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

114 CHAPTER 5
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multitap-CCD
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4-tap CCD
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Fast detection of light 2 Fast separation of charge carriers
Repetitive integration for
sufficiently large signal
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Storage of
sampling points
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1-tap CCD
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Light sensitive area
Figure 5.8 Local sites of physical demodulation requirements:
Energy conversion, fast separation, addition and storage of charge
carriers.
For the multitap structure the charge carriers have to travel 4 gates each of about
15 µm length to reach the next storage site. Since the fringing field influence on the
charge transfer decreases more and more with increasing gate length for very long
CCD gates, the complete transfer will be dominated by the diffusion process.
Therefore, we expect a mean transfer time of about 68 ns per gate for the multitap-
CCD pixel. Since this is only a statistical value we postulate a 10 times higher
transfer time for a complete charge transfer of all charge carriers (important,
because the remaining charge carriers would contribute to a wrong sampling point).
Hence, the transfer time from one sampling point to the next should be about 3 µs
(4⋅10⋅68 ns), resulting in a sampling frequency of about 350 kHz. Real
measurements show a slightly better performance than this rough estimate, which
completely neglected fringing field influences. The multitap structure can be used
up to 2 MHz sampling frequency, which, however, only enables a modulation
frequency of 500 kHz.