Digital implementation of a second-order Costas loop demodulator.

摘要

This project describes the digital implementation of a Second-Order Costas Loop demodulator using simulation software SystemView by Elanix. The digital design of the system is based on direct transformation of every analog component of the Costas Loop to its respective discrete-time and subsequent digital domain. The relation between every component in the analog system that has to do with carrier tracking and synchronization is based on analog phase-locked loop theory. We used Costas Loop theory, which describes the Costas Loop as two phase locked-loops operating in quadrature phase to each other, to develop a linear model practically identical to that of the analog phase-locked loop. The difference between these two models consists of the phase detector gain, which is evident when establishing the phase detector gain of each system in relation to the amplitude of the input and output signals. We then used this model and phase-locked loop theory to develop the analog design of the Costas Loop and its subsequent transformation to the digital domain. The design of the arm filters, however, did not follow the derived linear model. Instead, we used conventional Costas Loop theory and the filter design tools of SystemView to design these two filters. The final digital implementation of the system was then realized using the design and simulation tools of SystemView in addition to fixed-point arithmetic theory to represent the parameters and the values of the signals processed.