In this paper an FPGA-based System-on-Chip (SoC) implementation of a flexible digital radio is presented and evaluated. The implemented transmitter is able to perform a direct up-conversion of a baseband signal to RF operating in the gigahertz frequency range as well as it enables the simultaneous transmission of two different carriers, each one having a different modulation, bandwidth, etc. The developed architecture is fully integrated into a single FPGA device and allows software programmability by including an embedded microprocessor for control operations. The high flexibility of this architecture allows to easily change the frequency of the carriers as also the spectral masks, making it interesting in the scope of white spaces exploration and for software radio-based applications.