Abstract:
Samples of a light radar (“LIDAR”) return signal are stored in an analog circular buffer following the transmission of a LIDAR pulse. Sampling continues for a fixed period of time or number of samples during a post-trigger sampling period after the occurrence of a trigger signal from a trigger circuit. The trigger circuit indicates the receipt of a return pulse associated with a target object based upon one or more return signal characteristics. Following the post-trigger sampling period, the stored analog samples are sequentially read out and converted to digital sample values. The digital sample values may be analyzed in a digital processor to further confirm the validity of the returned LIDAR pulse, to determine a time of arrival of the LIDAR pulse, and to calculate a distance to the target object. Some versions include multiple circular buffers and capture clocks, enabling the capture of samples from multiple return pulses.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/896,536 titled “ANALOG MEMORY BASED LIDAR SYSTEM” filed on Oct. 28, 2013 and incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments described herein relate to apparatus and methods associated with light pulse based distance measuring, including return signal capture and processing. 
       BACKGROUND INFORMATION 
       [0003]    Length or distance measurement techniques, once limited to mechanical devices such as tapes and sticks of known length to stretch across or otherwise traverse a distance to be measured, have evolved. Newer techniques include the transmission of a pulse of energy (“transmitted pulse”) at a time of transmission (“TOT”) to a target at the far end of the distance to be measured. A return pulse of energy (“return pulse”) reflected from the target is received at the point of transmission at a time of arrival (“TOA”). A time difference between the TOA and the TOT is calculated. The round trip distance between the point of transmission and the target is then calculated as the time difference multiplied by the speed of transmission of the pulse of energy through a medium such as air. A sonic pulse is transmitted at the speed of sound through the medium across the distance, for example, and a light pulse by the speed of light through the medium. The latter case is often referred to as LIght RaDAR or “LIDAR,” and is referred to by that acronym herein. 
         [0004]      FIG. 1A  is a block diagram of a prior-art LIDAR return signal processing apparatus  100 . The apparatus  100  is enabled following transmission of the transmitted pulse and detects electromagnetic energy at wavelengths including those associated with the transmitted pulse at a photo-detector  105 . The photo-detector  105  converts the light energy into an electrical signal that is amplified and otherwise analog processed in an amplifier  107 . The processed signal appears at an output  110  of the amplifier  107  and is referred to hereinafter as the “return signal.” 
         [0005]      FIG. 1B  is a prior-art waveform diagram of an example LIDAR return signal  113 . The timeline  114  associated with the waveform diagram includes a partial time segment  115  during which the outgoing pulse (not shown) is transmitted at t=0. Referring back and forth between  FIG. 1A  and  FIG. 1B , a comparator  118  compares the magnitude of the return signal  113  to a threshold reference voltage  125 . The threshold reference voltage  125  is generated by a threshold reference voltage source  120  and is preset to distinguish noise from a return pulse  135  corresponding to a target object at the distance to be measured. 
         [0006]    An output  140  of the comparator  118  is triggered when the magnitude of the return signal  113  exceeds the threshold reference voltage  125 . A clock in a time measurement unit (TMU)  145  is started at outgoing pulse TOT, t=0. The clock is stopped at the TOA  150  of the return pulse  135  corresponding to the positive-going threshold crossing event. A processing circuit  155  converts the TOA  150  to a distance-to-target  160  from the round trip distance  165  based upon the speed of light through the traversed medium as described above. 
         [0007]      FIG. 2  is a block diagram of a prior-art LIDAR return signal processing apparatus  200 . The LIDAR apparatus  200  includes a photo-detector  105  and an amplifier  107  to generate the return signal  113  as described above with reference to the apparatus  200 . However, the apparatus  200  also includes a fast analog-to-digital converter (“ADC”)  205 . The ADC  205  continuously generates digital samples of the return signal  113  and presents the samples to a processing apparatus  210 . A processing apparatus  210  performs various signal analysis operations on the digital samples. The signal analysis operations enable the LIDAR apparatus  200  to better distinguish the portion  135  of the return signal  113  corresponding to the target object from other portions of the return signal  113  and to obtain and deliver from the portion  135  the distance-to-target  160  at one or more output terminals. 
