DIGITALLY CONTROLLED DC OFFSET FOR LO LEAKAGE IN RF TRANSMITTERS

The goal of this thesis is to improve the reliability of a predistortion testbed which was developed for an educational lab in a previous Master thesis. The reliability of a system is one of the main concerns in any engineering design. Good engineers have to be able to provide designs which are both robust and reliable. Moreover, practicality has to be taken into account. It is not sufficient to have a system that can complete the task but is neither reliable nor practical. In Summary the goal for any designer should be to make the system designed robust, reliable, user friendly, easy to set up, and cost efficient. Therefore even though the previous predistortion linearization design was working properly, it lacked in reliability. That is one of the main problems when a design relies solely on analog components. Analog components are bound to exhibit mismatches and to be sensitive to the outside environment. In order to handle all of the problems mentioned, a digital system will be introduced as the solution of the reliability and tuning problem. The advantages of digital electronic systems are legendary. Not only is it easy to manipulate but also is produces very consistent results. One of the goal of this thesis is to replace the dc offset control circuit of a baseband circuit in the educational testbed from analog to digital. However, the robustness will come at a price. The digital system has to be accompanied with controlling software. The control software provided with the circuit blocks acquired was used for the new design to work properly. Two major building blocks were switched to digital control: (1) the variable gain amplifiers for conditioning the baseband signals and (2) the variable digital potentiometers for the LO leakage control. The variable gain amplifiers also perform as BALUN signal converters. That is, they generate differential signals from a single ended signal source. The digital potentiometers were used as a dc offset controller. The dc offset circuit developed rely on the fundamental concept of the voltage divider to generate the tuning voltages required by the IQ modulator. Buffer amplifiers were also implemented to avoid any loading effect as the tuning voltage is also used to bias the IQ modulator. A successful suppression of the LO leakage by up to 42 dB was demonstrated using this programmable dc offset circuitry.