INTERFACE APPARATUS FOR RECEIVING AND MONITORING PILOT SIGNALS WHICH CONTROL A TIMING SIGNAL GENERATOR

摘要

A master oscillator is phase locked to a wave derived from the countdown of a pilot signal received over either a principal or a standby pilot line. An interface unit for each pilot line compares the running count of the downcounter at the beginning of a cycle of the master oscillator wave with a stored count obtained in a prior cycle. If there is a substantial comparison match, the running count is overwritten into the store to compensate for slow drift between the frequenices of the pilot and oscillator. If a substantial mismatch is encountered, a pilot malfunction is presumed and the oscillator switches to the standby pilot or free runs if the standby pilot status is bad. The initial mismatch is presumed to be a momentary malfunction, such as a noise hit, and the stored count is "stuffed" into the downcounter at the beginning of the oscillator wave cycle to restore the prior phase relation between the downcounter operating cycle and the oscillator wave. Repeated mismatches are presumed to indicate a prolonged malfunction of the pilot and the downcounter is phase aligned with the downcounter of the other unit if the other pilot status is good or phase aligned with the oscillator wave if the other pilot status is bad.