Microwave lattice mixers at 4.5 Ghz

Abstract

The theory, development and construction is described, of a type of microwave mixer which has a very good noise performance due to a low conversion power loss. The conditions under which this performance is obtained are derived from a theory based on practical diode laws and operating conditions. The effect of input and output terminations on the conversion power loss are then analysed, and the open-circuit image rejection type is shown to be capable of the best performance. Realisation of the open-circuit image termination is difficult at microwave frequencies because of losses involved in the passive input network. Part 1 of this thesis concerns the theory and development of a new type of T.E.M. mode cavity especially suited to these conditions. Performance of the cavity is analysed using lumped equivalent circuits, and the results confirmed on a 400 MHz experimental cavity. A double-balanced system is then described, with a new type of frequency independent phase inversion. This combination is shown to suit the requirements of the type of mixer considered, and a total of six experimental mixers at 4.5 GHz are constructed using this type of cavity. The results obtained are shown to be in accordance with the theories put forward, and noise measurements are used to support the values for conversion power less put forward. In the last chapter, the practical and commercial implications of a high performance mixer at microwave frequencies are considered, especially with regard to the intermediate frequency amplifier design.