A 21–48 GHz Subharmonic Injection-Locked Fractional-N Frequency Synthesizer for Multiband Point-to-Point Backhaul Communications

This paper presents a mm-wave subharmonic injection-locked (SHIL) fractional-N frequency synthesizer for wireless multiband point-to-point backhaul communications. The SHIL synthesizer implements a low-phase-noise 4.5-6.1 GHz PLL and injects its output to a ÷3/÷4 dual-modulus divider followed by an ultra-wideband injection-locked frequency-multiplier (ILFM) chain to achieve excellent phase noise over an ultra-wide frequency tuning range. The proposed ILFM chain employs higher-order LC tanks to generate a rippled phase response around 0 ^° over a wide frequency range to significantly enhance the locking range and to eliminate expensive mm-wave frequency calibration loops. Fabricated in a 65 nm CMOS process, the synthesizer prototype measures a continuous output frequency range from 20.6 to 48.2 GHz with frequency resolution of 220 kHz and output phase noise between -107.0 and -113.9 dBc/Hz at 1 MHz offset while consuming 148 mW and occupying 1850 × 1130 μm ².