Borehole acoustic telemetry clock synchronization system

Abstract

In a borehole telemetry system for acoustically transmitting data over a pipe suspended in a borehole, the level of noise in the data stream is inherently high, making the use of discrete frequencies advantageous to provide useful data. Any drift in electrical circuits which might affect or be affected by the use of precise frequencies is undesirable. A clock synchronization or bit rate clock circuit in a data signal processing system is operated by a local crystal oscillator which is operated at substantially the same precise frequency as a crystal oscillator that is used to generate and transmit the data signal. The synchronization of the bit rate clock is arranged to make only small corrections in order to keep the frequencies fully synchronized at all times thus false signals will not shift the clocks out of synchronization. The bit rate clock has an early late detector that determines if received data is expected early or late with respect to the data clock that is transmitting the data and shifts a window in the circuit accordingly. Another feature includes a fast search mode that causes the clock systems of the transmitted data signal and the clock system in the data signal processing circuit to move rapidly into synchronization when the signals are far out of synchronization, bypassing arrangements in the circuit that make only small corrections. The output of the bit rate clock operates a transmitter section to transmit the data signal at any desired frequency but at the same bit rate.