Design of a power-efficient widely-programmable Gm-LC band-pass sigma-delta modulator for SDR

This paper presents the design and implementation of a fourth-order band-pass continuous-time ΣΔ modulator intended for the digitization of radio-frequency signals in software-defined-radio applications. The modulator architecture consists of two Gm-LC resonators with a tunable notch frequency and a 4-bit flash analog-to-digital converter in the forward path and a non-return-to-zero digital-to-analog converter with a finite-impulse-response filter in the feedback path. Both system-level and circuit-level reconfiguration techniques are included in order to allow the modulator to digitize signals placed at different carrier frequencies, from 450MHz to 950MHz. A proper synthesis methodology of the loop-filter coefficients at system level and the use of inverter-based switchable transconductors allow to optimize the performance in terms of robustness to circuit errors, stability and power consumption. The circuit, implemented in 65-nm CMOS, can digitise signals with up to 57-dB SNDR within a 40-MHz bandwidth, with an adaptive power dissipation of 16.7-to-22.8 mW and a programmable 1.2/2GHz clock rate1.