Costa's loop questions, design, tracking and limitations

1. How does Costas loop work?
2. What type of feedback is used in Costas loop and why?

How does Costas loop work?

The Costas loop1 is based on a pair of quadrature modulators - two multipliers fed with carriers in phase-quadrature. These multipliers are in the in-phase (I) and quadrature phase (Q) arms of the arrangement. Each of these multipliers is part of separate synchronous demodulators.

What type of feedback is used in Costas loop and why?

Fig 1 -- In the basic phase-locked loop structure the system relies on feedback formed by mixing a reference signal with a received waveform. received signal are equal in phase and frequency they are locked in sync.