Lidar Luminance Quantizer
- Tuesday, 29 November 2011
Commercial applications include 3D terrain mapping, urban reconnaissance, facial recognition, and robotic vision.
This innovation addresses challenges in lidar imaging, particularly with the detection scheme and the shapes of the detected signals. Ideally, the echoed pulse widths should be extremely narrow to resolve fine detail at high event rates. However, narrow pulses require wideband detection circuitry with increased power dissipation to minimize thermal noise. Filtering is also required to shape each received signal into a form suitable for processing by a constant fraction discriminator (CFD) followed by a time-to-digital converter (TDC). As the intervals between the echoes decrease, the finite bandwidth of the shaping circuits blends the pulses into an analog signal (luminance) with multiple modes, reducing the ability of the CFD to discriminate individual events.
Lidar Luminance Quantizer block schematic." class="caption" align="right">The lidar luminance quantizer (exemplified
in Figure 1) resolves the amplitude,
amplitude slope, and temporal
characteristics of a luminance echo at
the instants the signal crosses a plurality
of thresholds. The system is comprised of an n-bit flash quantizer with 2n–1 highspeed,
non-latched comparators, and a kbit
time-to-digital converter (TDC) with
2k quanta. Each comparator’s output is
encoded into amplitude (SA+, SA–) and
temporal (ST) data, the latter relative to
the illuminating laser flash instant. SA+
and SA– are the encoded amplitudes of
positive and negative slewing points
respectively. When a comparator detects
that a threshold has been crossed, it
latches the outputs of the amplitude
encoder and the TDC as binary words
SA+ or SA– , and ST. The interval ST is the
round-trip time from laser emission to
detection by a particular comparator. In
Figure 1, the SA and T registers are cascades
of parallel k-bit temporal data, nbit
amplitude data, and 1-bit slope sign
data, clocked by the associated comparator
output so that the data is streamed
asynchronously between laser flash
instants. The ensemble of measurements
allows decomposition of a scene’s
luminance versus time so that the modes
can be resolved resulting in greater
image detail. This scheme differs from
photon-counting lidar detection systems
in that the echoes are quantized in both
amplitude and time. An exemplary timing
diagram for a complex wave-shape is
shown in Figure 2 for a 3-bit amplitude
Temporal Quantization of an arbitrary analog signal." class="caption" align="right">The invention is unique in that it combines a slew sensitive flash digitizer function with a temporal encoder to digitize the echoes in the amplitude, derivative, and time domains. Its operation is synchronous with respect to the laser flash instants, but the measured amplitude and temporal points output from the encoder occur asynchronously between flashes. Each threshold crossed is paired with a TDC output value for a plurality of outputs per clock period, unlike a traditional flash digitizer where only one word is output per clock period. This function can be viewed as an equivalent ultra high-speed digitizer in that the amplitude measurements within a clock period can be resolved temporally by the TDC at a very fine level.
This work was done by Gerard Quilligan, Jeffrey DuMonthier, and George Suarez of Goddard Space Flight Center. GSC-15815-1
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