A 6μm-Precision Pulsed-Coherent Lidar with a 40-dB Tuning Range Inverter-Based Phase-Invariant PGA

Lidars are becoming common components for remote sensing in many emerging applications such as autonomous vehicle and facial recognition. To accurately visualize the point cloud for further data processing, metrics such as precision, sampling rate, and linearity are important. The concept of a sub-sampling pulsed-coherent lidar system [1] has demonstrated high precision and fast acquisition. The segmented time-of-flight (ToF) algorithm measures the phase shift of the carrier as fine ToF and counts the arrival time of the low-frequency mask's envelope as intermediate and coarse ToF. Fig. 1(a) shows the down-conversion architecture in [1] using a simple LO generation which only uses dividers and a mixer to minimize the clock jitter. The performance based on the coherent detection, however, is limited by several factors. (1) The crosstalk between the reference clock and the echoed signal at the same frequency lowers the sensitivity and the linearity. Specifically, the reference leaks to the incoming signal path hence introducing an in-band spur. At the same time, the received signals with different power pull the clock phase and cause input-dependent phase shifts. (2) I/Q IF channels and two high-speed ADCs are required to reconstruct the mask's envelope leading to high power consumption. Additionally, I/Q mismatch calibration process must be done prior to the normal acquisition. In this work, a local PLL synthesizes the LO to suppress the in-band reference leakage and crosstalk. A two-step down-conversion with digital down-mixing is implemented which improves the power efficiency and immunity to PVT variation without additional calibration. We also propose an inverter-based phase-invariant programmable gain amplifier (PI-PGA) that improves the linearity with a 40-dB tuning range. This calibration-free system achieves 6-μm precision and 30-μm INL
_Max
with a 5-MSa/s sampling rate.