Characterisation of the effect of passive components on single-balanced mixer performance, their compact design and load-pull mixer optimisation

<p dir="ltr">The increasing demand for high data-rate wireless communications has resulted in significant growth in the use of the electromagnetic spectrum such as E-band (60 to 90 GHz), W-band (75 to 110 GHz), and pushing through to D-band (110 to 170 GHz), and beyond. The research and development of miniature electronics to support these applications is an ongoing effort in the research community, aided by the release of faster and more elaborate monolithic microwave integrated circuit (MMIC) technology.</p><p dir="ltr">Mixers play a key role in the translation of the signal of interest between different frequency bands. It is often beneficial for a significant portion of the mixing function to be implemented by analogue components, rather than relying solely on digital signal processing (DSP), which can be expensive in terms of size, weight, power and cost (SWaP-C). There is a continuing need for research in high-performance mixers that also have minimum circuit size. Although studies have been conducted to miniaturise the size of on-chip baluns and power dividers, the effect of practical non-idealities in these passive circuits on the mixer’s performance characteristics (such as conversion loss and isolation) have not yet been reported in the open literature. This thesis investigates these effects for a single-balanced mixer (SBM) by deriving simple mathematical equations. The theoretical derivations are supported by electromagnetic simulation results. One of the key findings is that both magnitude and phase imbalances in the LO balun and the RF power divider affect the LO-RF isolation of the SBM. This highlights the importance of maintaining the imbalances in the passive circuits at a minimum level. In this context, three compact power divider designs with minimum imbalances are implemented using different techniques including stepped impedance transmission lines (SITL), defected microstrip structures (DMS) and shunt capacitive loading. The best overall compactness of 82% in the power divider is achieved by incorporating SITLs and shunt capacitive loading with magnitude and phase imbalances of less than 0.06 dB and 1.1◦ respectively. Overall compactness of 64 % is achieved using DMSs with F-shaped slots which is the best compactness so far of its kind.</p><p dir="ltr">Compact structures for a transformer balun and a Marchand balun are designed along with an analysis to find the optimal physical parameters. A key finding is that the optimal coupled-line length of the transformer balun for varying coupling coefficient is in the order of λ/10, which is much shorter than λ/4 coupled lines needed for the Marchand balun. The designed transformer balun achieves magnitude and phase imbalances less than 0.5 dB and 0.5◦ respectively.</p><p dir="ltr">The derived equations demonstrating the impact of passive circuits on mixer performance are experimentally verified by implementing two SBMs; one with the folded transformer balun and the other one with the spiral Marchand balun. High LO-RF isolation of better than 41.5 dB is achieved in the SBM with folded transformer balun due to minimum imbalances. It is also demonstrated that a high LO-RF isolation of an SBM can be achieved by utilising passive circuits with minimum imbalances without requiring any additional techniques, that are time-consuming and resource intensive, such as selective gate biasing.</p><p dir="ltr">Furthermore, an experimental passive load-pull technique to characterise a resistive FET single-ended up-conversion mixer is demonstrated for the first time. A step-by-step approach to find an optimum LO source and RF load impedance combination that lead to minimum conversion loss is presented. This technique will enable the designer to benchmark a realistic target for the mixer’s minimum conversion loss that can be achieved using a particular device size in a particular process technology.</p>