A 2.5GHz DDFS-PLL with 1.8MHz bandwidth in 0.35um CMOS

A wideband frequency synthesizer architecture is presented. The proposed topology employs a direct digital frequency synthesizer (DDFS) to control the output frequency of an offset-PLL. In this way, the synthesizer features a very fine frequency resolution, 24 Hz, as in delta-sigma fractional-N PLLs, but without being affected by the quantization-induced phase noise. This, in turn, allows enlarging the loop bandwidth. The frequency synthesizer is designed to be employed as a direct modulator for Bluetooth transmitter in a low-cost 0.35um CMOS technology. At 2.5 GHz it achieves 1.8-MHz bandwidth, while the settling time within 30 ppm for an 80-MHz step is 3 mu s. The integrated phase noise gives less than 1 degree of rms phase error and the worst-case spur is -48 dBc at 1 MHz, well below the specifications. Power dissipation is 120 mW for the PLL core, 50 mW for the DDFS plus DACs, and 19 mW for the GFSK modulator.