Dual frequency direct time-of-flight measurement

Abstract

The invention relates to a time-to-digital conversion system comprising: a laser pulse trigger signal generator configured to generate laser pulse trigger signals; a laser for emitting light pulses to a target, the laser being configured to operate at least at a first pulse rate and a second, different pulse rate, the pulses to be triggered by the laser pulse trigger signal generator; a sensor for receiving the light returned from the target and for outputting signals, in particular the sensor comprising a multiple single-photon avalanche diode (SPAD); a time-to-digital converter (TDC) having one or a plurality of TDC channels for receiving the signal of the sensor; a bin recording module configured to continuously record bins, the recording enabled by the TDC, wherein the recorded bins are sorted into a histogram according to a laser pulse sequence.

Description

[0001] Dual Frequency direct Time-of-Flight Measurement

[0002] DESCRIPTION

[0003] TECHNICAL FIELD

[0004] The invention relates to a time-to-digital conversion system, a method for time-to-digital conversion of light sensor signals, typically multiple single-photon avalanche diode (SPAD) signals, and readout electronics for time-to-digital conversion of the signals, typically multiple single-photon avalanche diode (SPAD) signals.

[0005] BACKGROUND

[0006] In the field of time-to-digital conversion systems, direct Time-of-Flight (dToF) systems usually have been developed to measure the time taken for light to travel from a source to a target and back to a detector. These systems typically employ multiple time-to-digital converter (TDC) channels to process the received signals from single-photon avalanche diodes (SPADs) .

[0007] Various methods have been employed to address the problems associated with the conventional dToF systems. One such method involves cyclic operation of TDC based on PLL, back- to-back measurement operation support, and inherent dynamic element matching by the choice of oscillator size in relation to histogram size. However, these methods may not fully address the challenges faced in achieving accurate time-to- digital conversion, especially when dealing with extended range .

[0008] Prior art systems have attempted to address some of these issues. For example, US20180063652A1 discloses a circuit arrangement for generating a time-to-digital converter (TDC) output signal and a method for generating a TDC output signal. The circuit arrangement includes a time-to-digital converter (TDC) for generating a TDC output signal based on a reference clock signal and an input signal.

[0009] Increasing the range of a direct time-of-f light sensor with the state-of-the-art approach goes along with a linear increase of histogram size (and corresponding chip area) and with the reduction in laser pulse frequency to ensure a unique signal response over the complete range. As longer range additionally involves stronger signal attenuation on the optical return path, multiple effects act together on limiting the system performance. Extension of the range by operation with two measurements of different laser pulse frequencies resolves these dependencies and allows to keep the pulse frequency high while the memory demand for histogram storage remains unchanged or is even reduced. The described procedure enables to retain the distance accuracy achieved in the close range seamlessly across the fully extended range.

[0010] The distance measurement using dToF is typically restricted to a certain distance range. For the extension of the distance interval dToF sensor and subsequent derivatives based on the architecture one known solution is to measure sub-ranges sequentially at reduced pulse rates. The measurement of thus subranges is describes with Fig. 6. However, this approach is relatively slow and inefficient.

[0011] In automotive LiDAR, huge histogram memories of 2048 bins per channel, e.g., have been reported to cover long distance range. Using huge histogram memories increases costs for the processing units and slows the process down.

[0012] Thus, there is a need for covering a long-distance range, allowing fast and efficient operations- with a low memory and area requirement.

[0013] SUMMARY

[0014] There exists a need for distance range coverage. These and other problems are solved by the subject matter of the attached independent claims.

[0015] The present invention provides a time- to-digital conversion system and method that addresses the aforementioned problems by employing a light sensor, a time-to-digital converter (TDC) having one or a plurality of TDC channels and a laser pulse trigger signal generator configured to generate laser pulse trigger signals, a laser for emitting light pulses to a target, the laser being configured to operate at least at a first pulse rate and a second, different pulse rate. A sensor, in particular comprising a photodiode, for receiving the light returned from the target and for outputting signals, in particular the sensor comprising a multiple single photon avalanche diode (SPAD) . In particular, the TDC is configured for receiving the signals of the sensor. A single time base for multiple TDC channels might be chosen. By choosing a single time base for multiple TDC channels, good bin to bin matching across the channels is achieved. The system furthermore comprises a bin recording module configured to continuously record bins, the recording enabled by the TDC, wherein the recorded bins are sorted into a histogram according to a laser pulse sequence. In particular the TDC is a continuously working TDC. The system may be configured for back-to-back measurement operation support.

