\chapter*{Summary}
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\addcontentsline{toc}{chapter}{Summary}

In \autoref{chapter:lidar_theory}, general principles and components of \ac{DToF} \ac{LIDAR} systems were discussed. Scanning and flash \ac{LIDAR} technologies were compared based on performance or commercial viability, laser sources and the constraints put on them by atmospheric effects or eye safety levels were discussed. A large part of the chapter was dedicated to \acp{SPAD}, as these devices enable \ac{LIDAR} technology in the first place. Their implementation, operation and non-idealities were discussed in sufficient detail necessary to understand the key performance characteristics and trade offs of \ac{LIDAR} systems. The chapter closed by discussing advanced \ac{DSP} techniques used for retrieving the \ac{ToF} of photons with picosecond level of precision even in noisy daylight environments.

The \autoref{chapter:tdcs} was dedicated to \acp{TDC}, the key components of \ac{DToF} \ac{LIDAR} systems, as the actual \ac{ToF} measurement is performed by these components. Common techniques optimizing \ac{TDC} implementations such as the reverse timing scheme or sliding scale technique were discussed. Then, various types of \ac{TDC} implementations were presented and compared, starting from counter based \acp{CTDC}, propagation delay based \acp{FTDC} and even sub-gate delay based \acp{SFTDC}. Since many \acp{FTDC} and \acp{SFTDC} implementations rely on \acp{DLL}, several pages were dedicated to their operation as well. At the end of the chapter, clock distribution schemes for \ac{LIDAR} system \ac{TDC} arrays were compared, highlighting the issue that many schemes trade off clock signal integrity and uniformity for power efficiency or vice versa. It was foreshadowed that \acp{ILO} can be used to optimize this trade off.

In \autoref{chapter:ilo_theory}, \acp{ILO} were discussed and explained. LC tank based \ac{ILO} model was used, enabling phasor analysis to achieve intuitive understanding of the injection locking phenomenon. Afterwards, a time-domain based mathematical model of an \ac{ILRO} was presented and the equation for its locking range was derived. This simple mathematical model was, however, unable to predict the behaviour of \acp{ILRO} when important non-idealities such as the injection clock duty cycle variation were considered. Finally, \ac{ILO} based \ac{TDC} architecture was presented, optimizing the performance trade offs discussed in \autoref{chapter:tdcs}.

In order to understand the behaviour of \acp{ILRO} better, a \textsc{Matlab} Simulink macro model of an \ac{ILRO} was created in \autoref{chapter:model}. Various types of analyses were performed using the model, confirming the findings derived in \autoref{chapter:ilo_theory} and expanding on the previous knowledge with new insights such the exploration of the sensitivity of the \ac{ILRO} to the injection clock duty cycle variation. It was found that asymmetric injection clock leads to sub-optimal locking range span, but this sensitivity can be diminished if the injection pulses are wide enough to cover the so-called \enquote{sensitive window} of the \ac{ILRO}. This, however, increases the total energy spent by the injections, which is an important consideration for \ac{DToF} \ac{LIDAR} systems which contain hundreds or thousands of \acp{TDC}. Therefore, one of the design trade offs was identified before the start of the design phase.

In the final \autoref{chapter:design}, a \ac{DLL}-biased \ac{ILRO} capable of achieving \SI{50}{\pico\second} time resolution has been designed and simulated. First, the processing technology ONK65 was introduced and a key design decision on the \ac{ILRO} operating frequency and the number of stages was explained. Afterwards, several subsections were dedicated to the discussion of overall architecture of an \ac{ILRO} based \ac{LIDAR} \ac{TDC} signal chain, because without the understanding of architecture, educated decisions with regards to design trade-offs are not possible. The design started with the \ac{ILRO} itself, focusing on the optimization of the \acp{CCDLU}, the biasing mirror or the injection circuit. The second phase of design focused on the \ac{DLL} blocks such as the \ac{FPD} or the \ac{V-to-I} converter. A detailed subsection was dedicated to trimming, which is necessary in order to ensure proper operation of the circuits in all operating conditions.

The designed circuits were verified by various types of simulation. Apart from a single \ac{PVT} corner, the designed \acp{ILRO} are able to lock to the injected frequency properly. The problematic \ac{PVT} corner, while statistically very unlikely and having minimal negative impact on the manufacturing yield, was analysed, and possible solutions of the problem were discussed along with their drawbacks. The time resolution offered by the \acp{ILRO} has been simulated to be$\SI{50}{\pico\second} \pm 50\%$ (at $3\upsigma$), which meets the assigned specification. A transient noise simulation confirmed the expected improvement in spectral purity of the generated clock phases, which is another key benefit of the \ac{ILRO} based \acp{TDC}. In the final section, some alternative techniques discovered during the design and potentially leading to better performance were briefly presented.