A receiver TDC chip set for accurate pulsed time-of-flight laser ranging

A receiver TDC chip set for accurate pulsed time-of-flight laser ranging Juha Kostamovaara, Sami Kurtti, Jussi-Pekka Jansson University of Oulu, Department of Electrical Engineering, Electronics Laboratory, FINLAND Abstract The pulsed time-of-flight (TOF) laser distance measurement method is based on measurement of the transit time of a short laser pulse (typically 3-4 ns) to an optically visible target and back to the receiver. This method is considered to be a potential solution for applications such as the measurement of levels and geometrical shapes in silos and containers, the positioning of tools and vehicles, velocity measurement, anticollision radars, proximity sensors and perception systems in traffic and in robot vision, for example. In this work we have developed an integrated chip set that realizes the receiver channel and time interval measurement functionalities of a pulsed time-offlight laser radar. The input to the receiver channel chip (0.35 m BiCMOS) is the current signal from the APD (avalanche photo detector) and the output an accurate logic-level timing signal for the time-to-digital converter circuit (TDC). The main systematic error in the pulsed time-of-flight sensor is the dependence of the timing point on the amplitude of the laser echo (timing walk). This error is compensated for here by measuring not only the position of the received pulse with respect to the transmitted pulse but also its width with a multi-channel TDC IC and then by using the known relation (calibration) between the walk error and the timing pulse width. The receiver achieves an input noise current of about 100nArms with a bandwidth of 300MHz and a timing error of less than 50ps in a range of more than 1:10000. The multi-channel timeto-digital converter chip (0.35µm CMOS technology) uses a low-frequency crystal as a reference and measures the time intervals with counter and delay line interpolation techniques. The TDC circuit is used to determine the time intervals between the transmitted laser pulse and the resulting optical echo pulses (up to three) and also the widths or rise times of the echoes. The circuit offers a measurement precision of about 10ps and a measurement range of up to 74µs. In terms of laser distance measurement its performance is equivalent to millimetre-level precision within an 11 km range. These circuit techniques are being used in several R&D projects aiming at the development of new accurate and highly integrated generations of laser scanners and radars. CDNLive! EMEA 2012 1 A receiver TDC chip set

Pulsed time-of-flight laser range-finding techniques The pulsed time-of-flight (TOF) laser distance measurement method is based on measurement of the transit time of a short laser pulse to an optically visible target and back to the receiver. Since the velocity of light is known with high accuracy, the distance to target can be calculated from R c T 2 A block diagram of a pulsed time-of-flight laser radar is shown in Figure 1. Its basic blocks are a laser transmitter, consisting typically of a laser diode pulsed with a bipolar transistor switch working in the avalanche breakdown regime, a receiver channel with an avalanche photo detector, and a time interval measurement unit calculating the time difference between the start and stop pulses.. Figure 1. Block diagram of a pulsed TOF laser range-finder One advantage of an optical radar over a microwave radar, for example, is the possibility for collimate the transmitted optical beam easily with optics. As a result not only the longitudinal but also the lateral accuracy, or spatial accuracy, can be very high, at the level or centimetres or even millimetres in a range of tens or hundreds of metres to non-cooperative targets. Moreover, the components used in laser radar electronics and in the related optics are typically low-priced and thus this technology is potentially interesting for high-volume applications. Compared with other optical distance measurement technologies, pulsed time-of-flight techniques are typically considered to be especially advantageous when the measurement speed should be high (>1000 results/s) and/or the dynamic range of the back-scattering laser echo is large (>1:1000). In addition to some traditional laser radar applications (such as proximity sensors, height measurement in silos and containers, the positioning of tools and vehicles and the measurement of shapes and dimensions) new applications are emerging which are characterized by precisely the above features. One particularly interesting application of this kind is environment perception in traffic applications, robot vision and games. As noted above, for practical reasons the laser pulse length used is typically in the range of 3-4ns. This corresponds to 0.5 metre in air. On the other hand, practical applications CDNLive! EMEA 2012 2 A receiver TDC chip set

typically call for an accuracy of a few centimetres, so that a stable pulse shape and the detection of a specific point in the received laser pulse are obviously needed. These demands are alleviated somewhat by the fact that the amplitude of the received laser echo may vary in a range of 1:10 000 or even more. In this work we have developed two integrated full-custom circuits which realize the main functionalities of a pulsed time-of-flight laser scanner. The receiver chip (REC) produces a digital timing pulse for the time-to-digital converter from the weak optical echo. A leading edge timing discrimination principle is used in the receiver channel, as this allows the signal to be clipped and thus gives potentially a very wide dynamic range for the input signal amplitude. The timing comparator gives a timing signal for the TDC as the signal crosses a constant threshold voltage (V th ) at the input to the comparator. Unfortunately, the leading edge timing discrimination principle would produce a relatively large timing walk error in its basic configuration for the received optical echo pulses, the amplitudes of which vary greatly due to changes in the reflectivity, orientation and distance of objects. The timing error could be in the nanosecond range (several tens of centimetres), which would be unacceptable in most practical cases. In the receiver-tdc configuration designed here the timing walk error is compensated for by means of the known relation (calibration) between the walk error generated and the measured pulse length, as is shown in Fig. 2. Figure 2. Time-domain timing walk error compensation based on pulse width measurement The multichannel time-to-digital converter (TDC) measures the time intervals between the electrical timing pulses and converts the results to digital words. The TDC is able to digitize the pulse positions and widths of three separate successive stop pulses. As explained above, this measurement mode can be used to correct the timing walk error produced by the varying timing pulse amplitudes. A low-frequency (~10MHz) crystal is the only external component needed for the TDC circuit, which together with the onchip oscillator that controls it, forms the reference signal for the measurement core. All the delays participating in the measurement are locked to the cycle time of the reference clock with delay locked loop (DLL) techniques in order to prevent the resolution from fluctuating with process, voltage and temperature (PVT) variations. CDNLive! EMEA 2012 3 A receiver TDC chip set

