CA1119686A

CA1119686A - System for extracting timing information from a modulated carrier

Info

Publication number: CA1119686A
Application number: CA000277997A
Authority: CA
Inventors: William N. Waggener
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 1976-05-10
Filing date: 1977-05-09
Publication date: 1982-03-09
Also published as: BR7702901A; EG13329A; AU513642B2; GR63199B; DE2721023A1; ES458606A1; IT1101653B; FR2351549A1; IE45139L; AR222004A1; IE45139B1; AU2486077A; GB1579488A; NL7705119A; FR2351549B1; OA05657A; NO771514L

Abstract

ABSTRACT OF THE DISCLOSURE
The disclosure relates to a technique 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. An adaptive carrier-aided symbol tracking loop is provided in which signals derived from a symbol-coherent carrier maintain and aid symbol synchronization during periods of no symbol transitions.

Description

23.257 BACKGROUND OF THE INVEMTION
___ ____~__ _ This invention relates to digital signalling system6 and, more particularly, to the extr-action o~ timing lnformation from digital signalling systems.
~ hen digital data is transmitted by electrical, acoustic, mechanical or other means, it is necessary at the receiving end to extract timing ini~ormatlon from the recelved signal in order to identify the start time epoch of each trans-mitted digital symbol. Typically, at the transmitter end a carrier signal was modulated with digital symbols, the symbol rate being related to the carrier f'requency, generally either as a multiple thereo~ or as a ratio of integers. Xn systems where the recei~ed signal has become noisy~ the problem of accurately obtaining timing information is particularly acute.
For example, one type of system wherein a noisy transmission path is experienced is a so-called t'logging-while-drilling" system wherein well logging data is transmitted to the surface of a borehole via a drill pipe during the drilling operation. In such instance~ it is difficult to transmit acquired data ~o the surface electrically unless the drill pipe is provided with a special insulated oonductor including means ~or forming appropriate connectlons for the conductor at the drill pipe joints. Accord-ingly, there have been proposed varlous systems for transmitting the logging data~acouskically, either through the drill pipe or ~; in drilling liquid. Typically, the data is converted to digital form and then used to modulate a carrier signal, such as by "phase-shift keying" ("PSK"), "frequency shift keyi~g" ("F'SK"), or "amplitude-shift keying" ("ASK"). At the surface, the ::: ::

; ~
~ -2-~ ~ , 23.257 acoustic slgnal ls detected and demodulated in order to provlde the desired readout lnformation (see e.g. U.S. Patent No. 3,886,495).
In this type of system, or any other wherein the transmission medium is less than ideal, the signal experiences substantial noise and this renders it difficult to extract symbol timlng information. Some techniques for extracting symbol timing were described in an article entitled "Recent Advances in Symbol Synchronization" by W. N. Waggener, which appeared in Yolume 12, No. 1 of Instrument Society of America Transaction~, at page 7.
.
When digi-tal data is transmitted by modulating a carrier and the carrier frequency is coherently rolated to the symbol rate, timing in~ormation from the carrier wave can be used to aid in extracting symbol timing h~ormatlon. By way o~
a numerical example, if the symbol rate were, say, 600 symbols per second, and the modulated carrier ~requency was, say, 2400 hertz, there are four carrier cycles per symbol period. Simple division of the carrier ~requency by a ~actor of 4 ~ould yield a clock ~requency equal to the symbol rate. Although the divided rrequency would be substantially correct, the division process produces an ambiguous phase which must be resolved.
Symbol timing information could also be extracted ~rom the digital data independently o~ the carrier frequency, as is disclosed in the above-referenced publication. Symbol tlming information can only be obtained, howeverg when a symbol changes from one value to another value. Thus, extended periods without such a transition render it difficult to maintain symbol synchronlzation. Also, where unusually high noiselevels are experienced during transmission, the problem of maintaining synchronization is intensified.

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, ' It is an ob~ect o~ the present lnvention to provide a system which minimizes errors during detection of timing information.

