A High Image-Rejection SC Quadrature Bandpass DSM for Low-IF Receivers

The quadrature bandpass Delta-Sigma modulator (QBDSM) is widely used in low-IF wireless receivers. In general, every coefficient in a QBDSM is complex. For a complex coefficient realized by real circuit blocks, there are always two circuit paths (I and Q) for both its real and imaginary parts (if non-zero). Mismatches between the two circuit paths of any complex coefficient in the QBDSM can lead to the following problems: 1) interference at the image frequency to fold into the signal band; 2) the unshaped quantization noise at the image frequency to leak into the signal band; and even 3) the mirrored image of the desired signal around its center frequency (self-image) to appear in some implementations. This paper analyzes the I/Q mismatch effects in a switched-capacitor QBDSM and proposes chopper and circuit multiplexing techniques to mitigate these problems. A prototype QBDSM was fabricated in a 0.18 μm CMOS, with a measured image rejection ratio no less than 73 dB, a self-image rejection ratio of 72 dB, and no observable image quantization noise.