3D Time-of-flight distance measurement with custom - Universität ...

152 CHAPTER 6
6.1 Camera electronics
6.1.1 Digital sequencer board
The task of the digital sequence unit is to generate bit patterns that control both the
TOF-sensor chip and the modulated illumination unit. Since no delay lines are
used, the four phase relations between sensor timing and illumination timing (0°,
90°, 180°, 270° and 1 st to 4 th sampling point respectively), have to be generated
digitally. For our targeted modulation frequency of 20 MHz we therefore need
flexible digital circuitry that can be operated at 80 MHz and beyond.
One possible solution is the realization of a RAM-based sequencer, where a
complete bit-pattern is loaded into a memory, which is then repeatedly read out at
80 MHz frequency (Tbit=12.5 ns). Assuming a 5 Hz frame rate (Tframe=200 ms),
however, a memory size of 2 megabytes per digital channel (200 ms/12.5 ns
=16,000,000 bit) would be necessary, i.e. 48 megabytes for 24 channels
(@ 80 MHz and 200 ms frames). Such a RAM-based sequencer solution is surely
possible but appears rather overkill for the required task. Also the generation and
download of 48-megabyte bit patterns into the RAM would be time consuming and
complicated.
A closer look at the required timing shows that the necessary bit patterns actually
contain far less information than 48 MB. The reason why such a large memory
would be necessary is the quickly alternating modulation signal of 20 MHz
frequency at four different phase relations. All other signals, such as those for
addressing, reset, readout and frame synchronization of the external A/D converter,
are slower (