A highly reconfigurable RF-DPLL phase modulator for polar transmitters in multi-band/multi-standard cellular RFICs

A multirate, fractional-N RF digital phase-locked loop (DPLL) phase modulator implementation for polar transmitter supporting cellular communication standards up to 4G LTE-A is demonstrated for the first time. The RF-DPLL integrates LC-tank-based digital-controlled oscillator (DCO) cores with ΔΣ-noise shaping and fractional sample rate conversion to account for a broad range of frequency bands and spectral emission requirements. A two-point modulation with different sampling rates and signal scaling is applied to optimize the system for operation in narrow-band and wide-band phase modulation. DCO predistortion and DCO gain estimation is implemented to achieve sufficiently low in-band distortion. Measurement results of the RF-DPLL system as part of a polar transmitter implemented in 28-nm CMOS are shown, fulfilling 3GPP specifications for LTE-A uplink.