An injection-locked frequency-tracking [SIGMA][DELTA] direct digital frequency synthesizer

A bandpass (BP) sigma-delta modulator ([SIGMA][DELTA]M)-based direct digital frequency synthesizer (DDS) architecture is presented. The DDS output is passed through a single-bit, second-order BP[SIGMA][DELTA]M, shaping quantization noise out of the signal band. The single-bit BP[SIGMA][DELTA]M is then injection locked to an LC-tank oscillator, which provides a tracking BP filter response within its locking range, suppressing the BP[SIGMA][DELTA]M out of band quantization noise. The instantaneous digital frequency control word input of the DDS is used to tune the noise shaper center frequency, achieving up to 20% tuning range around the fundamental. The BP[SIGMA][DELTA]M-based synthesizer is fabricated in a 0.25-[micro]m digital CMOS process with four layers of metal. With a second-order BP noise shaper and a 44-MHz LC tank oscillator, an SFDR of 73 dB at a 2-MHz bandwidth and phase noise lower than--105 dBc/Hz at a 10-kHz offset is achieved. Index Terms--Direct digital frequency synthesizer (DDS), injection locking, oscillator, phase noise, sigma-delta modulator ([SIGMA][DELTA]M).