SUMMARY OF THE INVENTION

In accordance with this and other ob~ects, one aspect of the present invention is directed to a method for extractlng timing lnformation from a carrier signal modulated with dlgital symbols, the symbol rate being relaked to the carrier frequency, wherein a phase-locked loop generates an error signal and includes an oscil~ator responsive to said error signal, and comprlsing the steps of: generating a first signal in response to said carrier signal at substantially the symbol rate; generating a second signal responsive to symbol transltions; and combining said ~irst signal, said second signal and a signal derived from said oscillator to generate said error signal.
Another aspect of the present invention is directed to an ;apparatus for extracting timing information from a carrier slgnal modulated with digital symbols, the symbol rate being related to the carrier frequency, comprising: a phase-locked loop including an oscillator and error signal generating means for controlling said oscillatorJ means responsive to said carrier for generating a ~irst signal at substantially the symbol rate; means responsive to symbol transitlons ~or generating a second signal, and said error signal generating means bein~ responsive to said first signal, said second signal and a signal derived from said oscillator ~or generatlng an error signal to control said oscillator.

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23.257 BRIEF DRSCRIPTIOM OF THE DR~WINGS

FIG. 1 is a schematlc block diagram o~ a system in accordance with an embodiment of the invention.
FIG. 2 is a schematic block diagram of a system in accordance with another embodiment of the invention.
FIG. 3 is a diagram, partially in block form, showing the present lnvention incorporated ln a system for well logging.

~ 3.257 DESCRIPTION OF THE PREF~RRED EMBODIMENTS

Referring to FIG. 1, there is shown a schematic block diagram of a system in accordance with the inventlon for extracting timing in~ormation from a received input carrier slgnal modulated with dlgital symbols. The basic tlming ls provided by a phase-locked loop 20 which lncludes, inter alla~ a voltage controlled oscillator 21, a loop filter 22, and an error slgnal generating means 23. In the present embodiment, the error signal generatlng means ls a summlng amplifier comprising an operatlonal amplif'ler 24 arranged in conventional manner to serve as a summing amplifier;
i.e., with appropriate ~eedback reslstance and a summing ~unction at lts invertlng input. The inputs to the summing ~unction of summing amplifier 23, via weighting resistors Rl and R2~ are the outputs of phase comparators 31 and 32 respectively.
Phase comparator 31 receives as one of its lnputs a signal having a characteristic frequency fl which is derived from the oscillator 21 by a digital clock di~ider 400 The other input to comparator 31 is derived from the carrler of the received signal.
A carrier clock ls obtained ~rom the carrier and ls input to a counter 50 whose characteristic count cycle is set at the ratio between the carrler frequency and the symbol rate, this ratlo assumed to be an integer N ~or ease of illustration. The output of the counter 50, whlch is accordingly at substantially the symbol rateg is ooupled to the phase comparator 31 via AND gate 55.

;:
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23.257 One lnput to the phase comparator 32 ls a signal at a reference ~requency f2, which is also derived from osclllator 21 by clock divlder 40 and will generally be the same as fl- The other input to phase comparator 32 is the output o~ a transltion detector 60 whose output is a measure of the detected symbol transitions of the symbol-modulatecL carrler input signal. In particular, the input to the transi.tion detector 60 is derlved from the received input data by conventional filtering and limiting circuitry, so as to obtain a "clean" version o~ the received symbol data. I'he output o~ loop filter 22 which is indicative o~ the error level, is coupled to a threshold detector 70 whose outpuk is a logical '101' if a prescribed threshold level is e~ceeded. Conversely, the threshold detector output is a logical "l" if the khreshold level is not e~ceeded. The output of threshold detector 70 is the second input to the AND gate 55.
Operation of the system o~ FIG. ~ is as ~ollows:
The output o~ the transition detector is phase compared to the frequency f2 which is derived from the phase-locXed loop oscil-lator 21. The resultant signal (re~erred to herein generally as the "second error component signal") is weighted by R2 and applied as an error signal which drives the phase-locked loop to the ~requency and phase of khe symbol transitions. ~he output of the transition detector 60 also resets the counter 50. As the system is initially activated, counter 50 can randomly contain any count. The initial reset signal ~rom transitlon detector 60 thus synchronizes it wikh the symbol signals so that the carrier can sultably replace the symbol transitionsshould the latter stop for a time. Once the loop 20 is in phase-lock, the error signal in the loop, as measured at the output o~ the loop filter 22, will be small enough that the output of the threshold detector 70 will become a ~ ~9 ~ ~ 23.257 logical "1" which, in turn, enables the AND gate 55. Now, clock pulses ~rom the counter 50 are ~ed to phase comparator 31 to be phase compared wlth the signal at frequency fl.
The phase error indicated at the output of comparator 31 (re~erred to herein generally as the "first error component signal") is weighted by Rl and summed by sur~ning ampli~ier 23 with the measured phase error based on symbol transitions.
A ~eature of the invention is that if there are no symbol transitions for a period of time, the carrier clock counter 50 maintains loop synchronization. When symbol transltions occur, the phase error contribution based on symbol transi-tions is more heavily weighted than the phase error contri-bution based on the carrier clock by virtue o~ the different value resistors Rl and R2, so the loop tends to readJust the loop oscillator phase to match the actual symbol transltion timing. The resistor Rl is variable to facllitate adJustment of the weightlng ratio, when desired. If the loop loses lock, the lncrease in loop error signal will cause the error contribution due to the carrier clock 50 to be inhibited by virtue o~ AND gate 55 being disabled. This inhibiting action continues until the occurrence of symbol transitions which relock the phase-locked loop. The necessary æystem timing is obtained, for example, from clock divider 40.
The voltage controlled qscillator 21 is pre~erably set to run at a frequency which is a multiple o~ the symbol rate so that the digltal clock divider 40 can provide :
multiple phases of the reference frequency. This permits the optimum clock phase to be selected for the two phase comparators and permits compensation for any known fixed :: :
;~:::