         [0008]    Generally speaking, LIDAR return signal processing apparatus such as the apparatus  100  utilize lower-cost components and consume relatively less power than full return signal digitizing and processing LIDAR apparatus such as the apparatus  200 . However, the processor  210  of the apparatus  200  has access to samples from the complete LIDAR return signal and can statistically process the samples using digital signal processing (“DSP”) techniques. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is a block diagram of a prior-art LIDAR return signal processing apparatus. 
           [0010]      FIG. 1B  is a prior-art waveform diagram of an example LIDAR return signal. 
           [0011]      FIG. 2  is a block diagram of a prior-art LIDAR return signal processing apparatus. 
           [0012]      FIG. 3  is a flow diagram of a method of sampling and processing a LIDAR return signal according to various example sequences. 
           [0013]      FIG. 4  is a schematic diagram of a LIDAR return signal processing apparatus according to various example embodiments. 
           [0014]      FIG. 5  is a waveform diagram of a LIDAR return pulse illustrating capture and readout clock timing according to various example embodiments and sequences. 
           [0015]      FIG. 6  is a block diagram of a multi-pulse LIDAR return signal processing apparatus according to various example embodiments. 
           [0016]      FIG. 7  is a schematic diagram of a LIDAR return signal processing system according to various example embodiments. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0017]    Structures and methods disclosed herein receive a LIDAR return signal and discriminate based upon magnitude or other criteria to recognize one or more portions of the return signal associated with a target object. Samples of the return signal are captured and stored in an analog circular buffer to be digitized when the return pulse is detected. The disclosed structures and methods thus provide digital sample values suitable for further analysis while conserving ADC, processor, and power resources by limiting digitization to portions of the LIDAR return signal determined to be associated with the target object. 
         [0018]    The dual approach of return pulse detection in the analog domain and digitization for further analysis may overcome two disadvantages of prior-art level detection only methods as understood by the inventors of the disclosed subject matter. Reducing the threshold voltage level  125  of  FIG. 1  enables the detection of weaker returns from far away objects, highly light absorbing objects or highly specular objects that may reflect the light in a direction different from the location of the LIDAR receiver. However, reducing the threshold level  125  may also increase the rate of false triggers due to noise in the signal and/or noise on the threshold voltage  125  itself. Also, the slope of the leading edge of the return pulse is finite and may vary (e.g., due to a slanted reflective surface). Consequently, the intercept point of the leading edge with the threshold level  125  can vary in time for several objects at the same distance, even in the absence of noise. 
         [0019]    In order to achieve higher detection efficiency and higher accuracy, structures herein described generate samples from key portions of the LIDAR return signal rather than basing TOA on a single intersection point. Digital signal processing (“DSP”) techniques are then applied to the samples to extract a more accurate estimate of the TOA and thus more accurately determines the distance to the target object. Such dual approach avoids the penalty of continuously digitizing the entire LIDAR return signal at very fast rates. Consequently, apparatus and systems herein may be implemented with lower-cost, lower-power ADCs and DSP components. 
         [0020]    Apparatus, systems and methods herein store samples of a LIDAR return signal in an analog circular buffer after transmission of a LIDAR pulse to a target object. Storage locations associated with a circular buffer are overwritten on a first in, first out basis after the buffer is first filled. Sampling continues for a fixed period of time or number of samples after the occurrence of a trigger signal (“post-trigger sampling period”). The trigger signal is generated when the LIDAR return signal crosses over a predetermined magnitude threshold, indicating a potential LIDAR return pulse associated with a target object. Some versions and sequences disclosed herein may generate the trigger signal based upon other criteria indicating receipt of a return pulse from the target object. A modestly-sized circular buffer thus captures samples of the return pulse in the analog domain, including pre-trigger samples and post-trigger samples. Buffer space is conserved by storing only a portion of the LIDAR return signal associated with the return pulse from the target object. 
