A Highly Reconfigurable RF-DPLL Phase Modulator for Polar Transmitters in Cellular RFICs

Abstract

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. The RF-DPLL integrates LC-tank-based digitalcontrolled oscillator (DCO) cores with --noise shaping and fractional sample rate conversion to account for a broad range of frequency bands and spectral emission requirements. A twopoint 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 digital predistortion and DCO gain estimation in combination with open-loop gain auto adjustment and delay calibration are 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 4G, LTE-A uplink and legacy cellular communication standards like 3G, UMTS/HSPA+ and 2G, GSM/EDGE.

A Novel Digital-Intensive Hybrid Polar-I/Q RF Transmitter Architecture

“…10. A more detailed description of a highly reconfigurable implementation for a polar TX supporting cellular communication standards up to 4G LTE-A is for example shown in [20]. The phase modulation is realized by a frequency modulation of the DCO whereas the frequency modulation signal f MOD,n is injected at two points of the RF-DPLL in a feed-forward fashion to avoid a disturbance of the control loop.…”

“…In addition, the RF-DAC cells are operating on twice the channel frequency which results in reduced drain efficiency and limits the applicability with respect to higher channel frequencies. Recently, a digitalintensive multi-band, multi-standard polar TX implemented in 28-nm CMOS utilizing switched capacitor, signed magnitude RF-DACs [19] with external PAs and two-point phase modulated RF digital phase-locked loop (DPLL) [20] has been shown. The polar TX supports 40 MHz contiguous intra-band carrier aggregation in LTE uplink while also covering 2G and 3G cellular standards with FDD and TDD bands ranging from 700 MHz up to 3.8 GHz.…”

Multilevel outphasing power amplifier system with a transmission-line power combiner

“…The phase modulator (PM) has commonly been implemented with a phase-locked loop (PLL) [27,28], but recently structures based on digital-to-time converters (DTCs) have also been utilized to overcome the limited PLL bandwidth [29,30]. As a method of applying the amplitude modulation, analog-intensive supply modulation [31,32] has been increasingly replaced by various structures labeled as digital PAs (DPAs), which can modulate the output amplitude directly based on a digital signal.…”

Analysis of Spectral Degradation and Error Compensation in 5G NR Digital Polar Transmitters

“…The phase modulation signal φ [n] is fed into an RF-phase modulator which is generating a phase modulated square-wave carrier where the positions of rising and falling edges in every RF cycle are controlled. The RF-phase modulator is usually realized by a phaselocked loop (PLL) [19] or via the usage of digital-to-time converters (DTCs) [20]- [24]. In the last step the amplitude signal is applied together with the phase modulated squarewave carrier to a radio frequency digital-to-analog converter (RF-DAC) [25], [26] generating the RF output signal at the desired carrier frequency f c .…”