A 2.4-GHz Extended-Range Type-I $\Sigma \Delta$ Fractional- $N$ Synthesizer With 1.8-MHz Loop B

Low-power low-loop-bandwidth (BW) integer-N frequency synthesizers with low phase noise have been reported previously. However, achieving similar power/phase-noise performance for a fractional-N synthesizer with a wide loop BW along with excellent spur performance has been challenging. A conventional fractional-N synthesizer is clocked by a crystal oscillator operating at a reference frequency (f_ref) less than a few tens of megahertz. An attractive alternative is to replace the low-frequency crystal oscillator with an integer-N phase-locked loop operating at an f_ref of a few hundreds of megahertz. The advantages and challenges of designing such a wide-loop-BW fractional-N synthesizer for low phase noise, spur, and power consumption are considered, and an extended-phase-range type-I ΣΔ fractional-N frequency synthesizer is implemented with an optimal f_ref of 290 MHz. Measurement results show that the synthesizer operating at 2.4 GHz with a wide loop BW of 1.8 MHz attains an in-band phase noise of -110 dBc/Hz and a worst case fractional spur of -69 dBc. The digital-intensive 0.18-μm CMOS design consumes 14.1 mW. No quantization noise cancellation or charge pump linearization techniques are used.