         [0021]    Following the post-signal sampling period, the disclosed apparatus, systems, and methods sequentially read out the stored samples from the circular buffer into an ADC for conversion to digital sample values. The digital sample values may be analyzed in a digital processor to further confirm the validity of a LIDAR return pulse. The latter method may result in more accurate identification of LIDAR return pulses than a comparator-only method which relies on a single point of the LIDAR return signal for identification. Also, the digital processor may analyze the digital sample values using methods such as “center of gravity” to determine a return pulse TOA. The use of several samples may increase the accuracy of the TOA calculation relative to prior-art comparator-only methods even under conditions of return signal noise or pulse variations. It is noted that the accuracy of the TOA as determined using methods and structures herein described is independent of the trigger accuracy for a given return pulse. The digital processor also calculates the target distance from the return pulse TOA. 
         [0022]    Some versions of the disclosed structures and methods present the digital samples from the ADC at one or more output terminals. Some versions may include a digital processor and/or activities performed by a digital processor. Some versions may also present the return pulse TOA and/or time stamps of sampling times related to the return pulse at one or more output terminals. Some versions may include multiple circular buffers. The latter case permits the capture of samples in one circular buffer while another circular buffer is being read out, enabling the capture of multiple return pulses. Some versions include analog processing circuitry positioned in series between the circular buffer and the ADC to pre-process the read-out analog samples. Such pre-processing may be similar to that done by a digital processor, with the advantage of reducing the number of analog values prior to digital conversion. Reducing the number of analog values may reduce the number of A/D conversions and the length of the read-out period (“dead time”) during which return signal capture is suspended. Consequently, slower, less expensive A/D converters and/or a fewer number of circular buffers may be used. Analog processing may, in some versions, reduce the number of analog samples to a single analog value proportional to the TOA. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 3  is a flow diagram of a method  300  of sampling and processing a LIDAR return signal according to various example sequences. The method  300  commences at block  302  with receiving a TOT of a LIDAR pulse transmitted through a medium. The method  300  proceeds at block  305  with receiving the LIDAR return signal following the transmission of the LIDAR pulse. 
         [0024]    The method  300  includes storing samples of the LIDAR return signal in a set of N analog storage elements organized as a circular buffer, at block  310 . “Circular buffer” as used herein means a set of N storage elements capable of being sequentially written to and overwritten when full on a first-in, first-overwritten basis. When the circular buffer becomes full, the oldest, first-stored data is subsequently sequentially overwritten as additional samples of the LIDAR return signal are clocked into the circular buffer while the LIDAR return signal is being received. Each stored sample is an analog representation of a magnitude of the LIDAR return signal at a corresponding sample time. 
         [0025]    The method  300  continues at block  315  with determining whether a magnitude of the LIDAR return signal has exceeded a predetermined threshold. If not, samples of the LIDAR return signal continue to be stored in the circular buffer, at block  310 . If and when the magnitude of the LIDAR signal exceeds the predetermined threshold, a trigger signal is generated at a trigger time, at block  318 . The method  300  includes continuing to store LIDAR return signal samples in the circular buffer for a predetermined period of time following the generation of the trigger signal (the “post-trigger period”). The method  300  also includes terminating the storage of LIDAR return signal samples following the post-trigger period, at block  320 . 
         [0026]    Following the post-trigger period, the method  300  includes sequentially reading out the stored LIDAR return signal samples, at block  325 . Some version of the method  300  may also include analog processing the read-out LIDAR return signal samples, at block  326 . Such analog processing may result in a decreased number of analog samples. Analog processing computes the TOA from the analog samples using techniques such as “center of gravity” implemented in analog circuitry. The method  300  also includes A/D converting the LIDAR return signal samples to generate a set of digital sample values representing the LIDAR return signal magnitude samples, at block  328 . In the case of analog processing activities at block  326 , the method  300  includes A/D converting the analog processed set of analog samples to generate a set of analog processed digital sample values, at block  328 . 
         [0027]    Some versions of the method  300  may also include digitally processing the set of digital sample values and sampling times associated with the analog samples of the LIDAR return signal, at block  330 . Such digital processing operations obtain the TOA of a return pulse portion of the LIDAR return signal representing a reflection from a target object. 
