Design and Implementation of the Digital Costas Loop Based on Software Defined Radio

The frequency offset is the main problem of the conventional SDR receiver for demodulation. In order to overcome this drawback, the new methods are applied to improve the conventional Costas loop which is used to extract carrier. Firstly, the integration and accumulator is designed to realize the function of low-pass filter, which takes up fewer hardware resources. Secondly, the more accurate arctangent phase detector is used to detect the phase. It replaces the multiplier phase detector, which only obtain the approximate phase difference. Based on these two improvements, the realization of each module about the improved loop is presented. In this paper, first of all, the designed algorithm is simulated on the plat of MATLAB and Modelsim. The simulation results show that the design in this paper is feasible and the method is very effective. Moreover the system has been implemented and tested using field programmable gate arrays (FPGA). The test results indicate that the system has many advantages, including the design structure is simple, the convergence speed and the stability are better, and it can eliminate the frequency offset effectively. Therefore, this system can be widely applied in the engineering.