An Injection-Locked Frequency-Tracking ΣΔ Direct Digital Frequency Synthesizer

A bandpass (BP) sigma-delta modulator (ΣΔM)-based direct digital frequency synthesizer (DDS) architecture is presented. The DDS output is passed through a single-bit, second-order BPΣΔM, shaping quantization noise out of the signal band. The single-bit BPΣΔM is then injection locked to an LC-tank oscillator, which provides a tracking BP filter response within its locking range, suppressing the BPΣΔM 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 BPΣΔM-based synthesizer is fabricated in a 0.25-μm 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.