A 6.8-14.4GHz Calibration-Free Fractional-N RO-Based Harmonic-Mixing Frequency Synthesizer Achieving −80.7dBc Fractional Spur

A calibration-free fractional-N synthesizer is presented with octave tuning capability and low fractional spurs, which is fabricated using a 65-nm bulk CMOS process. The synthesizer is based on the harmonic-mixing (HM) architecture, featuring a unity gain for the quantization error transfer to achieve low fractional spur without any calibration. To support octave-tuning, a ring-oscilator(RO)-based first stage is utilized to save area with negligible jitter deterioration. Moreover, a variable sub-sampling ratio technique is employed to relax the tuning range requirement for the RO. The measurements demonstrate an octave-tuning synthesizer with −80.7 dBc fractional spur, 71.7% tuning range and −254.6 dB FoMT.