| 摘要 |
1. A data transmission system comprising, signal generating means for generating and applying to a signal circuit a signal which represents data to be transmitted, wherein the signal generating means comprises a reference signal generating means and modulation means for modulating the reference signal to thereby encode the data to be transmitted, the modulation means comprising effective impedance modulating means for modulating the effective impedance of the signal circuit, and the effective impedance modulating means comprising switch means which is switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state, wherein the switch means comprises charge storage means arranged to enhance the difference in effective impedance between the first and second states. 2. A data transmission system for use with a pipeline system having at least one electrically conductive pipe arranged for carrying a fluid or gas, comprising reference signal generating means for applying a reference signal to a signal circuit, effective impedance varying means for varying the effective impedance of the signal circuit in dependence on data to be transmitted, and monitoring means for monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit to thereby extract data, wherein the reference signal generating means is arranged for applying a signal to a pipe in the pipeline system whereby the signal circuit comprises said pipe and effective impedance varying means comprising switch means which is switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state, wherein the switch means comprises charge storage means, providing varying the effective impedance of the signal circuit in dependence on the state of the switch means. 3. A data transmission system according to Claim 2 wherein the signal circuit comprises a signal channel along which data is transmitted and a return conduction path via earth. 4. A data transmission system according to Claim 2 or 3 wherein the reference signal generating means comprises a constant current source. 5. A data transmission system according to Claim 4 wherein the current sources is arranged to generate a constant dc current signal. 6. A data transmission system according to Claim 4 or 5 wherein the monitoring means is arranged for monitoring changes in the potential difference, due to the reference signal, between a terminal of the current source and earth. 7. A data transmission system according to any one of Claims 4 to 6 wherein the monitoring means is connected across the terminals of the current source. 8. A data transmission system according to any one of Claims 4 to 6 wherein a separate reference earthing point is provided acid the monitoring means is arranged to monitor the potential difference between an output of the current source and the reference earthing point. 9. A data transmission system according to any one of Claims 2 to 8 which is arranged for use in a well having downhole structure, the downhole structure comprising the pipe to which the reference signal generating means is connected. 10. A data transmission system according to any one of Claims 2 to 9 wherein the effective impedance varying means is an effective impedance modulation means for modulating the reference signal to encode data to be transmitted and the monitoring means is arranged for decoding data transmitted. 11. A method of data transmission for use with a pipeline system having at least one electrically conductive pipe arranged for carrying a fluid or gas comprising the steps of generating and applying a reference signal to a signal circuit as part of the signal circuit and applying the reference signal to said pipe, varying the effective impedance of the signal circuit in dependence on data to be transmitted, and monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit to thereby extract data, wherein for varying the effective impedance of the signal circuit the switch means is used which is switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state, wherein the switch means comprises charge storage means providing varying the effective impedance of the signal circuit in dependence on the state of the switch means. 12. Data transmission apparatus for use in a data transmission system according to any one of Claims 1 to 10, the apparatus comprising reference signal generating means for applying a reference signal to a signal circuit, effective impedance varying means for varying the effective impedance of the signal circuit in dependence on data to be transmitted, and effective impedance varying means monitoring means for monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit to thereby extract data, reference signal generating means is arranged for applying a signal to a pipe in the pipeline system, means comprising switch means which is switchable between two states, the effective impedance of the signal circuit when the switch means is in the first state being greater than the effective impedance of the signal circuit when the switch means is in the second state, wherein the switch means comprises charge storage means arrange providing varying the effective impedance of the signal circuit in dependence on the state of the switch means, providing varying the effective impedance of the signal circuit in dependence on the state of the switch means. |
