A quadrature modulator with enhanced harmonic rejection filter

When using a current-commutating quadrature modulator in a wireless transmitter, undesired spurs are generated at 3f_LO, 5f_LO, and so on, thus requiring a good filter to suppress those unwanted signals. By using the non-ideal characteristic of the non-zero output resistance of the amplifier in a Sallen-Key filter, a second order low-pass filter with transmission zeros is proposed. In this paper, the filter is designed and simulated using a 0.35 μm SiGe BiCMOS model provided by TSMC. The designed 4^th-order low-pass filter with a parasitic pole, implemented by cascading two biquads and an amplifying stage, outperformed a 9^th-order AC-coupled RC low-pass filter not only in size, but also in suppression characteristics and current consumption.