All-digital sigma-delta RF modulator for software defined radio applications

This paper presents an implementation of an alldigital transmitter architecture for software defined radio (SDR) applications. This transmitter uses a first order, sigma-delta modulator for one-bit quantization and noise shaping. The binary signal is passed through high-speed multiplexing circuits for frequency upconvertion directly from base-band to the desired carrier frequency [1]. This digital implementation has interesting features such as reconfigurability as well as preserving a good signal quality, at an additional expense of the high speed components required to perform oversampling and multiplexing. The behavior of the all-digital transmitter is simulated and measured using a baseband signal having 1.94 MHz bandwidth up-converted to 2.5 GHz carrier frequency. The implementation steps are shown along with the obtained simulation and measurement results. The overall linearity performance are given in terms of gain response, signal-to-noise-and-distortion ratio (SNDR) and the adjacent-channel-power ratio (ACPR).