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A direct digital frequency synthesizer (DDFS) using an analog-sine-mapping technique is presented in a 0.35-μm SiGe BiCMOS process. We intend to apply the translinear principle to develop a triangle-to-sine converter (TSC) that can achieve outputs with low harmonic content. The TSC is introduced for the DDFS to translate phase data to sine wave. Using this analog-interpolating technique, the DDFS, with 9 bits of phase resolution and 8 bits of amplitude resolution, can achieve operation at 5-GHz clock frequency and can further reduce power consumption and die area. The spurious-free dynamic range (SFDR) of the DDFS is better than 48 dBc at low synthesized frequencies, decreasing to 45.7 dBc worst case at the Nyquist synthesized frequency for output frequency band (0-2.5 GHz). The DDFS consumes 460 mW at a 3.3-V supply and achieves a high power efficiency figure of merit (FOM) of 10.87 GHz/W. The chip occupies 1.5 × 1.4 mm2.