[0016] The switching between the first pulse rate and the second pulse rate allows to extend the distance coverage as the turn-over of the histogram is different for different histogram sizes. Furthermore, due to the continuous operation of the TDC with high time resolution, all measurement relevant timing signals can be generated from one source, allowing for laser pulse generation with delay compensation and highest accuracy with respect to the TDC operation. Additionally, by selecting the oscillator size with respect to the histogram size range for the first and for the second laser pulse rate, inherent dynamic element matching can be achieved . DETAILED DESCRIPTION

[0017] The above objects of the invention are achieved by a time-to- digital conversion system and a method for time-to-digital conversion of sensor signals, in particular photo-diode signals, in particular multiple single-photon avalanche diode (SPAD) signals.

[0018] Preferred embodiments may be taken from the dependent claims, and, beyond that, from the following description, in particular comprising various embodiments as covered and described in the annexed claims.

[0019] The skilled person will understand that any embodiment described in the following description is covered and comprised by the subject matter covered by the annexed claims .

[0020] The embodiments, features and combination of features as described herein in connection with the invention, as well as the combination of features as given in the annexed claims, but also any combination of features as mentioned and described in connection with the embodiments shall be considered as being disclosed herein, at least, however, shall be considered to be derivable by the skilled person.

[0021] In particular, each feature and each combination of features in the embodiments as described herein may for example be claimed in a different combination, in particular different claim category, at least because the skilled person will recognize that each and every combination of the features mentioned herein is suitable for contributing to solving the underlying problem.

[0022] Further, each feature and each combination of features in the claims and used in the description below may be used and claimed independently from the respective claimed subject matter, independently from claim dependencies and back- references, and independently from the claim category in which the feature is claimed. For example, in an arbitrary combination selected from one or more claims, one or more embodiments as set forth herein below and / or from the annexed figures may be envisaged.

[0023] Within the present application, terms such as "lateral" or "laterally", "rear", "frontal", "upper", "lower", "bottom", "opposite", "inner", "outer" or the like, as used herein, which de-scribe the position of a first object relative to another object, preferably refer to the relative position of a respective part or object with regard to its position fully mounted for its intended use.

[0024] A time-to-digital conversion system according to the present invention comprises a laser pulse trigger signal generator configured to generate laser pulse trigger signals; a laser for emitting light pulses to a target, the laser being configured to operate at least at a first pulse rate and a second, different pulse rate, the pulses to be triggered by laser pulse trigger signal generator; a sensor for receiving the light returned from the target and for outputting signals, in particular the sensor comprising a multiple single-photon avalanche diode (SPAD) ; a time-to-digital converter (TDC) having one or a plurality of TDC channels for receiving the signal of the sensor; a bin recording module configured to continuously record bins, the recording enabled by the TDC, wherein the recorded bins are sorted into a histogram according to a laser pulse sequence.

[0025] One or more TDC channels are configured to receive a signal from the sensor. A laser pulse trigger signal generator is configured to generate laser pulse trigger signals, optionally from the single PLL time base, the laser being configured to operate at least at a first pulse rate and a second, different pulse rate. A bin recording module is configured to continuously record bins, the recording enabled by the TDC, in particular a continuously operating TDC, wherein the recorded bins are sorted into a histogram according to a laser pulse sequence. The histogram may be evaluated for extracting a distance measurement from the peaks present in the histogram. The system may be further configured for back-to-back measurement operation support.

[0026] This allows the system to perform continuous measurements without any delay between them. In particular, this allows to cycle through the histogram a plurality of times.

[0027] In particular, the bins of the histogram are associated with a certain time of flight of the light pulse. Signals from a certain time of flight are recorded into a certain bin. The bins having a predetermined time width. However, one bin is not necessarily associated with only one time of flight of the light pulse but may also associated with a time of flight associated with spaced apart by a multiple of the time range covered by the histogram or by a multiple of the time range covered by the histogram and a measuring gap or delay.

