The design of a direct-conversion paging receiver quadrature converter for wrist watch applications

Direct conversion architecture is highly desirable in a watch pager receiver where space is absolutely at a premium and external components must be minimized. Current consumption is also severely constrained, making it necessary to realize each RF block with less than 100 μA. This paper examines the issues in direct conversion receivers, especially LO re-radiation and 90° phase shifter. It is pointed out that insufficient gm due to insufficient current makes many existing solutions impractical. It is also shown that RC-CR circuits have excellent phase but poor amplitude characteristics. The latter can be remedied by saturating amplifiers and very low current, high performance quadrature converters can thus be realized. The same argument holds for CMOS, which has low gm even if more current is used at 1 GHz