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A downhole telemetry system that transmits a burst-QAM uplink signal to the surface of the well is disclosed. In a preferred embodiment, a downhole instrument coupled to a pair of conductors in a wireline or composite tubing string transmits a burst-QAM uplink signal to a surface system. The burst-Q

A downhole telemetry system that transmits a burst-QAM uplink signal to the surface of the well is disclosed. In a preferred embodiment, a downhole instrument coupled to a pair of conductors in a wireline or composite tubing string transmits a burst-QAM uplink signal to a surface system. The burst-QAM signal preferably comprises a series of data frames carrying telemetry data. Each data frame is preferably preceded by a quiet interval (when no signal is present), a timing synchronization sequence, and a training sequence. The timing synchronization sequence is designed for easy timing recovery at the surface, and the training sequence is designed to aid the adaptation of the equalizer. The data frame itself preferably includes a synchronization field, a data count, and a checksum in addition to the data. Direct digital synthesis is preferably employed to modulate the uplink signal.

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1. A well comprising:a wireline cable having a telemetry conduit;a surface system coupled to the telemetry conduit; anda downhole instrument coupled to the surface system via the telemetry conduit, wherein the downhole instrument is configured to transmit telemetry information to the surface system

1. A well comprising:a wireline cable having a telemetry conduit;a surface system coupled to the telemetry conduit; anda downhole instrument coupled to the surface system via the telemetry conduit, wherein the downhole instrument is configured to transmit telemetry information to the surface system using burst quadrature amplitude modulation (burst-QAM). 2. The well of claim 1, wherein the burst-QAM uses a signal constellation of 16 points. 3. The well of claim 1, wherein the wireline cable includes multiple electrical conductors, of which one or more serve as the telemetry conduit. 4. The well of claim 1, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein the data frames are interspersed with quiet periods. 5. The well of claim 1, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein each data frame is preceded by a respective timing synchronization sequence. 6. The well of claim 1, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein each data frame is preceded by a respective training sequence. 7. The well of claim 4, wherein each data frame consists of a plurality of synchronization words, a data count field, a plurality of data words, and a check word. 8. The well of claim 1, wherein the downhole instrument includes a transmit path having a preamble generator configured to transmit a preamble before each data frame. 9. The well of claim 8, wherein the transmit path includes a data scrambler configured to combine data frame data words with a mask to randomize the data words. 10. The well of claim 9, wherein the transmit path includes a direct digital synthesis modulator configured to modulate the scrambled data words using quadrature amplitude modulation. 11. The well of claim 10, wherein the transmit path includes a high pass filter coupled between the direct digital synthesis modulator and an isolation transformer. 12. The well of claim 9, wherein the surface system includes an uplink receive path having:a timing recovery module configured to generate a clock signal locked to a timing synchronization sequence preceding each data frame in the telemetry signal;an adaptive equalizer configured to update filter coefficients in response to a training sequence preceding each data frame in the telemetry signal;a framing module configured to strip the timing synchronization sequences and the training sequences from the telemetry signal; anda descrambling module configured to combine the data frames with the mask to reverse a downhole scrambling operation. 13. The well of claim 12, wherein the timing synchronization sequences and training sequences are repeated at least 10 times per second. 14. A well comprising:a tubing string that extends into a well bore;a downhole instrument mounted on a lower end of the tubing string; anda wireline cable that extends along the tubing string through an interior space, wherein the downhole instrument is configured to transmit telemetry information to an upper end of the tubing string via telemetry conduits in the wireline, using burst quadrature amplitude modulation (burst-QAM). 15. The well of claim 14, wherein the wireline cable includes multiple electrical conductors, of which one or more serve as the telemetry conduit. 16. The well of claim 14, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein the data frames are interspersed with quiet periods. 17. The well of claim 14, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein each data frame is preceded by a respective timing synchronization sequence. 18. The well of claim 14, wherein the downhole instrument is configured to transmit the telemetry information in data frames, and wherein each data frame is preceded by a respective training sequence. 19. The well of claim 16, wherein each data frame consists of a plurality of synchronization words, a data count field, a plurality of data words, and a check word.