Abstract: In a phase controlled demodulator, a modulated digital input signal is demodulated into quadrature signals with a carrier recovered by a voltage controlled oscillator. A phase difference between the quadrature signals is detected by a phase detector of Costas loop and applied through a loop filter to the voltage controlled oscillator to control the frequency and phase of the recovered carrier when the frequency deviation between the received and recovered carriers is within a phase control range. When the frequency deviation exceeds the phase control range, the output of the phase detector is a beat of the two carriers and a frequency sweep control voltage is applied to the VCO to search for the missing carrier. The magnitude of the beat is detected and compared with a predetermined threshold value.