In an arrangement comprising a phase control circuit the phase comparator, when receiving input pulses with which the clock pulses are in phase, will produce an output signal having a pulse frequency which is twice the pulse frequency of the input pulses. The ripple on the control-voltage for the oscillator...http://www.google.co.uk/patents/US4689577?utm_source=gb-gplus-sharePatent US4689577 - Circuit for synchronizing an oscillator to a pulse train