[0028] A person skilled in the art will immediately acknowledge that this system according to the present invention advantageously allows the system to provide consistent and accurate time-to- digital conversion of signals and allows for a high distance range. In particular by using a single, continuously operating high precision time source, the system can generate all required signals for conversion, reducing the potential for errors and discrepancies that could occur with multiple or intermittent time sources. This leads to improved performance, reliability, and efficiency in the time-to- digital conversion process.

[0029] In an advantageous embodiment of the inventive system, the system further comprising a high precision time source for generating signals required for time. to digital conversion of sensor signals, the time source being in particular a a single continuously operating high precision time source, preferably a quartz oscillator. Alternatively or additionally, the system may further comprise a ring oscillator configured to generate one or multiple clock phases for the time-to digital conversion of the sensor signals.

[0030] Using a single continuously operating high precision time source allows the system to maintain consistency and precision while performing time- to-digital conversion of multiple SPAD signals. A reference oscillator may optionally be used to generate all signals, which enhances absolute accuracy. This reduces the potential for errors and discrepancies that could occur with multiple or intermittent time sources. As a result, the performance, reliability, and efficiency of the time-to-digital conversion process are improved .

[0031] In an advantageous embodiment of the inventive system, the system comprises a single phase-locked loop (PLL) time base. In particular, the single phase-locked loop (PLL) time base includes a voltage-controlled oscillator.

[0032] In an advantageous embodiment, the system is further configured for back-to-back operation.

[0033] In an advantageous embodiment of the inventive system, the ring oscillator comprises a prime number of delay elements. Alternatively, the ring oscillator may comprise a number of delay elements with only a few factors.

[0034] In an advantageous embodiment of the inventive system, the system is configured to switch between a first laser pulse rate, used in a first time interval, and second laser pulse rate, used in a second time interval, the second time interval following, immediately or with some delay, on the first time interval.

[0035] In an advantageous embodiment of the inventive system, the system is configured to switch between more than two distinct laser pulse rate, in particular a first laser pulse rate, a second laser pulse rate.

[0036] In an advantageous embodiment of the inventive system, the system is configured for sequentially switching between the first and second laser pulse rate and optionally further laser pulse rates.

[0037] In a further advantageous embodiment of the inventive system, the ring oscillator comprises a prime number of delay elements. This advantageously enables to avoid an uneven bin distribution. In particular this allows for choosing the first and second laser pulse rate according to the desired distance range and avoids having the same peak configuration within the desired range.

[0038] In a further advantageous embodiment of the inventive system, the system is provided with or connected to a processing module for determining a peak-position within a histogram, and / or determining a difference between peak positions within different histograms and / or determining a distance of an object from one or more of the following data: peak-position of a first histogram, peakposition of a second histogram, difference of peak-positions of different histograms.

[0039] The processing module might be integrated into the system or connected to the system by a wired or non-wired connection. If the system is connected to a mobile device, such as a smartphone, the processor of this device might be used as processing module.

[0040] The problems described above are also advantageously solved by a method for time-to-digital conversion of light sensor signals, in particular photodiode signals, in particular multiple single-photon avalanche diode (SPAD) signals, the method comprises the following steps: generating time-signals required for time- to-digital conversion; sequentially generating laser pulse trigger signals at a first laser pulse rate and at a second laser pulse rate; receiving the generated signals at one or more TDC channels histogram processing of the one or more TDC channels. The TDC channels may be summed prior to histogram processing.

[0041] The method may be configured for back-to-back measurement operation support. In particular, the method can be carried out by using a system as described above.

[0042] It shall be noted that the steps given above do not necessarily have to be carried out in the given order. The provided steps may be carried out in any other suitable order .

[0043] However, the order as given above may apply for particular variants of the method.

[0044] In an embodiment of the method to support back-to-back measurement operations, this ability can significantly improve the efficiency and throughput of the system. This feature allows for continuous measurements without any delay between them, which is particularly beneficial in applications that require rapid or real-time data collection and analysis.

