TABLE OF CONTENTS Preceding sections have shown the importance of coupling structures such as 90 and 180 hybrid couplers, baluns, power splitters and combiners in balanced mixers. In MMICs, it is desirable to make the overall mixer circuit as small as possible. Generally, the coupling structures are the circuits that require the most area. This is particularly true for distributed circuits at low frequencies. Therefore, it is common practice to use either a folded version of the distributed circuit or a lumped approximation, in order to reduce the overall size. At millimetre-wave frequencies, distributed circuits are already quite small and commonly used. An alternative to the passive distributed and lumped-element coupling circuits is an active circuit. Active coupling circuits have the potential to be much smaller than the distributed or lumped circuits, but although they can have gain, the noise performance of most circuits investigated to date is poorer. Often, the asymmetry of an active balun or splitter means that the noise figure to each output port is not the same, even though the gains may be equally balanced. This can be quite acceptable in some receiver architectures where the mixer is preceded by a low noise amplifier. However, although insertion of a low noise amplifier in front of a mixer can decrease the overall noise figure of the combination, it also decreases the dynamic range. Recently, balanced oscillators [79], which have two equal amplitude outputs in anti-phase, have been demonstrated at microwave frequencies. They have the potential of directly...