23 .257 f~ 3~8~i phase shift between the carrier transltlons and the symbol transi~
tions (by using approriate fl and f2). Preferably~ the carrier frequency should be a relatlvely large multiple of the symbol rate so as to minimize the phase offset due to the resolutlon o~
the counter 50. In cases when the carrier frequency is only a small multiple of the symbol rate, a coherent frequency rnultlplier may be employed to increase the carrier reference clock frequency.

; _g_ 23.257 ~ 3 ~

A less complex version of the system of FIG. 1 is shown in FIG. Il. In this embodiment, the symbol transition detector 60 resets the counter 50 each time a symbol transition occurs. A single phase comparator 31 is employed and deri~Jes an error signal by comparison o~ the output o~ the clock divider 40 with the output of colmter 50. When symbol transitions are absent, the counter ~Ifree runs" ancL maintains loop lock.
It is readily apparent from ~I~. 4 that counter 50 provides an output pulse ln response to the occurence of a reset pulse and/or when it reaches lts preset count. Should a syn chronous counter implementation be selected ~or counter 50, these two events shou~ oocur within the same carrier cloc~ period so that both result in one counter output pulse. A disadvantage inherent in using a synchronous counter is due ko the ~act that the counter output is in phase with the carrler clock and not necessarily with the data signal. Thus, although this imple-mentation is less complex than the system of FIG. 1, the resolution of counter 50 is a limiting ~actor on per~ormance.
The resolution error can be improved by using an asynchronous counter. It generates an output pulse in immediate response to a reset pulse when a symbol transition is detecked.
Consequently, the precise phase o~ the da~a signal is provided to phase comparator 31 to maintain the phase lock lcop accurately locked onko the symbol transitions. However, it should be noted thak i~ the carrier ~requency is much higher than the data fre-quency~ the phase di~ference between the carrier and the symbol transitlons is so small that it may be aoceptable even with the use of a synchronous counter.
The above-~discussed timing signal extraction system has been found useful in the field of well logging, and, more ::
:

