Abstract: A phase lock loop which locks a numerically controlled oscillator (NCO) to a non-continuous reference signal. The phase of the reference signal is ascertained by detecting the zero crossings of the signal. The zero crossing is provided to a phase detector which compares the phase of the NCO with that of the reference signal. The phase detector generates a phase error which is provided to a loop filter. The loop filter filters the phase error and provides a control signal to the NCO. The phase detector is enabled for the duration corresponding to the time when the reference signal contains valid phase information. A mechanism detects bad reference cycles and removes the phase error corresponding to these bad reference cycles from consideration. Another mechanism detects phase locked by the phase lock loop by measuring the width of the reference cycles within the duration when the reference signal is valid.