A digital-RF converter architecture for IQ modulator with discrete-time low resolution quadrature LO

A digital-to-RF converter (DRFC) architecture for IQ modulator is proposed in this paper. The digital-RF converter utilizes the mixer DAC concept but a discrete-time oscillatory signal is applied to the digital-RF converter instead of a conventional continuous-time LO. The architecture utilizes a low pass ΣΔ modulator and a semi-digital FIR filter. The digital ΣΔ modulator provides a single-bit data stream to a current-mode SDFIR filter in each branch of the IQ modulator. The filter taps are realized as weighted one-bit DACs and the filter response attenuates the out-of-band shaped quantization noise generated by the ΣΔ modulator. To find the semi-digital FIR filter response, an optimization problem is formulated. The magnitude metrics in out-of-band is set as optimization constraint and the total number of unit elements required for the DAC/mixer is set as the objective function. The proposed architecture and the design technique is described in system level and simulation results are presented to support the feasibility of the solution.