Circuit development Receiver IC, REC A simplified block diagram of the receiver channel is shown in Fig. 3 a). The optical echo is converted to a current pulse in an external avalanche photodiode (APD). The current pulse is converted to a voltage pulse in a trans-impedance pre-amplifier and then further amplified in a voltage-type post-amplifier. A constant threshold voltage is used at the input to the timing comparator generating logic-level timing signals for the multichannel TDC. The TDC measures the start (generated by the laser transmitter) - stop (echo or possibly multiple echoes from the target) delays and the timing pulse widths. In addition, the receiver chip includes the necessary bias blocks and an SPI interface block. Figure 3. a) Block diagram of the receiver channel, b) Layout of the IC receiver channel (0.35um BiCMOS) As the walk compensation takes place in the time domain, it is also operative beyond the range in which the receiver operates linearly [1]. Thus the linear range of the receiver (typically <1:50) does not limit the range over which the timing walk error can be compensated for. The measured timing error of the receiver as a function of the input current pulse amplitude, starting from the minimum level of 1 A (giving an SNR of app. 10), is shown in Figure 4. Figure 4. Timing error of the receiver for a laser pulse with a width of app. 3.5ns. CDNLive! EMEA 2012 4 A receiver TDC chip set

Time-to-digital converter IC, TDC The architecture of the TDC circuit is presented in Fig. 5. The time digitizing is based on a counter and two-level stabilized delay line interpolation. The 14-bit counter counts the full reference clock cycles between the timing signals, thus making a long μs-level measurement range possible. The interpolators find the locations of the timing signals within the reference clock cycles with ~10 ps resolution [2,3], and an ALU decodes the raw data and calculates the time intervals between the Start and Stop pulses. The data interface to and from the TDC is a 100MHz SPI. Figure 5. a) Block diagram of the 7-channel TDC, b) Layout of the TDC (0.35um CMOS) As an example of the performance available, Fig. 6 shows the distributions of the single-shot measurements (average value µ and standard deviations σ) for three successive stop pulses generated with the Tektronix AWG 2021 signal generator. As seen in the figure, the single-shot precision is better than 10ps. The measurement range of the TDC is +/-74 s and temperature drift without external calibration is <0.5ps/ C due to delay locking of on-chip delays to the external reference. Figure 6. Single-shot measurement distributions (average value µ and standard deviations σ) CDNLive! EMEA 2012 5 A receiver TDC chip set

Conclusions The development of this laser radar receiver-tdc chip set is believed to be an important step towards the realization of a pulsed time-of-light laser radar as a high-performance micro-module for a variety of emerging applications. One example of a research effort utilizing the circuit techniques developed here is the Minifaros project (http://www.minifaros.eu/), which aims at opening up the Advanced Driver Assistance System market to small and medium-sized cars and broadening the range of possible applications by developing a new low-cost, low-power, miniature LaserScanner characterized by high performance. The project will develop and demonstrate a totally new type of laser sensor for enhanced environment perception in terms of omnidirectional optics, a MEMS solution for replacing the scanning mirror, integrated electronics and the ability to serve numerous automotive applications and others besides. The construction of the prototype Laser Scanner is shown in Figure 7a (dimensions 8*8*12 cm 3 ). Another application is the development of a new laser scanner system for measuring refractory lining thickness in the molten metals industry. By virtue of the small laser beam diameter, high repetition rate and accurate receiver-tdc circuitry, the system is able even to detect small features (cracks) in the lining profile, see Figure 7b. Figure 7. a) A LaserScanner for environment perception, b) 3-D rendering of a ladle produced by the ScanTrax laser scanner (right) and digital imaging (left), courtesy of Process Metrix References S. Kurtti, J. Kostamovaara, An Integrated Laser Radar Receiver Channel Utilizing a Time-Domain Walk Error Compensation Scheme, IEEE Transactions on Instrumentation and Measurement, vol 60, Issue 1, 2011, pp. 146-157. J. Jansson, A. Mäntyniemi, J. Kostamovaara, A CMOS Time-to-Digital Converter with Better than 10 ps Single-Shot Precision, IEEE Journal of Solid-State Circuits, Vol. 41, No. 6, June 2006, pp. 1286-1296. J. Jansson, A. Mäntyniemi, J. Kostamovaara, Synchronization in a Multi-level CMOS Time-to-Digital Converter, IEEE Transactions on Circuits and Systems--I: Fundamental Theory and Applications, Volume 56, Issue 8, Aug. 2009, pp. 1622-1634. CDNLive! EMEA 2012 6 A receiver TDC chip set