[0045] It will be immediately acknowledged by a person skilled in the art that a feature, embodiment, effect or advantage described herein in connection with the inventive system, may also be a feature, embodiment, effect or advantage of the inventive method or sensor, respectively, and vice versa. In an advantageous embodiment of the inventive method, the method further comprises a step of generating multiple clock phases using a ring oscillator to achieve a high time resolution in the TDC.

[0046] This advantageously allows the method to achieve a high time resolution in the time-to-digital converter (TDC) , which can significantly enhance the precision and accuracy of the time- to-digital conversion process. By generating multiple clock phases using a ring oscillator, the method can finely tune the timing of the conversion process, allowing for more precise measurements. This can be particularly beneficial in applications that require high-resolution timing data, such as high-speed digital systems or precision measurement instruments .

[0047] In a further advantageous embodiment of the inventive method, the method further comprises a step of calibrating the single phase-locked loop (PLL) time base to ensure accurate time-to- digital conversion.

[0048] This advantageously ensures the accuracy of the time-to- digital conversion process. In an embodiment an open loop ring oscillator may be trimmed using a reference timing signal or a reference clock. A mismatch of bins may be addressed by inherent dynamic element matching instead of individual trimming. The external reference in both cases improves the accuracy in terms of scale error.

[0049] In a further advantageous embodiment of the inventive method, the method further comprises a step of: continuously recording bins with high precision using the continuously operating TDC, wherein the sorting of the bins into a histogram is performed according to a laser pulse sequence and / or a step of processing the summed TDC channels using a histogram processor to generate a histogram of the time-to-digital conversion .

[0050] This advantageously allows for continuous, high-precision recording of bins, which can significantly enhance the accuracy and resolution of the time-to-digital conversion process. The continuously operating TDC ensures that there are no interruptions or inconsistencies in the bin recording, leading to more reliable and accurate results.

[0051] In a further advantageous embodiment of the inventive method, the ring oscillator comprises a prime number of delay elements to ensure the full effect of dynamic element matching .

[0052] In a further advantageous embodiment of the inventive method, the method further comprises:

[0053] Performing a first step comprising:

[0054] - setting a first laser pulse rate and continuously recording bins with high precision using a continuously operating PLL TDC and sorting the bins into a histogram with a first size.

[0055] Performing a second step, comprising:

[0056] - switching to a second laser pulse rate and continuously recording bins with high precision using the continuously operating TDC and sorting the bins into a histogram with a second size. In particular, the first and second step are performed alternatingly .

[0057] The method might be extended by a further step of

[0058] - switching to a third or further laser pulse rate and continuously recording bins with high precision using the continuously operating TDC and sorting the bins into a histogram with a third or further size .

[0059] The use of further laser pulse rates permits even longer distance ranges. In a further advantageous embodiment of the inventive method, the histogram has X bins for the first laser pulse rate and X-Y bins for the second laser pulse rate, wherein X and Y are natural number and X > YA peak in the histogram with X bins is associated with a certain time between the emission of the pulse and the recording of the pulse and thus associated with a certain distance d. However, it is as well associated to a distance dl+n*dX, wherein dX corresponds to total distance covered by the histograms having X bins. If n is 0, it is a first order histogram, if n equals 1, the peak is within the second order of the histogram etc.

[0060] A peak in the histogram with X-Y bin can be associated to a certain distance d to the light source corresponding to a time-of-f light t. However, it is as well associated to a distance d2+m*dX-Y, wherein dX-Y corresponds to the total histogram covered by the histogram with X-Y bins. Thus, X and Y can be chosen in such a way, that the distance between the peaks for the first and the second histogram allows to identify n resp. m by the distance of the peaks for the same distance of the target from the source. As an example, X might be chosen to be 40 and Y=8.

[0061] In a further advantageous embodiment of the inventive method, the first laser pulse frequency is in the range of 200 to 300 MHz and the second laser pulse frequency is in the range of 30 to 100 MHz less or more than the first laser pulse frequency, in particular wherein the first laser pulse frequency is 250 MHz, and the second laser pulse frequency is 200 MHz.

[0062] In particular the bin size and the laser frequency is adapted to each other.