         [0028]    The method  300  may also include calculating a distance-to-target, at block  333 . The distance-to-target is calculated as a difference between the TOA and the TOT multiplied by a speed of light through the medium divided by two: 
         [0000]      Distance-to-target=( TOA−TOT )* c/ 2 
         [0000]    The method  300  may also include presenting the distance-to-target and/or the set of digital sample values at one or more output terminals, at block  335 . The method  300  may conclude with presenting one or more return pulse characteristics extracted from the LIDAR return signal samples at one or more output terminals, at block  340 . Pulse characteristics to be output may include pulse TOA, pulse height, pulse width, type of object from which the return pulse was reflected, etc. 
         [0029]      FIG. 4  is a schematic diagram of a LIDAR return signal processing apparatus  400  according to various example embodiments. The apparatus  400  receives a LIDAR return signal from a LIDAR return signal detector  405  at an input  408  of the apparatus  400 . The LIDAR return signal detector  405  includes a photo detector  105  and low-noise amplifier (“LNA”) circuitry  107 . The photo detector  105  may be a photo diode as shown, a photo transistor, an avalanche photodiode, a silicon photomultiplier or any other type of detector capable of detecting photons of a return signal in the wavelength range of a transmitted LIDAR signal. The LNA circuitry  107  may be implemented with a trans-impedance amplifier, charge-sensitive amplifier, voltage amplifier or other circuitry suitable for amplifying the LIDAR return signal. 
         [0030]    The apparatus  400  includes a circular buffer  410  coupled to the input  408 . The circular buffer  410  includes a predetermined number of analog sampling stages (e.g., the example sampling stages  413 ,  415 , and  418 ) coupled to the input  408 . Each analog sampling stage illustrated and described herein is a sample-and-hold circuit as is known in the art. It is noted that the apparatus  400  and other apparatus and systems described herein may employ sample-and-hold circuits with alternate architectures that perform the functions described. An input of each sampling stage receives the LIDAR return signal and stores a magnitude sample of the LIDAR return signal at a sampling time associated with the sampling stage. Sampling times are determined as further described below. 
         [0031]    Each sampling stage of the circular buffer  410  includes an analog storage element (e.g., the capacitors  422 ,  423  and  425  associated with the sampling stages  413 ,  415  and  418 , respectively). Each analog storage element stores a magnitude sample of the LIDAR return signal at the sampling time associated with the sampling stage. The example analog storage elements are shown as capacitors on  FIG. 4 . Each capacitor stores a LIDAR return signal sample magnitude as a voltage level. It is noted, however, that some versions of the circular buffer  410  may utilize other analog storage elements. 
         [0032]    Each sampling stage of the circular buffer  410  also includes a sampling switch (e.g., the sampling switches  425 ,  428  and  432 ). Each sampling switch is coupled to a respective analog storage element and to the LIDAR return signal detector  405  to receive the LIDAR return signal. Each sampling switch initiates storage of the LIDAR return signal magnitude sample at the sampling time associated with the respective sampling stage when closed and terminates the sampling time when opened. 
         [0033]    Each sampling stage of the circular buffer  410  also includes a readout switch (e.g., the readout switches  450 ,  455  and  458 ). Each readout switch is coupled to the analog storage element to present the stored LIDAR return signal magnitude sample at an output of the circular buffer at a readout time associated with the sampling stage when closed. 
         [0034]    The LIDAR return signal processing apparatus  400  also includes a trigger circuit  419  coupled to the LIDAR return signal detector  405 . The trigger circuit  419  generates a trigger signal upon sensing a return signal characteristic indicating a potential LIDAR return pulse associated with a target object. In some versions of the apparatus  400 , the trigger circuit  419  may include a voltage comparator  118  and a threshold reference voltage source  120 . A first input of the voltage comparator  118  is coupled to the threshold reference voltage source  120  to establish a predetermined threshold voltage V_TH. A second input of the voltage comparator  118  is coupled to the LIDAR return signal detector  405 . The voltage comparator  118  triggers when the magnitude of the LIDAR return signal is equal to or greater than V_TH, indicating a potential LIDAR return pulse from a targeted object. 
