A 250MHz-to-4GHz Δ-Σ Fractional-N Frequency Synthesizer with Adjustable Duty Cycle

A Delta-Sigma fractional-N Phase-locked Loops (PLL)- based frequency synthesizer (FS) with a frequency range of 250 MHz to 4 GHz is presented. By employing the Delta-Sigma fractional-N technique and utilizing the multi-phase clocks available in the ring-VCO of the PLL, the frequency synthesizer is capable of generating high-resolution and low-spur clocks with instant frequency switching. Moreover, the proposed FS has the ability to adjust clock duty cycle which is needed in applications such as time-interleaved ADCs, switched-capacitor circuits, and DC-DC converters.