A High Temperature RF Front-End of a Transceiver for High Speed Downhole Communications
Electronics are normally designed to operate at temperatures less than 125 oC. For high temperature applications, the use of those normal electronics becomes challenging and sometimes impractical. Conventionally, many industries tried to push the maximum operating temperature of electronics by either using passive/active cooling systems or tolerating degraded performance. Recently, there has been a demand for more robust electronics that can operate at higher temperature without sacrificing the performance or the use of any weighty, power hungry, complex cooling systems.
One of the major industries that need electronics operating at high temperature is the oil and gas industry. Electronics have been used within the field in many areas, such as well logging downhole telemetry systems, power networks, sensors, and actuators. In the past, the industry has managed to use the existing electronics at temperatures up to 150 oC. However, declining reserves of easily accessible natural resources have motivated the oil and gas industry to drill deeper. The main challenge at deep wells for downhole electronics is the high temperatures as the pressures are handled mechanically. The temperature in deep basins can exceed 210 oC. In addition, existing well logging telemetry systems achieve low data transmission rates of less than 2.0 Mbps at depth of 7.0 Km which do not meet the growing demand for higher data rates due to higher resolution sensors, faster logging speeds, and additional tools available for a single wireline cable. The main issues limiting the speed of the systems are the bandwidth of multi-conductor copper cable and the low speed communication system connecting the tools with the telemetry modem.
The next generation of the well logging telemetry system replaces the multi-conductor wireline between the surface and the downhole with an optical fiber cable and uses a coaxial cable to connect tools with the optical node in downhole to meet the growing needs for higher data rates. However, the downhole communication system between the tools and the optical modulator remains the bottleneck for the system. The downhole system is required to provide full duplex and simultaneous communications between multiple downhole tools and the surface with high data rates and able to operate reliably at temperatures up to 230 oC.
In this dissertation, a downhole communication system based on radio frequency (RF) transmission is investigated. The major contributions of our research lie in five areas. First, we proposed and designed a downhole communication system that employs RF systems to provide high speed communications between the downhole tools and the surface. The system supports up to six tools and utilizes frequency division multiple access to provide full duplex and simultaneous communications between downhole tools and the surface data acquisition system. The system achieves 20 Mbps per tool for uplink and 6 Mbps per tool for downlink with bit error rate (BER) less than 10-6. Second, a RF front-end of transceiver operating at ambient temperatures up to 230 oC is designed and prototyped using Gallium Nitride (GaN) high electron mobility transistor (HEMT) devices. Measurement results of the transceiver's front end are reported in this dissertation. To our knowledge, this is the first RF transceiver that operates at this high temperature. Third, current-voltage and S-parameters characterizations of the GaN HEMT at ambient temperatures of 250 oC are conducted. An analytic model that accurately predicts the behavior of the drain-source resistor (RDS) of the GaN transistor at temperature up to 250 oC is developed based on these characterizations. The model is verified by the analysis and the performance of the resistive mixer. Fourth, a passive upconversion mixer operating at temperatures of 250 oC is designed and prototyped. The designed mixer has conversion loss (CL) of 6.5 dB at 25 oC under local oscillator (LO) power of 2.5 dBm and less than 0.75 dB CL variation at 250 oC under the optimum biasing condition. Fifth, an active downconversion mixer operating at temperatures up to 250 oC is designed and prototyped. The proposed mixer adopts a common source topology for a reliable thermal connection to the transistor source plate. The designed active mixer has conversion gain (CG) of 12 dB at 25 oC under LO power of 2.5 dBm and less than 3.0 dB CG variation at 250 oC. Finally, a novel high temperature negative adaptive bias voltage circuit for a GaN based RF block is proposed. The proposed design comprises an oscillator, voltage doubler, and temperature dependent bias controller. The voltage offset and temperature coefficient of the generated bias voltage can be adjusted by the bias controller to match the optimum biasing voltage required by a RF building block. The bias controller is designed using a Silicon Carbide (SiC) bipolar junction transistor.