Phase and frequency locked loops are used concurrently in the control of an oscillator to expand a clock signal or generate a high frequency clock signal based on a reference clock signal. The frequency locked loop adjusts the frequency of operation of the oscillator as a coarse adjustment to oscillator operation. The phase locked loop provides a finer adjustment of the operation of the oscillator to align a characteristic feature, typically a leading rising edge, of the oscillator generated clock signal with the same characteristic of the reference clock signal.
The frequency locked loop and phase locked loop have independently sensed operation of the oscillator and also have independently adjusted operation of the oscillator. Since the frequency locked loop and phase locked loop independently sense operation of the oscillator, inconsistent indications of the status of the oscillator are possible. The corrective action taken by the two loops in response to the sensed inconsistent indications tries to adjust the frequency of operation of the oscillator in opposite directions until the oscillator operates at equilibrium. In this mode of operation, a condition known as false locking, the oscillator operates at a stable frequency that is not the desired frequency. False locking is a result of sensing the operation of the phase locked loop using one detector and sensing the operation of the frequency locked loop using a different detector.
Additionally, the frequency detectors previously used in the control of frequency locked loops had a very limited range of accuracy, beyond which incorrect assessment of relative frequency of the reference clock signal and the oscillator generated clock signal could be wrong more than half of the time.
It would be desirable to sense operation of the phase locked loop and operation of the frequency locked loop using the same detector to eliminate the potential discrepancy introduced by sensing operation of the phase locked loop and frequency locked loop by different sensors, and simultaneously provide a frequency detector with a wider range of accuracy.