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High-Efficiency PA Development
The high-efficiency power amplifier (PA) employs a robust architecture to provide increased power-delivery efficiency in a compact, rugged mechanical package. Increasing transmission efficiency results in a greater transfer of available energy from the transmitter’s principal power source to the transmission media. The maximization of energy transfer is especially critical downhole, where the transmitter’s principal power source is typically a battery. Efficient coupling of magnetic field energy into the low impedance transmission media present at the PA transmitting antenna results in a maximum amount of field energy remaining at the receive antenna after transit through the earth’s strata. Increased field energy at the receive antenna equates to increased recoverable signal amplitude; thus, the overall receiver signal-to-noise ratio is improved resulting in increased operational depth capability.
Two separate high-efficiency PA systems have been developed: a downhole tool power amplifier and a surface power amplifier. The downhole tool PA is compact, rugged and uses a solid-state switched-output driver circuit topology to efficiently convert batterystored energy into magnetic field energy. For maximum operational flexibility, the downhole PA can be reconfigured while deployed in-hole via a two-way, EM-based, communications link to the surface. Additionally, the downhole tool PA electronic-assembly was sized to fit in a 11⁄4-in. inside diameter (max) pressure vessel to allow integration into a working drillstring.
The downhole tool PA and receiver assembly consists of printed circuit boards mounted in a rigid backbone. Each circuit board is designed to utilize SOIC components provided by Honeywell. The components are designed for extended temperature range operation to 225°C and are provided in thruhole style integrated circuit packages.
The power source for the downhole electronic assembly consists of a battery pack constructed from a series of high-temperature (200°C), lithium-thionyl-chloride batteries arranged in an axial configuration (shown in the right-hand portion of Figure 2). The battery pack is assembled with insulators between each cell to prevent shorting during drilling operations and act as mini shock absorbers to absorb vibration. The entire
PA/receiver/battery assembly is contained within a 11⁄4-in. ID screw-on pressure housing (shown in the foreground of Figure 2). The downhole tool PA/receiver assembly provides the following capabilities:
• 15 W (max) delivered power: programmable via the EM downlink (eight adjustment settings);
• 2 Hz to 10 Hz adjustable uplink carrier frequency: programmable via the EM downlink (five adjustment settings);
• ability to measure received signal strength of the downlink and report via EM uplink;
• ability to measure gap-antenna impedance and report via EM uplink;
• ability to perform real-time systemhealth diagnostics and report status via EM uplink; and
• ability to record up to 30 minutes of time domain waveforms from the gap antenna.
The surface power amplifier (Figure 3), housed in a compact ruggedized chassis, allows downlink command-communications with the downhole tool.
Its electronics utilize the same basic solid state, switched output driver circuit topology as the downhole tool PA; however, the surface PA is capable of generating 1,000 Wrms of power to the load. In addition, the surface PA features safety interlocks so power delivery to the load is disabled if an abnormal impedance condition is sensed on the PA outputs. This interlock feature helps protect against the safety hazard caused by broken, loose or improperly connected antenna wires.
The surface PA provides an RS-232 serial communications interface to a laptop or desktop computer so information can be exchanged with the data fusion receiver. The serial communications link is used to relay the operator-downlink command packet to the PA for formatting and subsequent downlink transmission to the downhole tool. The surface PA also performs internal health/diagnostic functions such as measurement of the transmission antenna load impedance and the monitoring of various internal system-health metrics. This data is reported to the data fusion receiver GUI application via the RS- 232 serial communications interface.
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