Microwave differential structures optimization: application to a double balanced SiGe active down-converter design

This paper deals with the design of high performance microwave and millimeter wave balanced circuits. It focuses on the design methodology and the description of some original techniques which improve the balance of microwave differential amplifiers. Based on these structures, an original broadband active balun is proposed and applied as RF and LO power splitters of a K band double balanced down-converter. This converter moreover involves a simplified Gilbert mixing cell, and 3D interconnections have been developed to prevent any balance damage at the couplers/mixer interfaces. The overall function, which converts a 20 GHz RF single-ended signal into a 1 GHz IF one, has been implemented on a compact single chip using a 0.25 μm SiGe BiCMOS process. Measurements show an 18 dB conversion gain, a 12 dB double side band noise figure and a -1 dBm OP1dB. Moreover, as a consequence of its highly balanced configuration, this circuit features outstanding port-to-port isolations as well as a spurious-free IF output spectrum.