A 30-GHz Digital Sub-Sampling Fractional-N PLL With -238.6-dB Jitter-Power Figure of Merit in 65-nm LP CMOS

This article describes the implementation of a 30-GHz frequency synthesizer. The target is to reduce the gap in terms of jitter-power product that exists between millimeter- wave and RF synthesizers, using a low-cost 65-nm LP CMOS technology. The circuit is a digitally intensive fractional-N phase-locked loop, which combines a sub-sampling bang-bang phase detector, a low-power divider-by-six prescaler with a novel injection scheme, and a digital technique reducing the output range of the digital-to-time-converter. The synthesizer can operate between 30.4 and 34.2 GHz with a frequency resolution of 191 Hz and with an integrated rms jitter below 180 and 197.6 fs for the integer-N and fractional-N channels, respectively. The sub-sampling loop can synthesize fast sawtooth chirps around 33.4 GHz with peak-to-peak amplitudes up to 1.14 GHz. The fractional spurs, measured at the 5-GHz prescaler output, are below −54 dBc, even considering near-integer channels. The power dissipation of 35 mW from the 1.2-V supply leads to a −238.6 dB of jitter-power figure of merit for fractional-N channels.