Nrz phase-locked loop circuit with associated monitor and recovery

Abstract

Monitor circuitry is placed in the closed loop of a phase-locked loop (PLL) circuit and is capable of detecting the existence of an out of frequency window condition and outputting a control signal when the PLL is operating outside the predetermined narrow frequency window. Recovery circuitry, including a phase frequency detector with a wide acquisition range, temporarily replaces the normal phase detector of the PLL, responsive to the control signal from the monitor circuitry. When the monitor circuitry determines that the PLL is back within the predetermined narrow frequency window, the PLL phase detector is switched back into the PLL loop and the phase frequency detector is removed from the PLL loop. The PLL phase detector is then able to lock in on the incoming data stream. In the preferred embodiment, the monitor and recovery circuitry and connected to a specialized PLL circuit which utilizes a 180 degree phase difference between incoming NRZ data and the recovered clock to strobe the data in the center of the input data pulse, thus improving the data error rate.