Costas loop The Costas loop uses both in-phase and quadrature phase detectors to keep a VCO centered at the suppressed-carrier frequency, as shown in Figure 6.25. Let the input be b(t)cos (!ctþi), where b(t)¼1. Label the signals for each path in the block diagram and show that the Costas loop keeps the VCO centered at the suppressed-carrier frequency, insensitive to b(t), and thus provides an acceptable reference signal for the synchronization detector.