         [0035]    Some versions of the trigger circuit  419  may include a comparator monitor (not shown) coupled to the voltage comparator  118 . The comparator monitor senses the output of the voltage comparator  118  at periodic intervals for a number of times corresponding to the number N of sampling stages associated with the circular buffer  410 . The trigger  466  is generated if and when a combination of the voltage comparator outputs is positive, indicating that the return signal has remained above V_TH or returned to a level above V_TH for or during a predetermined period of time. The output of the comparator  118  may be checked at the same time that a new sample is stored in a sampling stage, for example. 
         [0036]    Some versions of the trigger circuit  419  may include a circular buffer scanner (not shown) coupled to the circular buffer  410 . The circular buffer scanner reads and analyzes the N analog values stored in the circular buffer  410  and, based on the result, generates the trigger  466 . In some versions, for example, the scanner may sum the N samples last-stored in the circular buffer and compare the result against a reference voltage. The circular buffer scanner generates the trigger  466  if and when the sum is equal to or greater than the reference voltage. The comparator monitor and the circular buffer scanner are examples of embodiments of techniques that may be used by the apparatus  400  to detect and trigger based upon return pulses whose amplitudes are close to the noise level while keeping false positives low. 
         [0037]      FIG. 5  is a waveform diagram  500  of a LIDAR return pulse  505  illustrating capture and readout clock timing according to various example embodiments and sequences. The waveform diagram  500  shows the rising edge of the LIDAR return pulse  505  at a time  510  when the rising edge crosses V_TH  515 . The waveform diagram  500  will be referred back to in subsequent discussions. 
         [0038]    Referring now to  FIGS. 4 and 5  together, the LIDAR return signal processing apparatus  400  also includes a capture clock phase logic module  435  coupled to an output of the trigger circuit  419 . The capture clock phase logic module  435  generates phases of a capture control clock (e.g., clock phases appearing at the outputs  437  of the capture clock phase logic module  435 ). Each phase of the capture control clock controls the sampling time associated with a sampling stage of the circular buffer  410 . 
         [0039]    An example set of capture control clock phases  517  are shown as arrows occurring at 500 picosecond intervals along the timeline of the waveform diagram  500 . The capture control clock phases  517  may occur with periodicities dependent upon a particular implementation of the apparatus  400 ; 500 picoseconds is merely an example. The capture control clock phases  517  sequentially enable each sampling stage during a pre-trigger period of time  520  when the trigger signal associated with the circular buffer  410  is inactive and for a predetermined post-trigger period of time  525  after the trigger signal becomes active. The pre-trigger and post-trigger periods are determined by the available number of sampling stages implemented to create the circular buffer, the shape of the LIDAR return pulse  505 , and the magnitude of V_TH or other trigger criteria based upon wave shape characteristics of the LIDAR return pulse  505 . The size of the circular buffer  410  and the magnitude of V_TH may be chosen such that samples of desired portions of LIDAR return pulses of most shapes are captured in the circular buffer  410  by the end of the post-trigger period  525 . Some versions of the capture clock phase logic module  435  may also include a time stamp generator (not shown) to generate a time of capture associated with one or more of the capture control clock phases  517 . Time of capture in this context is defined as the time at the end of a period between the TOT and a given clock phase of a given loop of the circular buffer. For example, if a circular buffer has twenty sampling stages, is clocked at one gigahertz and the time of the TOT is zero seconds, the time of capture of a sample into the tenth sampling stage during a second loop through the circular buffer after the TOT is t=30 nanoseconds. 
         [0040]    The LIDAR return signal processing apparatus  400  also includes a readout clock phase logic module  445  coupled to the output of the trigger circuit  419 . The readout clock phase logic module  445  generates phases  540  of a readout control clock (e.g., clock phases appearing at the outputs  447  of the readout clock phase logic module  445 ). Each readout clock phase controls the readout of a LIDAR magnitude sample stored at a sampling stage of the circular buffer  410 . The readout clock phase logic module  445  sequentially enables the readout of the LIDAR magnitude sample stored at each sampling stage. Each sampling stage is read out at a readout time corresponding to the sampling stage during a readout period  545  following the post-trigger period  525 . A sampling stage is read out each four nanoseconds in the example of the readout clock phases  540  of  FIG. 5 . A four nanosecond readout speed is shown merely as an example to illustrate that readout of captured LIDAR magnitude samples may occur at a much slower rate than the capture rate. 
