The number of different radio systems is currently increasing in portable devices like mobile phones due to the wider range of applications that require an optimized wireless communications channel to different services or other devices. To make this cost and power efficient, the transmitter must be adaptable to several different radio systems while keeping the complexity and cost reasonable. This article presents fundamental circuit techniques to enable low cost, low power, and low complexity transmitter architecture by combining digital-to-analog conversion, up-conversion, unwanted sideband rejection, power control, and part of the digital image rejection filtering into a single mixed-signal circuit block. The direct-digital RF-modulator architecture is the basis for a software-defined radio transmitter. It was demonstrated against WCDMA system requirements, but the versatility and the performance of this architecture enables support for other radio systems, such as GPRS and WLAN.