[0063] In a further advantageous embodiment of the inventive method, the method further comprises: Performing a third step comprising: - extracting a first peak position in the histogram with a first size

[0064] - extracting a second peak position in the histogram with a second size

[0065] - determining a difference between peak positions in the histogram with a first size and the histogram with the second size in order to evaluate the histogram order.

[0066] All described embodiments of the invention have the advantage that they provide a multi-channel Time-to-Digital Converter (TDC) architecture with high precision. This is achieved through a common time base for multiple TDC channels, ensuring good matching and enabling cyclic operation of the TDC. These features offer unique selling points, particularly for customers seeking accurate and efficient time-to-digital conversion solutions.

[0067] Thereby, the present invention is particularly useful, when applying dToF sensors in mobile devices, such as mobile phones, tablets, AR / VR glasses, or the like. In general, the present invention can be advantageously applied in proximity sensing, enhanced autofocus, 3D imaging, LiDAR, and other applications in the field.

[0068] BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Expedient embodiments are now described with reference to the accompanying drawings .

[0070] FIG. 1 show a diagram of an exemplary architecture using a single PLL.

[0071] FIG. 2 shows the basic architecture of a Multi-event TDC.

[0072] FIG. 3 shows a block diagram of a 20 channels, 49 stages TDC.

[0073] FIG. 4 shows schematically the use of two histograms with a different number of bins. FIG. 5 shows a flow diagram of the present invention.

[0074] FIG. 6 shows schematically the use of overlapping windows having different integration times.

[0075] Figure 1 illustrates a Phase-Locked Loop (PLL) based timing generation according to the present invention that is shared for all 20 Time- to-Digital Converter (TDC) channels. This setup allows for continuous acquisition of bins across the full frame, which is a significant improvement over the previous configurations.

[0076] In this embodiment of the invention a time-to-digital conversion system is designed for multiple single-photon avalanche diode (SPAD) signals as an example for photodiode signals. The system features a continuously operating high precision time source, preferably a quartz oscillator, that generates all required signals for the conversion process. Thereby, the system utilizes a single phase-locked loop (PLL) time base, which includes a voltage-controlled oscillator, for multiple time-to-digital converter (TDC) channels. These TDC channels are configured to receive the generated signals from the high precision time source and may include a digital- to-analog converter for signal conversion. The system also includes a laser pulse trigger signal generator that generates laser pulse trigger signals from the single PLL time base. A ring oscillator, comprising a number, in particular a prime number or number having only a few factors, of delay elements, is incorporated to generate multiple clock phases. Additionally, the system features a bin recording module that continuously records bins enabled by the continuously operating TDC. The recorded bins are sorted into a histogram according to a laser pulse sequence. The system is in the shown embodiment also configured to support back-to-back measurement operations.

[0077] In FIG. 2 the basic architecture of a Multi-event TDC is depicted. A PLL based ring oscillator generating 49 phases with a delay of 125 picoseconds is shown as (A) . This ring oscillator is a component in the system, providing multiple phases for precise timing control. Fig. 2 also depicts the conversion of the thermometer code of the ring oscillator to a IHot code, which propagates with a delay of 125 picoseconds. This conversion process is crucial for the subsequent processing steps. Thereby, the thermometer to 1 Hot encoder is depicted as (B) . A IHot signal is triggering the enable mode of latches for 125 picoseconds. During this period, a Single-Photon Avalanche Diode (SPAD) event will find only one latch being transparent at a time. This mechanism ensures precise timing control and synchronization.

[0078] The Time binning logic is depicted as (C) . FIG. 2 illustrates a single bin decoder as (D) that prevents the generation of multiple events from a single SPAD FE pulse. This feature is crucial for preventing signal overlap and ensuring accurate time-to-digital conversion.

