A 2.5-GHz DDFS-PLL with 1.8-MHz bandwitdth in 0.35-micron 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 0.35- m 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 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 120mW for the PLL core, 50 mWfor the DDFS plus DACs, and 19 mW for the GFSK modulator.