Coplanar Hybrids based on an Enhanced Inductor Model for Mixer Applications up to mm-Wave Frequencies

A designer of up-converters for mm-wave frequencies faces a problem, in that the LO frequency and RF frequency are spaced very close together. Therefore, the suppression of the LO signal is not automatically achieved with the matching networks. In order to suppress the LO signal at the output port, a filter with narrow bandwidth has to be applied or the mixer has to be set up as a balanced mixer. Normally, the second solution is more convenient. A balanced mixer can be set up by utilising 90° or 180° hybrids in conjunction with two identical single mixer stages. These hybrids consist of transmission line couplers or lumped elements. For a high packing density design lumped hybrids are the preferred solution since they utilise less chip area. Circuit design based on coplanar technique offers several advantages compared to microstrip technique as outlined by Pogatzki and Kramer (1). Accurate models for coplanar rectangular inductors which are suitable for interactive CAD were not available for mm-wave frequencies in the past. Therefore, large hybrids like Lange couplers or rat-race couplers were used to achieve the required phase conditions for balanced mixers. This paper introduces an enhanced inductor model based on Naghed and Wolff (2), Naghed et al (3), Kulke and Sporkmann (4), Pogatzki et al (5), which is valid for a wide range of geometries up to mm-wave frequencies and therefore, can be used to design 90° and 180° hybrids. These hybrids require only about 10% of the chip area that standard couplers occupy.