Carrier recovery loop for reception of quadraphase shift keyed signals

A carrier recovery loop for the reception of quadraphase shift keyed (QPSK) signals e.sub.1 of frequency f.sub.1 comprising:

• first and second phase detector means each responsive to the phases of two signals supplied thereto to generate an output signal whose amplitude varies substantially sinusoidally with a linear change in phase difference between the two signals supplied thereto and with the negative going transitions of said sinusoidal output signal being unstable null points;
• first frequency multiplier means responsive to e.sub.1 to supply a signal e.sub.2 of frequency 2f.sub.1 to said first phase detector means and a signal e.sub.3 of frequency 4f.sub.1 to said second phase detector means;
• a voltage controlled oscillator (VCO) having a frequency control input and an output signal e.sub.vco of frequency f.sub.vco ;
• second frequency multiplier means responsive to e.sub.vco to supply a signal e'.sub.vco of frequency 2f.sub.vco to said first phase detector means and a signal e".sub.vco of frequency 4f.sub.vco to said second phase detector means;
• switching means responsive to a control signal to connect the output of said first phase detector means to the frequency control input of said VCO when the output signal of said first phase detector means is greater than the output signal of said second phase detector means, and to connect the output of said second phase detector means to the frequency control input of said VCO during the remainder of the time;
• and
• amplitude responsive means responsive to the relative amplitudes of the output signals of said first and second phase detector means to generate said control signal.

A carrier recovery loop for the reception of a quadraphase shift keyed (QPSK) signal e.sub.1 of nominal frequency f.sub.1 and comprising:

• first means responsive to e.sub.1 to produce a signal e.sub.2 of frequency 2f.sub.1 and a second signal e.sub.3 of frequency 4f.sub.1 ;
• a voltage controlled oscillator (VCO) for generating a signal e.sub.vco of frequency f.sub.vco ;
• second means responsive to e.sub.vco to generate a signal e'.sub.vco of frequency 2f.sub.vco and a signal e".sub.vco of frequency 4f.sub.vco ;
• first and second phase locked loops comprising:
• first and second phase detector means, respectively, responsive to e.sub.2 and e'.sub.vco and to e.sub.3 and e".sub.vco respectively, to generate output signals e.sub.4 and e.sub.5, each of which varies substantially sinusoidally with a linear change in the phase difference of the two signals supplied thereto and further with e.sub.5 having twice the rate of sinusoidal variation as does e.sub.4, and further with the negative-going cross-over transitions of e.sub.5 constituting unstable null points in said second phase locked loop;
• and
• control means responsive to the comparative amplitudes of e.sub.4 and e.sub.5 to connect e.sub.4 to said VCO during predetermined time periods bracketing each unstable null of e.sub.5 and to connect e.sub.5 to said VCO during the remaining time.

In a system for receiving bursts of quadraphase shift keyed (QPSK) signals e.sub.1 of frequency f.sub.1, a method of recovering and maintaining the phase of the carrier signal comprising the steps of:

• generating a voltage controlled signal e.sub.vco of frequency f.sub.vco at the output of a voltage controlled oscillator (VCO);
• multiplying the frequency f.sub.vco of e.sub.vco by two and by four to produce signals e'.sub.vco of frequency 2f.sub.vco and e".sub.vco of frequency 4f.sub.VCO ;
• multiplying the frequency f.sub.1 of e.sub.1 by two and by four to produce signals e.sub.2 of frequency 2f.sub.1, and e.sub.3 of frequency 4f.sub.1 ;
• generating signals e.sub.4 and e.sub.5 indicating respectively the phase difference between the signals e.sub.2 and e'.sub.vco and between the signals e.sub.3 and e".sub.vco, and where e.sub.4 and e.sub.5 each varies in a substantially sinusoidal manner with linear variation in the phase difference between the signals e.sub.2 and e'.sub.vco and the signals e.sub.3 and e".sub.vco ;
• and where the rate of sinusoidal variation for e.sub.5 is twice that of e.sub.4 ;
• and
• supplying e.sub.4 to the VCO when e.sub.5 is within a predetermined angular distance from the negative-going cross-over transition of the sinusoidal variation of e.sub.5 ;
• and
• supplying e.sub.5 to the VCO when e.sub.5 is outside said predetermined angular distance.

In a system employing bursts of quadraphase shift keyed (QPSK) signals e.sub.1 of frequency f.sub.1, the method of recovering the phase of the carrier signal comprising the steps of:

• generating in a voltage controlled oscillator (VCO) a voltage controlled signal e.sub.vco of frequency f.sub.vco ;
• doubling and quadrupling f.sub.vco to produce signals e'.sub.vco of frequency 2f.sub.vco and e".sub.vco of frequency 4f.sub.vco ;
• doubling and quadrupling f.sub.1 to produce signals e.sub.2 of frequency 2f.sub.1, and e.sub.3 of frequency 4f.sub.1 ;
• generating signals e.sub.4 and e.sub.5 indicating respectively the phase difference between e.sub.2 and e'.sub.vco and between e.sub.3 and e".sub.vco, and where e.sub.4 and e.sub.5 each varies in a substantially sinusoidal manner with linear variation in the phase differences between e.sub.2 and e'.sub.vco and between e.sub.3 and e".sub.vco ;
• supplying e.sub.4 to the VCO when the amplitude of e.sub.5 is less than the amplitude of e.sub.4 ;
• and
• supplying e.sub.5 to the VCO when the amplitude of e.sub.5 is greater than the amplitude of e.sub.4.