Design and realization of a digital Delta Sigma modulator for fractional-nfrequency synthesis

The basic operation of a fractional-n frequency synthesizer has been publis hed, but to date little has been presented on the digital Delta Sigma modul ators which are required to drive such synthesizers, This paper provides a tutorial overview, which relates digital Delta Sigma modulation to other ap plications of Delta Sigma modulation where the literature is more complete. The paper then presents a digital Delta Sigma modulator architecture which is economical and efficient and which is practical to realize with commerc ially available components in comparison with other possible implementation s which require extensive custom very large-scale integration (VLSI). A demonstration is made of a 28-b modulator using the architecture presente d, which provides a 25-MHz tuning bandwidth and < 1-Hz frequency resolution . The modulator is demonstrated in an 800-MHz frequency synthesizer having phase noise of -90 dBC/Hz at a 30-kHz offset.