A 0.65-V 2.5-GHz Fractional-N Synthesizer With Two-Point 2-Mb/s GFSK Data Modulation

We present ultra-low-voltage circuit design techniques for a fractional-N RF synthesizer with two-point modulation which was realized in 90-nm CMOS using only regular V_T devices.; the voltage controlled oscillator, phase-frequency detector and charge pump operate from a 0.5 V supply while the divider uses a 0.65 V supply. The frequency synthesizer achieves a phase noise better than -120 dBc/Hz at 3 MHz, while consuming 6 mW. A calibration technique to equalize the gain between the two modulation ports is introduced and enables phase/frequency modulation beyond the loop bandwidth of the phase-locked loop. Measurement results for 2-Mb/s GFSK modulation are presented.