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~119~6 particularly, ~or the technique of "logging-while-drilling"
previously mentioned. An apparatus capable of performing thls technique is depicted in FIG. 3 in connection with a typical rotary drilling apparatus 100. Derrick 102 supports drill string 104 extending into the borehole 107 and suspended ~rom hook 105 by swivel 106. Drill string 104 includes bit 108, one or more drill collars 110, and a length of drill pipe 112. Pipe 112 is coupled to a kelley 114 which extends through rotary drive mechanism 116 driven by one or more motors 118.
Positioned near the entrance of borehole 107 is a conventlonal drilling fluid~ or mud, circulating system 115.
The circulating system includes a pump 120 whieh circulates mud from pit 122 into mud line 124 and then downwardly through kelley 114, drill string 104 and out the orifices in bit 108. The mud then returns upwardly in the annulus 126 and exits casing 128 through opening 130 into mud return line 132 back to mud pit 122.
A logglng^while-drilling system 134 is suitably po-sitioned downhole in proximity to bit 108. Perhaps the most promising approach to date for transmitting signals uphole is a system which includes a signal generator 136 controllably driven for selecti~ely interruptlng the flow of the mud to thereby impart an acoustic signal to it. ~he acoustic signal travels upward through the circulating mud in drill string 104 and is detected at the surface by transducer 138 attached to mud line 124.
Cartridge 140 is provided ~or sensing the various downhole con-ditions and for driving signal generator 136 in accordance with the values of the sensed condltions. Power supply 142 serves to energize cartridge 140 and lncludes a turblne positioned within the flow of the clrculating mud. The details o~ such a system : :~
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23.257 ~ 6 are disclosed in U.S. Patent Mo. 3,309,656, for example.
The timing signal extraction circuit descrlbed above is positioned at the surface to receive at its input khe filtered signal from transducer 138. A pump-noise filter 144 removes the mud disturbances caused by pump operation which would other-wise seriously distort the signal from generator 136, as dis-closed in U.S. Patent No. 3,742,443. Processing circuit 146 receives the filtered signal and may include further means to improve the signal quality such as a bandpass filter and an automatic gain control. However, the primary function of pro-cessing circuit 146 is to recover a carrier clock as well as to demodulate the output of filter 14LI~ These tasks are per~ormed in a well known manner by available circuitry. The above described timing reconstruction circuik 148 (see FIGS. 1 and 2) receives two inputs 149 and 150 ~rom processing circuit 146 corresponding, respectively, to the carrier clock and the de-modulated data signal. A suitable clock signal in phase with the data is produoed by circuit 148 in accordance with the de-tailed discussion provided above and then input to data reconskruction circult 150. The reconstructed clock is utilized by circuit 152 to recover the information in the data signal on line 150 which is indica~lve of the measured downhole condition.
If PSK modulation is used, circuit 152 may operate to produce a bit decision signal from the input 150 which is then passed through a d1fferentia1 decoder which indicates the occurrence of a trans~tion.
A decommutator may also be included which functions under the timing control o~ c-ircuit 148 to determine the beginning of a ~, ~

~ -12-:; : ~
:

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~ 3~ ~ 23.257 data word. Circuitry of this sort is well known and no further details are there~ore deemed necessary. (For a presentatlon of the proces~ing and data reconstruction circuits see Viterbi, A.J~, Principles o~ Coherent Communication, McGraw-Hill, 1966 pgs. 286--Z92). The data words and/or measurements derived from the decommutator output may be selectively displayed and/or recorded on a suitable devlce 154.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for extracting timing information from a carrier signal modulated with digital symbols, the symbol rate being related to the carrier frequency, wherein a phase-locked loop generates an error signal and includes an oscillator responsive to said error signal, comprising the steps of:
generating a first signal in response to said carrier signal at substantially the symbol rate;
generating a second signal responsive to symbol transitions; and combining said first signal, said second signal and a signal derived from said oscillator to generate said error signal.

2. The method of claim 1, wherein the digital symbol rate is an integral submultiple of the carrier frequency.

3. The method of claim 1, wherein the signal derived from said oscillator is an integral submultiple of the oscillator frequency.

4. The method of claim 1, 2, or 3, wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions.

5. The method of any one of claims 1 to 3, wherein said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator.

6. The method of any one of claims 1 to 3, wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator to produce third and fourth signals; summing the third and fourth signals.

7. The method of any one of claims 1 to 3 wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator to produce third and fourth signals; and further comprising weighting the third and fourth signals to emphasize the effect of said fourth signal relative to that of the second signal; summing the third and fourth signals.

8. The method of any of of claims 1 to 3, wherein said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator and wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the step of disabling said first signal in response to a predetermined condition.

9. The method of any one of claims 1 to 3, wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator produce third and fourth signals; summing the third and fourth signals and wherein said combining step com-prises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further compris-ing the step of disabling said first signal in response to a predetermined condition.

10. The method of any one of claims 1 to 3 wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator to produce third and fourth signals; and further comprising weighting the third and fourth signals to emphasize the effect of said fourth signal relative to that of the second signal; summing the third and fourth signals and wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the step of disabling said first signal in response to a predetermined condition.

11. The method of any one of claims 1 to 3, wherein said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator and wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions, and further comprising the step of disabling said first signal in response to a predetermined condition, said disabling step including detecting the output level of said error signal and disabling said first signal when said output level exceeds a preselected level.

