Linearisation techniques for microwave direct-carrier transmitters

• Main Author: Chongcheawchamnan, Mitchai
• Published: University of Surrey 2002
• Subjects: 621.3813
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.395404

A high bandwidth-efficiency modulation scheme is demanded for supporting a high-data rate communications, and so a highly linear transmitter is needed. Applying a linearisation technique to the transmitter can achieve this goal. In this thesis, there are three main topics which are investigated. They are mixer linearisation, the vector modulator based on the reflection-type attenuator and transmitter linearisation. For the first topic, there are two contributions in this thesis. The first technique is the application of the feedforward (FF) technique to linearise a downconversion mixer. It is shown for the first time that the FF technique for a mixer is simplified to a be a single loop rather than the conventional doubleloop structure, leading to a lower complexity and a high-linearity mixer. The second proposed technique applied to a mixer is a harmonic injection technique. The technique simply injects the difference-frequency tone to the input of a mixer. It is shown from the simulations and the experiments that the technique can improve the linearity significantly without trading off the power efficiency. Apart from the mixer linearisation topic, there are three contributions concerning with the reflection-type attenuator (RTA). The first is the feedback reflection attenuator based on Field-Effect Transistors (FETs). It has been found that applying resistive feedback can improve the attenuation range of the RTA and also the phase-distortion by trading-off the input and output return losses. The RTA size is comparable to the size of the conventional RTA which suffers from the phase-distortion. For variable attenuator applications, this structure can improve the attenuation range over the conventional RTA. For bi-phase modulator application, the structure is 50% smaller than the balanced structure based on the conventional RTA, which is needed for correcting the phase-distortion. The second contribution for this topic is the demonstration of an improved structure for a vector modulator (VM) based on the RTA. To avoid the phase distortion problem, the full balanced structure is needed and hence a large chip area is consumed. A simple technique to compensate the phase distortion and balance the amplitude for the whole control voltage range is proposed by adding an extra source inductor and a shunt resistor at the MESFET's drain. The aforementioned problems are overcomed and the circuit size is 50% of the balanced VM. In addition, the baseband signals for the proposed structure are reduced to 2, compared to 4 channels for the balanced VM. The third contribution is the study of the nonlinearity distortion in the RTA, The analysis technique is based on the power series model, The results provide the criteria for selecting the active devices to obtain the small nonlinearity distortion. Linearisation techniques for the whole transmitter are also under the research in this thesis. There are three contributions to the topic; The first technique is applying the FF technique for the whole transmitter. The advantage of the technique is the capability of reducing the distortion not only from the main power amplifier but also for the modulator. The second technique is a proposed topology for a low-cost millimetre-wave transmitter. The structure has low complexity since it composes of only 3 main parts, i.e. a VM, a medium/high-power oscillator and a DSP processor. The DSP part provides multi-functionality to the structure. These functions include baseband predistortion, channel filtering, modulation technique, to name the few. The last contribution of this topic is the improved LINC structure, so called adaptive predistortion LINC, to correct the phase/gain imbalances. The proposed technique shows the capability to overcome this effect, which can degrade the distortion performance in LINC.