Phase-locked loops

A system for providing in phase synchronism between a first signal and a second signal comprising

• means for comparing the frequency of a reference signal with the frequency of a feedback signal derived from said second signal to produce a first difference signal representing the frequency difference therebetween;
• filter and amplifying means;
• switching means between said frequency comparing means and said filter and amplifying means for supplying said first frequency difference signal to said filter and amplifying means, said latter means thereby providing a first control signal representing said frequency difference;
• oscillator means responsive to said first control signal for producing said second signal, the frequency of said second signal being controlled by said first control signal so that said frequency difference is minimized so that the frequency of said feedback signal derived from said second signal is in substantial synchronism with the frequency of said reference signal;
• means for comparing the phase of said second signal with the phase of said first signal to produce of second difference siganl representing the phase difference therebetween;
• means for actuating said switching means to supply said second phase difference signal to said fixed filter and amplifying means and to prevent said first frequency difference signal from being supplied thereto when said substantial frequency synchronism has been achieved, said filter and amplifying means thereby providing a second control signal;
• said oscillator means being responsive to said second control signal so that said phase difference is minimized and the phase of said second signal is in substantial in-phase synchronism with the phase of said first signal.