The Impact Of Loop Filter In Phase Locked Loop

Abstract

Phase Lock Loop is one of the most essential techniques today and it is used for many different purposes. Typical cases are frequency synthesizer, radio receiver, clock recovery and FM demodulation. Especially as a frequency synthesizer, PLL is a core component in wireless communication devices. One of the most important components of the PLL is loop filter since it gives a decisive impact to the whole system operation. This thesis is focused on specifying the effect of loop filter by designing two different analog loop filters and comparing their outputs. There are a lot of different loop filters that can be used for various PLL systems. Two representative loop filter models were selected, and they were designed to make proper operation with other components. Among many various types of PLL, this thesis covers the analog PLL, therefore loop filters were implemented on the printed circuit board with analog components. Pole and zero points were tried to be designed to be equal so that the other factors can be measured. Other analog components that construct PLL are from MiniCircuit. Hold-in, Pull-in range and phase noise were measured with spectrum analyzer. Especially, the method of hold-in range calculation is introduced and calculated and measured results are compared. With suitable connections of components, whole PLL systems were implemented. The importance of loop filter is specified with the results although it presented considerable discrepancy between expected and practical results. Further studies might be improved by adding frequency divider on the feedback loop and measuring transient response that depends on the transfer function of the system. Designing loop filters with different pole and zero points would give some different results in the aspect of phase noise.