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

OPTICAL TOF RANGE MEASUREMENT 47
2.2.4 Summary
To summarize, demodulation pixels can be realized by giving them the ability to
sample the incoming light in the time domain synchronously with its modulation
frequency. With the DFT it is only possible to measure discrete frequencies. In
order to enlarge the SNR, the sampling process must be repeated a large number
of times, integrating the small signal amounts of each sampling point to a larger
signal value. This successive integration of the sampling points (1) sharpens the
system’s sensitivity to discrete frequencies only: spectral selectivity, (2) lowers the
(unwanted) sensitivity to neighboring frequency components and (3) improves the
SNR by increasing the signal strength. Due to this frequency selective integrating
nature we also call our pixels “lock-in pixels”. Like a lock-in amplifier, they are only
sensitive to the modulation frequency itself.
With analytical calculations as well as some numerical simulations we have shown
that the 4-tap algorithm is not sensitive to quadratic system non-linearties and is
also insensitive to even harmonics of the modulation signal. Odd harmonics, as
present in a square wave for example disturb the measured phase. This is caused
by aliasing. In practice this is not a serious problem, since there is (usually) an
unambiguous relationship between the real phase and the measured phase, so that
the error can be corrected using a look up table (LUT). In the TOF application such
a LUT correction was not necessary due to the low pass characteristics of both the
demodulation pixels and the LED illumination. Although the LEDs are controlled by
a digital signal (ideally a square wave), their output at 20 MHz is more sinusoidal
(c.f. Chapter 5).