Current state-of-the-art software defined radio transmit techniques typically involve synthesis of signals at the Intermediate Frequency (IF) or, even in some cases, such as High Frequency (HF) and Very High Frequency (VHF), at the Radio Frequency (RF). Such synthesis demands data streaming to the Digital Analog Converter (DAC) input at very high rates, approaching 1 GHz in some cases. Supplying data at these rates is a serious challenge, even between components on the same board. To alleviate the problems of moving data at high speed and simplifying system design, DAC manufacturers are offering parts that include on-chip clock multipliers, Direct Digital Synthesis Numerically Controlled Oscillators (DDS/NCOs), interpolating filters, and mixers. The theory is that by digitally performing the final upconversion on the DAC chip, the user need only supply data at the baseband rate. Furthermore, the programmable NCO can bring substantial flexibility to the hardware, enabling software control for spread spectrum techniques such as frequency hopping. While these features provide obvious benefits, there are subtle caveats that should be noted when specifying DACs for phase-sensitive applications.