12. The method of any one of claims 1 to 3, wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator to produce third and fourth signals; summing the third and fourth signals and wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further com-prising the step of disabling said first signal in response to a predetermined condition, said disabling step including detecting the output level of said error signal and disabling said first signal when said output level exceeds a preselected level.

13. The method of any one of claims 1 to 3 wherein the combining step comprises comparing the phases of the first and second signals, respectively, with signals derived from said oscillator to produce third and fourth signals; and further comprising weighting the third and fourth signals to emphasize the effect of said fourth signal relative to that of the second signal; summing the third and fourth signals and wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the step of disabling said first signal in response to a predetermined condition, said disabling step including detecting the output level of said error signal and disabling said first signal when said output level exceeds a preselected level.

14. The method of any one of claims 1 to 3, wherein the first signal is derived from the carrier signal with a counter; said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and said first and second signals combining step comprises control-ling the reset of said counter with said second signal.

15. The method of claim 1, 2 or 3, wherein said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response to the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface.

16. The method of any one of claims 1 to 3, wherein said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface.

17. The method of claim 1, 2, or 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid, and said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response to the sensed parameter, modulating said carrier signal with said symbols, trans- mitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface.

18. The method of any one of claims 1 to 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid and said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface.

19. The method of claim 1, 2, or 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid; and said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response to the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface, said detection including the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal and reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter.

20. The method of any one of claims 1 to 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid and said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and sensing a given parameter during a borehole drilling operation, generating said symbols in response the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface; and detecting said transmitted modulated signal at the surface further comprising the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal and reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter.

21. The method of claim 1, 2, or 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid; and said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transitions; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response to the sensed parameter, modulating said carrier signal with said symbols, trans- mitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface, said detection including the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal and reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter; recording and/or displaying an output derived from the reconstructed symbols.

22. The method of any one of claims 1 to 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid and said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface, said detection including the steps of comprising the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal and reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter; recording and/or displaying an output derived from the reconstructed symbols.

23. The method of claim 1, 2, or 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically transmitted to the surface through said drilling fluid,and said combining step comprises comparing the phase of the signal derived from the oscillator with the phase of said second signal in the presence of symbol transitions and the phase of said first signal at least in the absence of symbol transition; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response to the sensed parameter, modulating said carrier signal with said symbols, trans- mitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface, said detection including the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal; reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter; and synchronizing said first and second signals.

24. The method of any one of claims 1 to 3, wherein drilling fluid is circulated from the surface downhole and then back to the surface during drilling; and said modulated signal is acoustically trans- mitted to the surface through said drilling fluid and said combining step comprises combining the first and second signals and comparing the phase of the resulting signal with the phase of the signal derived from said oscillator; and further comprising the steps of sensing a given parameter during a borehole drilling operation, generating said symbols in response the sensed parameter, modulating said carrier signal with said symbols, transmitting said modulated signal to the surface, and detecting said transmitted modulated signal at the surface, said detection including the steps of obtaining a carrier signal and a symbol transition signal from said detected modulated signal and reconstructing said symbols at the surface under the timing control of the signal derived from said oscillator to provide measurements of said parameter;
and synchronizing said first and second signals.

25. An apparatus for extracting timing information from a carrier signal modulated with digital symbols, the symbol rate being related to the carrier frequency, comprising:
a phase-locked loop including an oscillator and error signal generating means for controlling said oscillator;
means responsive to said carrier for generating a first signal at substantially the symbol rate;
means responsive to symbol transitions for generating a second signal; and said error signal generating means being responsive to said first signal, said second signal and a signal derived from said oscillator for generating an error signal to control said oscillator.

26. The apparatus of claim 25 wherein said first signal generating means comprises a counter.

27. The apparatus of claim 25 or 26 wherein said digital symbol rate is an integral submultiple of the carrier frequency.

28. The apparatus of claim 25, 26 or 27 wherein said signal derived from said oscillator is an integral submultiple of the frequency of said oscillator.

29. The apparatus of claim 26, wherein said signal generating means has two inputs and said first and second signal generating means are coupled to one input of the error signal generating means and said signal derived from said oscillator is applied to the other input.

