G - Physics – 01 – V
Patent
G - Physics
01
V
349/5, 333/50
G01V 1/22 (2006.01) E21B 47/12 (2006.01) E21B 47/18 (2006.01) H03L 7/10 (2006.01) H04L 27/227 (2006.01)
Patent
CA 1128187
ABSTRACT OF THE DISCLOSURE The disclosure is applicable for use in a logging- while-drilling apparatus for obtaining subsurface measure- ments during drilling in a fluid-filled borehole. Acoustic carrier waves are generated downhole in the borehole fluid and are PSK modulated in accordance with digital data representative of downhole measurements. The PSK modulation is obtained by momentarily unidirectionally either decreasing or increasing the frequency of the acoustic carrier signal until either a desired phase lag (for a decrease in frequency) or phase lead (for an increase frequency) is imparted to the acoustic carrier signal. An uphole receiving subsystem includes transducers for converting the modulated acoustic carrier waves to electronic signals and circuitry for de- modulating the electronic signals to recover the measurement information taken downhole. A variable loopwidth carrier tracking loop is provided for locking onto the carrier of the received signal. Timing signals from this tracking loop can then be utilized in demodulating the received signal. the tracking loop as disclosed herein comprises a phase- locked loop which includes an oscillator having a controlled input, error signal generating means for generating an error signal as a function of the phase difference between a signal derived from the oscillator and the input signal. A variable filter, having a plurality of different bandwidths, couples the output of the error signal generating means to the control input of the oscillator. A loopwidth controller is coupled to the variable filter and is operative to change the loopwidth of the filter as a function of the input signal. In the preferred embodiment of the invention, the variable filter of the carrier tracking loop includes a plurality of capacitors which are switchably coupled into and out of operation in the filter under control of the loopwidth control means. Those of the capacitors which are not currently operational in the filter are precharged so as to prevent loss of lock in the phase-locked loop when switching to another loopwidth.
322271
Schlumberger Technology Corporation
Smart & Biggar
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