The MT90401 is a digital phase locked loop (DPLL) that is designed to synchronize SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network) networking equipment. The MT90401 is used to ensure that the timing of outgoing signals remains within the limits specified by Telcordia, ANSI and the ITU during normal operation and in the presence of disturbances on the incoming synchronization signals. LOCK