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.