Phase locked loop having rapid tuning

An automatic frequency tuning system, comprising

• a fixed frequency oscillator,
• a first frequency divider having a fixed division factor and connected to be fed by the fixed frequency oscillator to produce a reference frequency,
• a variable frequency oscillator,
• a second divider having a variable division factor and connected to be fed by the variable frequency oscillator so as to produce a variable control frequency,
• phase comparison means repeatedly comparing the phases of the reference and control frequencies whereby to produce in response to each comparison a control signal dependent on the magnitude and direction of any phase error detected,
• integrating means connected to integrate the control signal,
• adjusting means responsive to the integrated control signal and connected to adjust the frequency of the variable frequency oscillator in dependence thereon,
• the integrating means integrating the control signal at such a rate that the adjusting means changes the variable frequency by an amount substantially twice as much as necessary to make the control frequency equal to the reference frequency so as to create a temporary phase error of opposite sign, and
• control means operative, in response to the temporary phase error having substantially compensated for the detected phase error before the next phase comparison, to make such substantially instantaneous reduction in the value of the integrated control signal as to cause the adjusting means to change the variable frequency in the opposite direction and by substantially half the said amount, whereby to tend to reduce the said phase error to zero.