[0079] In FIG. 3 a block diagram of a 20 channels, 49 stages TDC is shown. The figure depicts the main components of a time-to- digital conversion system. Thereby, at the heart of the system a Phase-Locked Loop (PLL) is provided that generates delays of 125 picoseconds. This PLL is used for timing control in the system. A ring oscillator with 49 stages is provided, which is locked to a 163 MHz clock. This ring oscillator generates multiple phases for the system. The system includes 20 Time-to-Digital Converter (TDC) channels, with 16 main channels and 4 reference channels for the Single-Photon Avalanche Diode (SPAD) array. These channels are responsible for processing the signals from the SPAD array. A 326 MHz clock generator, which provides the system clock for the entire setup is also provided. This clock generator ensures synchronization across all components of the system. Finally, a Vertical-Cavity Surface-Emitting Laser (VCSEL) clock generator is provided, responsible for generating the clock signals for the VCSEL, which is a type of semiconductor laser diode. In FIG. 4a shows a histogram of a first measurement of a distance. The histogram is selected to have 40 bin in a back- to-back measurement. The measurements shows a peak centered at bin 16. Fig. 4b shows an exemplary measurement of the distance between with a 40 bin histogram, thus X=40. The dashed line and the straight line are lying one over the other in a region of the first 49 TDC channels when using a system with 49 channels. Afterwards the summing into the histogram starts again at the first bin of the histogram, which is however not the first but the second run through. Thus, even though the distance evaluation by the histogram only would end at bin 40 of the histogram, a longer distance results in a peak in the histogram dl+n*dX where dX is the distance measurable with a single histogram.

[0080] FIG. 4c shows the measurement with a histogram of the same distance with a smaller diagram, having only 32 bins. Thus, Y is 8 in the shown case. The peak for this measurement is centered at bin 24. Thus, as the smaller histogram covers a smaller distance range dX-Y. The peak thus can be associated to several distances spaced apart by the maximal measurable distance with a single 32 bin histogram. Considering the 40 bin and the 32 bin histogram, the distance between the peaks in the first and second histogram is 8 bins indicating the pulse was received in the second cycle after pulse emission. Thus, the shown peak is a peak in the second order histogram. The shift of the peak positions depending on the distance range covered by the small and by the large histogram is shown Fig. 4d. By the shift between the peaks, it can be determined that m and n are 1, thus the distance is d+l*dX. In this example, after a distance corresponding to 160 bins the signal difference would start to repeat, which corresponds to the limit of the sensing range. At a laser bin size of 125 ps, the sensing range corresponds to 3 m.

[0081] FIG. 5 shows a flow diagram in a first step a first laser pulse frequency is set. This pulse frequency might be 250 MHz. The laser pulse is emitted, and the return signal is detected via a detector. The return signal is continuously recorded with the time-to-digital conversion system in recording bins. The bins are sorted into a histogram with a first size having first bin count. In particular, this might be the histogram as set out in Fig. 4a, having 40 bins, meaning X=40. The system is working in a back-to-back measurement operation, meaning, that after bin 40 the histogram is continued to be filled with bin 1 of the histogram. Thus, there is no gap resp. delay between the first order and the second order histogram. The second step is performed comprising setting a second laser pulse frequency different from the first one. The bins are sorted into a histogram with a second size having a second bin count. This might in particular be the histogram as set out in Fig. 4c, having 32 bins, meaning Y=8. The system is working in a back-to-back measurement operation, meaning, that after bin 32 the histogram is continued to be filled with bin 1 of the histogram. The range coverage of this histogram is different compared to the first one, meaning that a certain distance is read out at the first histogram as dl+n*dX and at the second histogram as d2+m*dX-Y while dl+n*dX= d2+m*dX-Y. The system might now return to the first step and set the laser again to the first laser pulse frequency .

[0082] In parallel or delayed to the first and the second step, the position of the peak(s) in the histogram with the first size and the position of the peak(s) in the histogram with the second size are determined. The difference of the peak positions is determined. This difference is a multiple n of Y for the same object. This difference is indicative for the order of the histogram and thus for the distance. Thus, depending on the difference between the peak positions and the position of the peak either in the diagram with the first or the second size the distance of an object might be determined. This method allows to determine the distance of objects within an extended range.

[0083] In FIG. 6 shows a data acquisition timing diagram according to the prior art. The distance range for the shown method is divided into a set of overlapping windows, in the present case window WO, window Wl, window W2, and window W3. For each distance window a distance measurement is performed one after the other. The given scene is therefore divided into a number of pixels MP, in particular corresponding to the pixel arrangement of a dToF detector, and the measurement is performed for each of the pixels MP for each of the windows. However, the measurement time can be different for each pixel MP in each of the windows to each time obtain a predetermined number of counts. Thus, for example, for the pixel MP (xO, yO) the second window needs a smaller measurement time with respect to the first window W(0) and the third window W(2) .

