| 摘要 |
In apparatus for obtaining sub- surface measurements during drilling in a fluid-filled borehole digital data representative of downhole measurements are PSK modulated by momentarily unidirectionally changing the frequency of an acoustic carrier signal. An uphole receiving subsystem converts the modulated acoustic carrier waves to electronic signals and includes a bandpass filter (110) which has its centre frequency offset from the nominal carrier frequency in the direction of the unidirectional frequency change applied during modulation, and which may be skewed in the same direction as the offset. The output signal from the filter (110) is coupled to a phase-locked loop (120) which locks onto the carrier of the received signal, and provides timing signals for the demodulation. The phase-locked loop (120) includes a variable bandwidth filter (300), and a controller (205A) to change the bandwidth of the filter (300) as a function of the input signal. The signal applied to the control terminal of the VCO (204) of the loop is compensated as a function of transitions in the received signal to account for the difference, arising from the unidirectional nature of the carrier modulation, between the nominal frequency of the carrier and the average frequency of the received signal. Capacitors in the variable filter (300) are precharged to appropriate voltages to prevent loss of phase- lock during switching between different loopwidths. <IMAGE> |
