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The dual-path ΔΣ fractional-N PLL provides linear charge pump gain and small VCO gain variation good for digital modulation but suffers from long settling time due to the narrowband coarse-tuning control path. By employing a dynamic gain-bandwidth control scheme for both paths, a high-gain coarse-tuning loop can be utilized for fast settling time. While maintaining a small control voltage range in the loop filter during frequency acquisition, the proposed method makes both coarse-tuning and fine-tuning paths active with compensated open-loop gain design, which results in a smooth transition from transient to normal mode. A 4 GHz dual-path ΔΣ fractional-N synthesizer with the nominal fine-tuning bandwidth of 60 kHz exhibits less than 68 μs transient settling time for 91 MHz frequency step.
Date of Publication: Aug. 2010