Schlumberger Technology Corporation

Sangamo Weston, Inc.

Loral Corporation

Gene F Schroeder

William N Waggener, Jr

Guy T Ellis, Jr

An improved digital demodulator capable of demodulating a frequency multiplexed input signal is disclosed. The frequency division multiplexed input signal is sampled in an analog-to-digital converter. The samples are translated by mixing with base band frequency signals to yield real and imaginary values corresponding to phase information in the original modulation signals. After translation, the translated samples are filtered in real and imaginary digital filters. After filtering, the original modulation information is recovered by analysis of the position of vectors in the complex plane represented by the real and imaginary values. In the preferred embodiment, a successive approximation technique is used to determine the angle of the vectors to the real axis. Low pass output filtering may be employed after recovery of the modulation signal for improvements in signal to noise ratio. In the preferred embodiment, the translation is performed by multiplying the input samples by digital values corresponding to sine and cosine values of local oscillator signals at base band frequencies. Preselect filtering may be employed prior to translation to decimate the input samples where possible to reduce subsequent processing requirements. In the preferred embodiment, the unit is operated in a first initialization mode in which a microprocessor supplies appropriate filter coefficients corresponding to the frequencies of the channels to be demodulated. Thereafter, the microprocessor does not directly control supply of samples or coefficients to the filters for evaluation. Very high data rates are thus made possible.

Digital demodulator for frequency-division-multiplexed signals

The disclosure is applicable for use in a logging-while-drilling apparatus for obtaining subsurface measurements 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 the 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 in 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 demodulating the electronic signals to recover the measurement information taken downhole. The uphole circuitry includes an improved carrier tracking loop which is useful, for example, in extracting the carrier from the modulated signal, the extracted carrier then being useful in the demodulation operation. The carrier tracking loop has a voltage controlled oscillator having a control terminal and a comparator for generating a control signal by comparing the phase of a signal derived from the received PSK modulated signal to the phase of a signal derived from the output of the voltage controlled oscillator. The control signal is applied to the control terminal of the oscillator. In accordance with an important feature of the invention, the signal applied to the control terminal is compensated as a function of transitions in the received signal to account for the difference between the nominal frequency of the carrier and the average frequency of the received signal. This difference results from the unidirectional nature of the carrier modulation.

Carrier tracking apparatus and method for a logging-while-drilling system

A variable filter having a plurality of different loopwidths is disclosed. The variable filter, which is typically employed in a phase locked loop, includes a plurality of capacitors which are switchably coupled into and out of operation in the filter in order to change loopwidths. 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.

Growing community of inventors

Sarasota, FL, United States of America

Gene F Schroeder