Fig. 1. Classic structure of Costas loop

Fig. 2. Structure of four phase Costas loop

Fig. 3. Structure of improved Costas loop

Fig. 4. Improved Costas loop phase tracking diagram

Fig. 5. Comparison of enlarged diagram of frequency capture and tracking

Fig. 6. Costas loop performance simulation diagram based on Matlab

Fig. 7. Comparison of data before and after Costas loop I and Q

Fig. 8. Simulation diagram of random source

Fig. 9. Constellation mapping simulation diagram

Fig. 10. Modulation simulation diagram

Fig. 11. Register transfer level (RTL) diagram of demodulator module

Fig. 12. Data simulation before and after demodulating LPF

Fig. 13. Costas loop output simulation diagram

Fig. 14. Simulation diagram of decision module

Fig. 15. Comparison of simulation results

Fig. 16. Comparison of Costas loop frequency tracking curves. (a) Frequency tracking response of improved Coatas loop; (b) frequency tracking response of traditional Coatas loop