Analysis of frequency detection capability of alexander phase detector

An analysis of a frequency detection capability of the Alexander phase detector is described. Based on the analysis, a simple modified implementation achieving unilateral frequency acquisition is proposed. The probability of the output occurrence and the frequency detection gain are calculated and verified by simulation. The gain curves show that the proposed phase-frequency detector (PFD) has a unilateral frequency detection capability without a range limitation. The capability is demonstrated through transient simulation of a PLL-based clock and data recovery (CDR) utilising the proposed PFD. The CDR achieves both phase and frequency acquisitions when the initial clock frequency is lower than the data rate. Moreover, the seamless transition from frequency locking to phase locking is obtained.