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

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.