Carrier recovery loop for reception of quadraphase shift keyed signals

Abstract

A carrier recovery loop for a burst type QPSK system. A QPSK signal source of frequency f.sub.1 has its output frequency doubled and then doubled again to produce a signal of 4f.sub.1 with the QPSK modulation thereby removed. Also provided are first and second phase locked loops comprising a common voltage controlled oscillator (VCO), and separate ones of first and second phase detectors, respectively, with each detector having a characteristic output which varies sinusoidally with linear variation of the phase difference of the two signals supplied thereto and with the negative-going cross-over transitions of the characteristic output constituting unstable nulls. A third x2 frequency multiplier connects the output of the VCO to the first phase detector, and a fourth x2 frequency multiplier connects the output of the third x2 frequency multiplier to the second phase detector. The output signals of the phase detectors are compared to generate a control signal indicating when the output of the second phase detector is near an unstable null at which time a switch responds to the control signal to connect the output of the first phase detector to the input of the VCO.