30. The apparatus of claim 26 wherein said digital symbol rate is an integral submultiple of the carrier frequency; wherein said signal generating means has two inputs; said first and second signal generating means are coupled to one input of the error signal generating means and said signal derived from said oscillator is an integral submultiple of the frequency of said oscillator and is applied to the other input.

31. The apparatus of claim 26, further comprising a first comparator means responsive to said first signal and said signal derived from the oscillator to generate a third signal; a second comparator means responsive to said second signal and a signal derived from the oscillator to generate a fourth signal; and a means for summing said third and fourth signal and for providing the summed signal to said error signal generating means.

32. The apparatus of claim 26 wherein said digital symbol rate is an integral submultiple of the carrier frequency; said signal derived from said oscillator is an integral submultiple of the frequency of said oscillator; and a first comparator means responsive to said first signal and said signal derived from the oscillator to generate a third signal; a second comparator means responsive to said second signal and a signal derived from the oscillator to generate a fourth signal; and a means for summing said third and fourth signal and for providing the summed signal to said error signal generating means.

33. The apparatus of claims 31 or 32 further comprising means for weighting the values of said third and fourth signals at the input to said summing means such that said fourth signal is applied with greater weight than the third signal.

34. The apparatus of any one of claims 26, 31 or 32 wherein said second signal is applied to the reset of said counter.

35. The apparatus of any one of claims 26, 29 or 30 wherein the output of said counter is connected to said error signal generating means and the reset of said counter is responsive to said second signal.

36. The apparatus of any one of claims 25,30 or 32 further comprising means for disabling said first signal in response to a predetermined condition.

37. The apparatus of any one of claims 26, 29 or 30 wherein the output of said counter is connected to said error signal generating means and the reset of said counter is responsive to said second signal and wherein said disabling means comprises means for detecting the output level of said error signal generating means and for disabling said first signal when the output level exceeds a preselected level.

38. The apparatus of claim 25, further comprising means for synchronizing said first signal generating means with said second signal.

39. The apparatus of any one of claims 25, 30 or 32 further comprising means for sensing a given parameter during a borehole drilling operation; means for generating said symbols in response to the sensed parameter; means for modulating said carrier signal with said symbols, means for transmitting said modulated signal to the surface; and means for detecting said transmitted modulated signal at the surface.

40. The apparatus of any one of claims 21, 30 or 32 wherein drilling fluid is circulated from the surface downhole and back to the surface during drilling; said transmitting means comprises means for acoustically transmitting signals through said drilling fluid; further comprising means for sensing a given parameter during a borehole drilling operation; means for generating said symbols in response to the sensed parameter; means for modulating said carrier signal with said symbols, means for transmitting said modulated signal to the surface; and means for detecting said transmitted modulated signal at the surface.

41. The apparatus of any one of claims 25, 30 or 32 further comprising means for sensing a given parameter during a borehole drilling operation; means for generating said symbols in response to the sensed parameter; means for modulating said carrier signal with said symbols, means for transmitting said modulated signal to the surface; means for detecting said transmitted modulated signal at the surface;
and filter means responsive to the detected modulated signal for reducing noise signals which are generated in the drilling fluid by the pump used for circulating said drilling fluid.

42. The apparatus of any one of claims 25, 30 or 32 further comprising means for sensing a given parameter during a borehole drilling operation; means for generating said symbols in response to the sensed parameter; means for modulating said carrier signal with said symbols, means for transmitting said modulated signal to the surface; means for detecting said transmitted modulated signal at the surface;
filter means responsive to the detected modulated signal for reducing noise signals which are generated in the drilling fluid by the pump used for circulating said drilling fluid;

said detection means including means for obtaining a carrier signal and a symbol transition signal from said detected modulated signal and for reconstructing said symbols under the timing control of a signal derived from said oscillator.

43. The apparatus of any one of claims 25, 30 or 32 further comprising means for sensing a given parameter during a borehole drilling operation; means for generating said symbols in response to the sensed parameter; means for modulating said carrier signal with said symbols, means for transmitting said modulated signal to the surface; means for detecting said transmitted modulated signal at the surface;
filter means responsive to the detected modulated signal for reducing noise signals which are generated in the drilling fluid by the pump used for circulating said drilling fluid;
said detection means including means for obtaining a carrier signal and a symbol transition signal from said detected modulated signal and for reconstructing said symbols under the timing control of a signal derived from said oscillator;
and means for recording and/or displaying an output derived from the reconstructed symbols.