A sixth-order continuous-time quadrature bandpass sigma-delta modulator for UMTS low-IF receivers and a study of the inherent excess loop delay effect

The first continuous-time (CT) quadrature bandpass ΣΔ modulator with a sixth-order architecture is presented. It provides a signal bandwidth of 3.84 MHz (according to the universal mobile telecommunication system (UMTS) standard), centered at a low-IF of 4 MHz, and achieves a simulated peak signal-to-noise-distortion ratio (SNDR) of 78.3 dB and a dynamic range (DR) of 78.6 dB at a clock frequency of 96 MHz. Furthermore, the excess loop delay effect is studied. This effect is one of the main problems in CT ΣΔ modulators. Previously presented papers investigate this effect for CT lowpass and bandpass ΣΔ modulators and for a second-order CT quadrature bandpass ΣΔ modulator. This paper, however, analytically describes and simulates the excess loop delay effect for the high-order CT quadrature bandpass ΣΔ modulator with a sixth-order architecture.