An IF-sampling SC complex lowpass ΣΔ modulator with high image rejection by capacitor sharing

Lowpass complex sigma-delta (ΣΔ) modulators that have a built-in mixer can be used to digitize narrowband intermediate frequency (IF) signals in radios and cellular systems. A well-known problem of the complex modulators is the mismatches between the in-phase (I) and quadrature phase (Q) channels. In this paper, a technique of sharing the critical sampling and feedback capacitors between the I and Q channels of the modulator is proposed. As demonstrated by circuit simulations, the mismatch effect can be greatly suppressed and thus the image rejection performance can be improved. A 3^rd order complex modulator is designed with a 0.35μm CMOS technology for a 10.7MHz IF input.