This application note discusses the use of clock distribution circuits in smart phone system design. As smart phones become more popular in the market, additional features such as A-GPS, Bluetooth, WLAN, and DVB-H are now included in cell phone designs. In all of these application modules, a master clock is needed, which is typically an external crystal oscillator. The master clock modulates the transmitted or received data to the proper frequency. This application note compares conventional clock trees that use discrete crystals for different modules or discrete buffer solutions to the clock solution using the integrated STCD1020/1030/1040 series circuits from STMicroelectronics (referred to as STCD10x0 throughout this application note, please refer to the STCD1020, STCD1030, STCD1040 datasheet). The benefits of using STCD10x0 are illustrated, and technical hints are given to help cell phone system designers use the STCD10x0 clock distribution solution.