A stable multi-mode fractional-N frequency synthesizer using charge pump current calibration

To solve the problem of stability of the multi-mode fractional-N synthesizer during mode transitions, a method of calibrating the charge pump current according to divider ratio is proposed in this paper to stabilize the loop bandwidth. This method keeps the variation of loop bandwidth within a smaller range in different divider ratios. A multi-mode fractional-N synthesizer using this method can operate stably in seven modes of three positioning systems. Simulation results show that the variation of normalized I_CP·K_VCO/ω_VCO in seven modes is in the range of -2.8% and 4%. For each mode, the in-band and out-of-band phase noises are no lager than -93dBc and -119dBc, respectively. The setting time is less than 24μs. The total power consumption is 15.2mW under 1.8V power supply.