Design Considerations for Image-Rejection Enhancement of Quadrature Mixers

In this letter, the influences of local oscillator (LO) reflections of the in-phase (I) and quadrature-phase (Q) mixers and limited isolations between the I/Q LO ports on the image-rejection ratio (IRR) performances of quadrature mixers are investigated and analyzed for the first time. The analysis reveals that, in addition to generating well-balanced quadrature LO signals, a good LO impedance matching and/or high electrical isolation between the LO ports of the I and Q mixers are required to obtain a high IRR over a wide bandwidth. To verify the analysis, a quadrature mixer with carefully designed wideband LC-ladder matching networks for the I/Q LO ports is implemented in a 65-nm CMOS technology. Under 1-V supply voltage and 3-dBm LO power, the proposed quadrature mixer exhibits 3.3±1.5 dB conversion gain and better than 30-dBc IRR across the 20-25-GHz band.