a phase - locked loop having improved off - frequency detection . a variable frequency output signal is to be precisely locked to the frequency and phase of an alternating input signal . this is accomplished by alternately utilizing two feedback loops . a reference signal , having a frequency approximately equal to the frequency of the input signal is input to an initialization feedback loop in which it is mixed with the output signal . the initialization loop produces a feedback signal for controlling a voltage controlled oscillator which generates the output signal . an off - frequency detector detects the frequency difference between the output signal and the reference signal . when this frequency difference decreases below a predetermined level , the off - frequency detector disables the initialization feedback loop and enables a primary feedback loop . in the primary feedback loop the phase of the output signal is compared to the phase of the input signal , and the difference signal controls the voltage controlled oscillator in such a manner that the phase of the output signal tracks the phase of the input signal . with the scheme , the initialization loop , which has a wide bandwidth , is used to control the voltage controlled oscillator over a wide range of frequencies of the output signal . when the frequency of the output signal has been brought within the capture range of the primary loop , operation is switched to the primary loop . the primary loop has a narrower bandwidth and therefore provides better noise rejection than the initialization loop .