A fractional-N frequency synthesizer with a 3-bit 4thorder Σ-Δ modulator

A fractional-N frequency synthesizer in a 0.5-µm BiCMOS technology is presented. A 3-bit 4th order Σ-Δ modulator employs an interpolative architecture with multiple feedback paths and metal-connected multipliers to implement the feedback coefficients, resulting in a simple hardware complexity, lower out-of-band phase noise, a frequency resolution of less than 10-Hz, and agile switching time. The experimental results show - 73dBc/Hz in-band phase noise and -142dBc/Hz out-of-band phase noise at 1MHz offset frequency. The fractional spurs are less than -80dBc/Hz at 200KHz offset frequency and the reference spur is -87dBc/Hz. The proposed synthesizer consumes 5.5mA from a single 3-V supply voltage and meets the requirements of most RF applications.