The recent boom in the mobile telecommunication market has trapped the interest of almost all electronic and communication companies worldwide. New applications arise every day, more and more countries are covered by digital cellular systems and the competition between the several providers has caused prices to drop rapidly. The creation of this essentially new market would not have been possible without the ap pearance of smalI, low-power, high-performant and certainly low-cost mobile termi nals. The evolution in microelectronics has played a dominant role in this by creating digital signal processing (DSP) chips with more and more computing power and com bining the discrete components of the RF front-end on a few ICs. This work is situated in this last area, i. e. the study of the full integration of the RF transceiver on a single die. Furthermore, in order to be compatible with the digital processing technology, a standard CMOS process without tuning, trimming or post-processing steps must be used. This should flatten the road towards the ultimate goal: the single chip mobile phone. The local oscillator (LO) frequency synthesizer poses some major problems for integration and is the subject of this work. The first, and also the largest, part of this text discusses the design of the Voltage Controlled Oscillator (VCO). The general phase noise theory of LC-oscillators is pre sented, and the concept of effective resistance and capacitance is introduced to char acterize and compare the performance of different LC-tanks.