Timing Recovery and Scramblers in Data Transmission

Abstract

This paper considers several problems associated with envelope-derived timing recovery, equalization, and scrambling in synchronous data transmission. Particular attention is focused on the time intervals in which periodic data sequences are transmitted, such as during start-up or when an idle code is being transmitted. It is shown that the standard envelope-derived timing-recovery system may be significantly improved by zonal filtering of the received passband signal prior to forming the envelope. For phase-modulated systems, we discuss the limitations of the “precession” technique employed for the purpose of providing a periodic timing wave when there is an input of short period. The advantages of using a phase-locked loop to filter the envelope instead of a narrow-band filter are also described. A study of scrambler operation has provided an extension of previous results concerning the relationship between the input and output period. It is shown that the output period of several scramblers connected in tandem does not necessarily double with the addition of a stage, and that if a particular stage does not lock up then no succeeding stage can lock up.