Quantization noise upconversion effects in mixer-first direct delta-sigma receivers

In this paper, we investigate the application of the direct ΔΣ receiver (DDSR) concept in a mixer-first architecture. Specifically, we analyze the degrading effects of quantization noise (Q_n) upconversion on DDSR sensitivity, which is a major concern in mixer-first DDSR architecture. We demonstrate that with the chosen approach, the mixer-first architecture is suitable for the DDSR despite the potential challenges arising from Q_n upconversion. A systematic modeling and understanding of Q_n upconversion effects is presented, which lead to simple design guidelines. The results demonstrate that a first-order low-pass Q_n filtering is sufficient in most cases for mixer-first DDSR implementations. Based on analytical results, we design a transistor-level mixer-first DDSR by merging the functionality of N-path capacitors both as channel select and Q_n filters. Simulations performed in a 28-nm complementary metal-oxideŰsemiconductor (CMOS) process show a mere 1.5-dB degradation from maximum signal-to-noise and distortion ratio (SNDR) for the worst-case scenarios arising from Q_n upconversion effects, validating the chosen approach.