Phase noise is a common hardware impairment, resulting from the frequency instability of voltage-controlled oscillators (VCO). To improve the phase noise performance of a VCO, they are typically connected to a control circuit. This control circuit is known as phase locked loop (PLL). It is commonly used as a frequency synthesizing circuit for the carrier frequency in mobile communication transceivers. Universal Software Radio Peripherals (USRP) are widely used in mobile communication research.