PLL from the three basic components, namely, phase, loop filter and VCO components. Phase for the role of an output voltage is produced, the voltage directly proportional to the average volume of the input signal and the two of the phase difference. Loop low pass filter is the role of filtered output current phase of the unwanted interference component and other components, to achieve the performance required by the loop, and to guarantee the stability loop. VCO is the role of a control voltage changes with the frequency of oscillation voltage. Filters are dependent on the frequency signal processing on the basis of a circuit. An active filter can only work in the normal operational amplifier within the scope of work. In electronic devices, the PLL in the Synthesis of, and other equipment and phase played an important role. Direct Digital Synthesis technology is a new synthesis of the frequency, frequency synthesis technology is a revolution, with electronic engineering and the practical needs of digital integrated circuits and microelectronic technology development, DDS technology increasingly demonstrating its superiority. Key words: PLL oscillator loop filter Synthesis (本人英语系毕业)