System and method for extracting timing information from a modulated

Abstract

The disclosure relates to an apparatus for receiving a carrier signal modulated with digital symbols, the symbol rate being related to the carrier frequency, such as by a submultiple or a ratio of integers. The invention is an adaptive carrier-aided symbol tracking loop in which signals derived from a symbol-coherent carrier maintain and aid symbol synchronization during periods of no symbol transitions. The invention is particularly applicable to systems wherein the received modulated carrier signal has been transmitted over a noisy transmission path, such as in a logging-while-drilling system. In accordance with the invention there is provided a phase-locked loop which includes a voltage controlled oscillator and an error signal generator for controlling the oscillator. In response to the carrier, a first signal is generated at substantially the symbol rate and, in response to symbol transitions, a second signal is generated which has a phase that depends upon symbol transitions. The error signal generator includes at least one phase comparator which is responsive to a signal derived from the oscillator and to the first and second signals for generating error signals that are applied to the oscillator. If symbol transitions are present, the second signal is used to maintain loop synchronization. If symbol transitions are absent, loop synchronization is maintained by deriving the error signal from the first signal; i.e., a signal at the symbol rate derived from the carrier.