         [0041]    In some versions of the apparatus  400 , the capture clock phase logic module  435 , the readout clock phase logic module  445 , or both may be implemented using a delay-locked loop (“DLL”). In such case, each capture clock phase and/or readout clock phase is generated at an output of a delay line element of the DLL. 
         [0042]    The LIDAR return signal processing apparatus  400  also includes an ADC  465  coupled to the readout switch associated with each sampling stage of the circular buffer. The ADC  465  is synchronized with the readout control clock to digitize the LIDAR magnitude samples as they are sequentially read out to create a set of digital sample values. Some versions of the LIDAR return signal processing apparatus  400  may present the trigger signal  466 , the time of capture of one or more LIDAR analog magnitude samples  467 , and/or the set of digital sample values  468  at one or more output terminals for further digital processing external to the apparatus  400 . Digital processing may include techniques for determining TOA from the set of digital samples  468  including “center of gravity” techniques. 
         [0043]    The readout and ADC conversion rate may be slower than the sampling rate (e.g., 250 MHz vs. 2 GHz for the examples shown) as previously mentioned. The triggered circular buffer architecture of the apparatus  400  thus enables the capture of N samples of a return pulse from a LIDAR return signal in the absence of a resource-intensive ADC and associated digital processing necessary to continually convert and process the entire LIDAR return signal at the sample rate. 
         [0044]    Some versions of the LIDAR return signal processing apparatus  400  may include a digital processor  470  coupled to the ADC  465 . The digital processor  470  receives the set of digital sample values and a TOT of an outgoing LIDAR pulse transmitted through a medium to the target object prior to sampling the LIDAR return signal. The digital processor  470  analyzes the set of digital sample values  468  to further differentiate the LIDAR return pulse as identified by the trigger from other portions of the LIDAR return signal. The digital processor  470  also calculates the TOA of the LIDAR return pulse by processing the set of digital sample values  468 . The digital processor  470  calculates a distance to the target object as a difference between the TOA and the TOT multiplied by the speed of light through the medium divided by two. In the case of having the digital processor embedded in the LIDAR return signal processing apparatus  400 , the apparatus  400  may present the distance to the target object  475  at an output terminal. 
         [0045]      FIG. 6  is a block diagram of a multi-pulse LIDAR return signal processing apparatus  600  according to various example embodiments. The apparatus  600  is organized architecturally and operates in ways similar to the LIDAR return signal processing apparatus  400  of  FIG. 4  as previously discussed. However, the apparatus  600  includes a number M of circular buffers (e.g., the circular buffers  605 ,  610 , and  615 ). Each circular buffer is coupled to a corresponding capture clock phase logic module (e.g., the capture clock phase logic modules  618 ,  622 , and  625 , respectively. Each capture clock phase logic module is coupled to a trigger circuit such as a voltage comparator  118 . It is noted that the trigger circuits of the apparatus  600  may be of types other than the voltage comparator type trigger as previously discussed in the context of the apparatus  400 . 
         [0046]    A data input terminal of each of the M circular buffers is coupled to the LIDAR detector  405  at an input terminal  408  and captures analog magnitude samples of the LIDAR return signal during a pre-trigger period and a post-trigger period associated with a given circular buffer. In some embodiments, the capture clock phase logic modules intercommunicate in order to avoid simultaneously triggering the post-sampling period associated with multiple circular buffers. In some embodiments, capture phase logic modules other than the first in the series (e.g. the modules  622  . . .  625 ) initiate capture by the associated circular buffer only after the immediately previous capture phase logic module has been triggered. The apparatus  600  is thus capable of capturing analog samples of multiple return pulses from the LIDAR return signal. 
         [0047]    A readout clock phase logic module  650  generates readout clock phases for a given circular buffer during a readout period following a post-trigger capture period associated with the circular buffer. Some embodiments may include multiple, lower-speed ADCs  465 , each coupled to a subset number of the circular buffers. The multi-buffer, multi-capture clock and multi-trigger architecture of the apparatus  600  enables the capture of samples of at least M return pulses from a single LIDAR return signal in the absence of a resource-intensive ADC and the associated digital processing necessary to continually convert and process the entire LIDAR return signal at the sample rate. 
