  Design of stable carrier recovery circuits are used in many applications:
wireless digital communication, optical communication, microwave devices and
other applications. Quadrature Phase-Shift Keying (QPSK, 4-QAM) is used as
modulation technique in many of these applications, since QPSK provides double
the data rate of classic Binary PSK (BPSK) modulation. Analysis of Costas loop
is a hard task because of its non-linearity. In this work we consider two
well-known modifications of 4QAM Costas loop circuits and discuss a new circuit
proposed by J. Ladvanszky. MATLAB Simulink models of the circuits are provided
and the noise analysis is performed.
