. 8  illustrates an embodiment similar to that of  FIG. 1 , but one that includes a controllable time delay  121  in the transmitted light signal path). The time delay  121  may be a hardware time delay circuit or a software-controlled time delay. The amount of the time delay implemented by the time delay  121  can provide additional control over the time amount of time delay between the transmitted light signal and the quad time windows. 
     In the waveform examples of  FIGS. 2 and 5 , the length of each time window C 1 -C 4  (designated in  FIG. 2  as T 1 ) represents a portion of the integration time for the measurements S 1 -S 4 . In the examples of  FIGS. 2 and 5 , the integration times for the various quad phase angles may be longer than T 1  (e.g., a few orders of magnitude longer than T 1 ) and may be the same. However, in other embodiments the integration time (or T 1 ) of one quad phase angle may be different than the integration time (T 1 ) of another quad phase angle. The integration times of the various quad phase angles is controlled by controller  114  and can be varied to further shape the noise region of each calculated phase angle θ. The direction in which there is more integration time will have a larger SNR, and the direction with less integration time will have a smaller SNR. One illustrative strategy can be for the controller  114  to assign more integration time to directions perpendicular to the expected vector defined by the angle θ and distance d. This is an alternative to varying the amount of noise in a given direction. This latter technique may not rotate the noise envelope, but it can change the shape of the noise envelope. So when the quad angles cannot be set to values accurate enough (e.g., due to hardware limitations), the controller  114  instead can control the integration time for quads in each direction. 
     Certain terms are used throughout the following description and claims to refer to particular system components. Different companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.