Twelve-bit 20-GHz reduced size pipeline accumulator in 0.25 μm SiGe:C technology for direct digital synthesiser applications

This article presents a 20 GHz, 12-bit pipeline accumulator with a reduced number of registers, suitable for direct digital synthesiser (DDS) applications. The accumulator is implemented in the IHP SG25H1 (0.25 μm) SiGe:C technology featuring heterojunction bipolar transistors (HBTs) with F_t/F_max of 180/220 GHz. The accumulator architecture omits the pre-skewing registers of the pipeline, thereby lowering both power consumption and circuit complexity. Some limitations to this design are discussed and the necessary equations for determining the phase jump encountered each time the control word (synthesised frequency) is changed are presented. For many applications employing signal processing after detection, this phase shift can then be corrected for. Compared to a full pipeline architecture (omitting the input circuitry for the most significant bit, as is customary for such designs), the implemented 12-bit accumulator reduces the number of registers by 55% and the power by approximately 32%, while obtaining the highest clock frequency for SiGe:C accumulators intended for DDS applications.