         [0048]    Some versions of the LIDAR return signal processing apparatus  600  may present the trigger signal  466 , the time of capture of one or more LIDAR analog magnitude samples, and/or the set of digital sample values  468  at one or more output terminals. 
         [0049]      FIG. 7  is a schematic diagram of a LIDAR return signal processing system  700  according to various example embodiments. The LIDAR system  700  may include a LIDAR return signal detector  405 , one or more circular buffers  410 , a trigger circuit  419 , one or more capture clock phase logic modules  435 , a readout clock phase logic module  445 , and an ADC  465 , all as coupled together and described in detail above with reference to  FIGS. 4 ,  5 , and  6 . 
         [0050]    The LIDAR return signal processing system  700  may also include an analog processing circuit  710  coupled between the output terminal(s) of the circular buffer(s)  410  and an input terminal of the ADC  465 . The analog processing circuit  710  performs calculations such as “center of gravity” calculations on the read out analog samples to determine the TOA. Doing so may enable the analog processing circuit  710  to reduce the number of analog samples to be digitized. Analog processing may, in some versions, reduce the number of analog samples to a single analog value proportional to the TOA. Reduction of the number of conversions associated with the read-out process also reduces the duration of “dead time” during which the buffer cannot capture additional the input signal. Doing so may reduce the complexity of the apparatus by reducing the speed requirement for the A/D converter  465  and/or the number of parallel circular buffers used to capture potential lost events. 
         [0051]    The LIDAR return signal processing system  700  may also include a digital processor  470  coupled to the ADC  465 . The digital processor  470  receives the set of digital sample values and a TOT of an outgoing LIDAR pulse transmitted through a medium to the target object prior to sampling the LIDAR return signal. The digital processor  470  analyzes the set of digital sample values  468  to further differentiate the LIDAR return pulse as identified by the trigger from other portions of the LIDAR return signal. The digital processor  470  also calculates the TOA of the LIDAR return pulse by processing the set of digital sample values  468 . The digital processor  470  calculates a distance to the target object as a difference between the TOA and the TOT multiplied by the speed of light through the medium divided by two. 
         [0052]    The LIDAR return signal processing system  700  may present the trigger signal  466 , the time of capture  467  of one or more LIDAR analog magnitude samples, the set of digital sample values  468  and/or the distance to the target object  475  at one or more output terminals. 
         [0053]    Apparatus, systems and methods described herein may be useful in applications other than capturing, storing, and processing portions of a LIDAR return signal. The examples of the method  300 , the apparatus  400  and  600 , and the system  700  described herein are intended to provide a general understanding of the structures of various embodiments and the sequences of various methods. They are not intended to serve as complete descriptions of all elements and features of systems and methods that might make use of these example structures and sequences. 
         [0054]    Structures and methods disclosed herein receive a LIDAR return signal and discriminate based upon magnitude or other criteria to recognize one or more portions of the return signal associated with a target object. Samples of the return signal associated with the target object are captured and stored in an analog circular buffer and subsequently digitized. The disclosed structures and methods thus provide digital sample values of one or more LIDAR return pulses suitable for further analysis while conserving ADC, processor, and power resources by digitizing only portions of the LIDAR return signal determined to be associated with the target object. 
         [0055]    By way of illustration and not of limitation, the accompanying figures show specific embodiments in which the subject matter may be practiced. It is noted that arrows at one or both ends of connecting lines are intended to show the general direction of electrical current flow, data flow, logic flow, etc. Connector line arrows are not intended to limit such flows to a particular direction such as to preclude any flow in an opposite direction. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be used and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense. The breadth of various embodiments is defined by the appended claims and the full range of equivalents to which such claims are entitled. 
         [0056]    Such embodiments of the inventive subject matter may be referred to herein individually or collectively by the term “invention” merely for convenience and without intending to voluntarily limit this application to any single invention or inventive concept, if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. 
         [0057]    The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b) requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In the preceding Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted to require more features than are expressly recited in each claim. Rather, inventive subject matter may be found in less than all features of a single disclosed embodiment. The following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.