Claims

CLAIMS1. A time-to-digital conversion system comprising: a laser pulse trigger signal generator configured to generate laser pulse trigger signals; a laser for emitting light pulses to a target, the laser being configured to operate at least at a first pulse rate and a second, different pulse rate, the pulses to be triggered by the laser pulse trigger signal generator; a sensor for receiving the light returned from the target and for outputting signals, in particular the sensor comprising a multiple single-photon avalanche diode (SPAD) ; a time-to-digital converter (TDC) having one or a plurality of TDC channels for receiving the signal of the sensor; a bin recording module configured to continuously record bins, the recording enabled by the TDC, wherein the recorded bins are sorted into a histogram according to a laser pulse sequence .

2. The system according to claim 1, the system further comprising a continuously operating high precision time source for generating signals required for time-to-digital conversion of the sensor signals, the time source being in particular a single continuously operating high precision time source, preferably a quartz oscillator, and / or a ring oscillator configured to generate one or multiple clock phases for the time-to-digital conversion of the sensor signals .

3. The system according to any one of claims 1 or 2, further comprising a single phase-locked loop (PLL) timebase, wherein in particular the single phase-locked loop (PLL) time base includes a voltage-controlled oscillator.

4. The system according to any one of claims 1 to 3, wherein the system is further configured for back-to-back operation .

5. The system according to any one of claim 2 and optionally claims 3 and 4, wherein the ring oscillator comprises a prime number of delay elements.

6. The system according to any one of claims 1 to 5, wherein the system is configured to switch between a first laser pulse rate, used in a first time interval, and second laser pulse rate, used in a second time interval, the second time interval following on the first time interval and / or wherein the system is configured for sequentially switching between the first and second laser pulse rate.

7. The system according to any one of claims 1 to 6, wherein the system is provided with or connected to a processing module for determining a peak-position within a histogram, and / or determining a difference between peak positions within different histograms and / or determining a distance of an object from one or more of the following data: peak-position of a first histogram, peakposition of a second histogram, distance of peak-positions of different histograms.

8. A method for time-to-digital conversion of light sensor signals, in particular multiple single-photon avalanche diode (SPAD) signals, the method comprising: generating time-signals required for time-to-digital conversion; sequentially generating laser pulse trigger signals at a first laser pulse rate and at a second laser pulse rate;receiving the generated signals at one or more TDC channels; histogram processing of the one or more TDC channels.

9. The method according to claim 8, wherein the method is performed with a system according to one of claims 1 to 7.

10. The method according to claim 8 or 9, wherein a distance measurement is extracted from the histogram.

11. The method according to any of claims 8 to 10, further comprisin :Performing a first step comprising:- setting a first laser pulse rate and- continuously recording bins with high precision using a continuously operating TDC and sorting the bins into a histogram with a first size;Performing a second step, comprising:- switching to a second laser pulse rate and continuously recording bins with high precision using the continuously operating TDC and sorting the bins into a histogram with a second size.

12. The method according to any of claims 8 to 11, wherein the histogram has X bins for the first laser pulse rate and X-Y bins for the second laser pulse rate, wherein X and Y are natural numbers and X > Y.

13. The method according to any of claims 8 to 12, further comprising : wherein the first laser pulse frequency is in the range of 200 to 300 MHz and the second laser pulse frequency is in the range of 30 to 100 MHz less or more than the first laser pulse frequency, in particular wherein the first laser pulse frequency is 250 MHz and the second laser pulse frequency is 200 MHz.

14. The method according to any one of claims 9 to 13, the method further comprising:Performing a third step comprising:- extracting a first peak position in the histogram with a first size- extracting a second peak position in the histogram with a second size- determining the difference of peak positions in the histogram with a first size and the histogram with the second size in order to evaluate the order of the histogram.

15. The method according to any one of claims 8 to 15, wherein the method is configured for back-to-back measurement operation support.

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